Your First ios Application

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Your First iOS Application

General

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No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, mechanical, electronic, photocopying, recording, or otherwise, without prior written permission of Apple Inc., with the following exceptions: Any person is hereby authorized to store documentation on a single computer for personal use only and to print copies of documentation for personal use provided that the documentation contains Apple’s copyright notice.

The Apple logo is a trademark of Apple Inc. No licenses, express or implied, are granted with respect to any of the technology described in this document. Apple retains all intellectual property rights associated with the technology described in this document. This document is intended to assist application developers to develop applications only for Apple-labeled computers.

Apple Inc. 1 Infinite Loop Cupertino, CA 95014 408-996-1010

App Store is a service mark of Apple Inc. Apple, the Apple logo, Cocoa, Cocoa Touch, Instruments, iPhone, Mac, Objective-C, and Xcode are trademarks of Apple Inc., registered in the United States and other countries. iPad is a trademark of Apple Inc.

IOS is a trademark or registered trademark of Cisco in the U.S. and other countries and is used under license.

Even though Apple has reviewed this document, APPLE MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESS OR IMPLIED, WITH RESPECT TO THIS DOCUMENT, ITS QUALITY, ACCURACY, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE. AS A RESULT, THIS DOCUMENT IS PROVIDED “AS IS,” AND YOU, THE READER, ARE ASSUMING THE ENTIRE RISK AS TO ITS QUALITY AND ACCURACY.

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Introduction 7

Organization of This Document 8

Chapter 1

Creating Your Project 11

Xcode 11

Application Bootstrapping 14 Recap 16

Chapter 2

Understanding Fundamental Design Patterns 17

Delegation 17

Model-View-Controller 17 Target-Action 18

Other Design Patterns 18

Chapter 3

Adding a View Controller 19

Adding a View Controller Class 19 Adding a View Controller Property 22 Creating the View Controller Instance 23 Setting Up the View 24

Housekeeping 25

Implementation Source Listing 26 Test the Application 27

Recap 28

Chapter 4

Inspecting the Nib File 29

A View Controller’s View Often Comes from a Nib File 29 File’s Owner 30

The View Outlet 31 Test the Application 32 Recap 34

Chapter 5

Configuring the View 35

Adding the User Interface Elements 35 Make Connections 39

Make the Text Field’s Delegate Connection 43 Testing 44

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Recap 45

Chapter 6

Implementing the View Controller 47

The User’s Name Property 47 The changeGreeting: Method 49

Configure the View Controller as the Text Field’s Delegate 50 Test the Application 51

Recap 51

Chapter 7

Troubleshooting 53

Code and Compiler Warnings 53 Check the Nib Files 53

Delegate Method Names 53

Chapter 8

Next Steps 55

Improve the User Interface 55

Create User Interface Elements Programmatically 55 Install the Application on a Device 56

Add More Functionality 56 Add Unit Tests 57

Appendix A

Code Listings 59

HelloWorldAppDelegate 59

The header file: HelloWorldAppDelegate.h 59

The implementation file: HelloWorldAppDelegate.m 59 MyViewController 60

The header file: MyViewController.h 60

The implementation file: MyViewController.m 60

Document Revision History 63

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Figures

Chapter 3

Adding a View Controller 19

Figure 3-1 MyViewController 20

Figure 3-2 MyViewController 21

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This tutorial shows how to create a simple iOS application. It has a text field, a label, and a button. You can type your name into the text field and tap the button to say hello. The application updates the label’s text to say “Hello, <Name>!”:

The goal is not to create a finely polished application, or on the other hand to get an application on screen as quickly as possible, but to illustrate the Three T’s:

● Tools: How you create and manage a project using Xcode

● Technologies: How to make your application respond to user input using standard user interface controls

● Techniques: The fundamental design patterns and techniques that underlie all Mac OS X development

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A secondary goal is to point out other documents that you must also read to fully understand the iOS development tools and techniques.

You should read this document if you are just starting development for iOS. You should use this tutorial to get started even if you intend to develop solely for the iPad. Although the tutorial shows the iPhone user interface, the tools and techniques you use are exactly the same as those you use to develop iPad applications. You must, though, already have some familiarity with the basics of computer programming in general and the Objective-C language in particular. If you haven’t used Objective-C before, read through at least Learning

Objective-C: A Primer.

Important: To follow this tutorial, you must have installed the iPhone SDK and developer tools available

from theiOS Dev Center.

This document describes the tools that are available for iPhone SDK v4.3 and later—check that your version of Xcode is at least 4.0.

To learn more about the technologies in Cocoa Touch, read iOS Technology Overview.

In this tutorial, little regard is given to the user interface. Presentation is, however, a critical component of a successful iOS application. You should read the iOS Human Interface Guidelines to understand how the user interface might be improved for a full-fledged application.

This tutorial does not discuss issues beyond basic application development; in particular, it does not describe how to submit applications to the App Store.

Organization of This Document

The tool you use to create Mac OS X applications is Xcode—Apple’s IDE (integrated development environment).

“Creating Your Project” (page 11) shows you how to create a new project and describes how an application launches.

Whereas Xcode is the application you use to create and manage the project, Cocoa is the name given to the collection of APIs that you use to develop programs for Mac OS X. You should typically use the highest level of abstraction available to accomplish whatever task you want to perform. To create a desktop application, you should use the Objective-C-based frameworks that provide the infrastructure you need to implement graphical, event-driven applications.

To use the Objective-C frameworks effectively, you need to follow a number of conventions and design patterns. Following the Model-View-Controller (or “MVC”) design pattern, the application uses a controller object to mediate between the user interface (view objects) and the underlying representation of a track (a model object).

● “Understanding Fundamental Design Patterns” (page 17) provides an overview of the design patterns you’ll use.

● “Adding a View Controller” (page 19) shows you how to customize a view controller class and create an instance of it.

You design the user interface graphically, using Xcode, rather than by writing code. The interface objects are stored in an archive called a nib file.

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Terminology: Although an interface document may have a “.xib” extension, historically the extension was

“.nib” (an acronym for “NextStep Interface Builder”) so they are referred to colloquially as “Nib files.”

The nib file doesn’t just describe the layout of user interface elements, it also defines non-user interface elements and connections between elements. The controller object is represented in the nib file. At runtime, when a nib file is loaded the objects are unarchived and restored to the state they were in when you saved the file—including all the connections between them.

● “Inspecting the Nib File” (page 29) introduces you to two important concepts: outlets, and the File’s Owner proxy object

● “Configuring the View” (page 35) shows you how to add and configure the button and text fields.

● “Implementing the View Controller” (page 47) walks you through implementing the view controller, including memory management, thechangeGreeting:method, and ensuring that the keyboard is dismissed when the user taps Done

● “Troubleshooting” (page 53) describes some approaches to solving common problems that you may encounter.

● “Next Steps” (page 55) offers suggestions as to what directions you should take next in learning about iOS development.

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In this chapter, you create the project using Xcode and find out how an application launches.

Xcode

The tool you use to create applications for iOS is Xcode—Apple’s IDE (integrated development environment). You can also use it to create a variety of other project types, including Cocoa and command-line utilities.

To create a new project . . .

1. Launch Xcode.

By default it’s in/Developer/Applications.

2. Create a new project by choosing File > New Project. You should see a new window similar to this:

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3. In the left pane, choose iOS application.

4. Select Window-based Application then click Next.

5. In the Device popup menu, make sure iPhone is selected.

Do not select the option to use Core Data. The tutorial does not use Core Data.

Do not select the option to use unit tests. The tutorial doesn’t discuss how to set up unit tests; however, you should consider using unit tests a best practice for Cocoa development.

6. EnterHelloWorldas the product name, and a suitable company identifier (if you don’t have one, use

edu.self).

Note: The remainder of the tutorial assumes that you named the projectHelloWorld, so the application delegate class is calledHelloWorldAppDelegate. If you name your project something else, then the application delegate class will be called YourProjectNameAppDelegate.

7. Click Next.

A sheet appears to allow you to select where your project will be saved.

8. Select a suitable location for your project (such as the Desktop or a custom Projects directory), then click Save.

You do not need to create a version control repository for this project, although in general you are encouraged to do so.

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You should see a new project window like this:

If you haven’t used Xcode before, take a moment to explore the application. You should read Xcode Workspace

Guide to understand the organization of the project window and how to perform basic tasks like editing and

saving files. You can now build and run the application to see what Simulator looks like.

To run your application in Simulator . . .

1. Make sure you have set the Active Scheme that uses the iPhone Simulator.

2. Choose Product > Run (or click the Run button in the toolbar).

The iOS Simulator application should launch automatically, and when your application starts up you should simply see a white screen:

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To understand where the white screen came from, you need to understand how the application starts up. For now, quit Simulator.

To quit Simulator . . .

● Choose Simulator > Quit.

Application Bootstrapping

The template project you created already sets up the basic application environment. It creates an application object, connects to the window server, establishes the run loop, and so on. Most of the work is done by the

UIApplicationMainfunction.

Themainfunction inmain.mcalls theUIApplicationMainfunction:

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int retVal = UIApplicationMain(argc, argv, nil, nil);

This creates an instance ofUIApplication; it also scans the application’sInfo.plistfile. TheInfo.plist

file is a property list that contains information about the application such as its name and icon. It contain the name of the nib file the application object should load, specified by theNSMainNibFilekey. Nib files contain an archive of user interface elements and other objects—you’ll learn more about them later in the tutorial. In your project’sInfo.plistfile you should see:

Value Key

MainWindow

NSMainNibFile

This means that when the application launches, the MainWindow nib file is loaded.

To look at the main nib file . . .

● ClickMainWindow.xibin the project navigator.

The file has the extension “xib” but by convention it is referred to as a “nib file”. Xcode displays the file in the editor area.

The nib file contains several items split into two groups by a dividing line. Above the line are placeholders:

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● The File’s Owner proxy object (the pale yellow cube).

The File’s Owner object is actually theUIApplicationinstance—File’s Owner is discussed later, in

“File’s Owner” (page 30).

● A First Responder proxy object.

The First Responder is not used in this tutorial but you can learn more about it by reading “The Event-Handling System” in iOS Application Programming Guide.

Below the line are actual objects:

● An instance ofHelloWorldAppDelegateset to be the application’s delegate.

Delegation is explained in more detail in“Understanding Fundamental Design Patterns” (page 17).

● A window.

The window has its background set to white and is set to be visible at launch. It’s this window that you see when the application launches.

You use the application delegate to perform custom configuration for the application as a whole, including displaying the window. It shouldn’t, though, manage any of the content on the screen beyond the window itself—that’s the job of view controllers. A view controller is a special controller responsible for managing a view—this adheres to the model-view-controller design pattern (explained in more detail in“Understanding Fundamental Design Patterns” (page 17)).

When the application object has completed its setup, it sends its delegate an

application:didFinishLaunchingWithOptions:message. This gives the delegate an opportunity to configure the user interface and perform other tasks before the application is displayed.

Recap

In this chapter you created a new project using Xcode and built and ran the default application. You also looked at some of the basic pieces of a project and a nib file, and learned how the application starts up. In the next chapter, you’ll learn about the design patterns you use when developing a Cocoa application.

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In this chapter, you learn about the main design patterns you’ll use in Cocoa applications. There’s not much practical to do in this chapter, but the concepts are fundamental, so allow some time to read and think about it.

The main patterns you’re going to use in this tutorial are:

● Delegation

● Model-View-Controller

● Target-Action

Here’s a quick summary of these patterns and an indication of where they’ll be used in the application.

Delegation

Delegation is a pattern in which one object sends messages to another object specified as its delegate to ask for input or to notify the delegate that an event is occurring. You typically use it as an alternative to class inheritance for extending the functionality of reusable objects.

In this application, the application object tells its delegate that the main start-up routines have finished and that the custom configuration can begin. For this application, you want the delegate to create an instance of a controller to set up and manage the view. In addition, the text field will tell its delegate (which in this case will be the same controller) when the user has tapped the Return key.

Delegate methods are typically grouped together into a protocol. A protocol is basically just a list of methods. If a class conforms to (or “adopts”) a protocol, it guarantees that it implements the required methods of a protocol. (Protocols may also include optional methods.) The delegate protocol specifies all the messages an object might send to its delegate. To learn more about protocols and the role they play in Objective-C, see the “Protocols” chapter in The Objective-C Programming Language.

Model-View-Controller

The Model-View-Controller (or “MVC”) design pattern sets out three roles for objects in an application.

Model objects represent data such as SpaceShips and Weapons in a game, ToDo items and Contacts in a

productivity application, or Circles and Squares in a drawing application.

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In this application, the model is very simple—just a string—and it’s not actually used outside of a single method, so from a practical perspective in this application it’s not even necessary. It’s the principle that’s important here, though. In other applications the model will be more complicated and accessed from a variety of locations.

View objects know how to display data (model objects) and may allow the user to edit the data.

In this application, you need a main view to contain several other views—a text field to capture information from the user, a second text field to display text based on the user’s input, and a button to let the user tell us that the secondary text should be updated.

Controller objects mediate between models and views.

In this application, the controller object takes the data from the input text field, stores it in a string, and updates a second text field appropriately. The update is initiated as a result of an action sent by the button.

Target-Action

The target-action mechanism enables a view object that presents a control—that is, an object such as a button or slider—in response to a user event (such as a tap) to send a message (the action) to another object (the target) that can interpret the message and handle it as an application-specific instruction.

In this application, when it’s tapped, the button tells the controller to update its model and view based on the user’s input.

Other Design Patterns

Cocoa makes pervasive use of a number of design patterns beyond those described above. The design patterns chapter in Cocoa Fundamentals Guide provides a comprehensive overview of all the design patterns you use in Cocoa application development—you should consider it to be essential reading. Understanding, and using, the patterns will make it easier for you to use the various technologies Cocoa provides, and transfer skills you’ve learned in one area to another. Following the patterns also means that your code is better able to take advantage of enhancements to the Cocoa frameworks in future releases.

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In this application you’ll need two classes. Xcode’s application template provided an application delegate class and an instance is created in the nib file. You need to implement a view controller class and create an instance of it.

Adding a View Controller Class

View controller objects play a central role in most iOS applications. As the name implies, they’re responsible for managing a view, but on iOS they also help with navigation and memory management. You’re not going to use the latter features here, but it’s important to be aware of them for future development. UIKit provides a special class—UIViewController—that encapsulates most of the default behavior you want from a view controller. You have to create a subclass to customize the behavior for your application.

To add a custom view controller class . . .

1. In Xcode, in the project organizer select either the project (HelloWorldat the top of the Groups and Files list) or theHelloWorldgroup folder.

The new files will be added to the current selection.

2. Choose File > New File and in the New File window. You should see a sheet like this:

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3. Select the Cocoa Touch Classes group, then selectUIViewControllersubclass.

4. Click Next.

You should see a sheet like this:

Figure 3-1 MyViewController

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5. Using the combo box, choose to create a subclass ofUIViewController.

6. Using the checkboxes, make sure that Targeted for iPad is not selected, and that With XIB for user interface is selected.

7. Click Next.

You should see a sheet like this:

Figure 3-2 MyViewController

Make sure the Add to targets check box is selected.

8. Give the file a new name such asMyViewController. By convention, class names begin with a capital letter.

9. Click Save.

Make sure that the files were added to your project.

If you look at the new source files, you’ll see that stub implementations of various methods are already given to you. These are all you need for the moment; the next task is to create an instance of the class.

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Adding a View Controller Property

You want to make sure that the view controller lasts for the lifetime of the application, so it makes sense to add it as a property of the application delegate (which will also last for the lifetime of the application). (To understand why, consult Memory Management Programming Guide.)

Perform the following tasks in the header file for the application delegate class (HelloWorldAppDelegate.h).

To add a view controller property . . .

1. Add a forward declaration for theMyViewControllerclass. Before the interface declaration forHelloWorldAppDelegate, add:

@class MyViewController;

The property will be an instance of theMyViewControllerclass. The compiler will generate an error, though, if you declare the variable but you don’t tell it about theMyViewControllerclass. You could import the header file, but typically in Cocoa you instead provide a forward declaration—a promise to the compiler thatMyViewControllerwill be defined somewhere else and that it needn’t waste time checking for it now. (Doing this also avoids circularities if two classes need to refer to each other and would otherwise include each other’s header files.) Later, you will import the header file itself in the implementation file.

2. Add a declaration for the view controller property. After the closing brace but before@end, add:

@property (nonatomic, retain) MyViewController *myViewController;

Important: Like C, Objective-C is case-sensitive.MyViewControllerrefers to a class;myViewController

is a variable that here contains an instance ofMyViewController. A common programming error in Cocoa is to misspell a symbol name, frequently by using a letter of an inappropriate case (for example, “tableview” instead of “tableView”). This tutorial deliberately usesMyViewControllerandmyViewControllerto encourage you to look at the name carefully. When writing code, make sure you use the appropriate symbol.

Properties are described in the “Declared Properties” chapter in The Objective-C Programming Language. Basically, though, this declaration specifies that an instance ofHelloWorldAppDelegatehas a property that you can access using the getter and setter methods myViewControllerandsetMyViewController:

respectively, and that the instance retains the property (retaining is discussed in more detail later). To make sure you’re on track, confirm that yourHelloWorldAppDelegateclass interface file (HelloWorldAppDelegate.h) looks like this (comments are not shown):

#import <UIKit/UIKit.h> @class MyViewController;

@interface HelloWorldAppDelegate : NSObject <UIApplicationDelegate> {

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}

@property (nonatomic, retain) IBOutlet UIWindow *window;

@property (nonatomic, retain) MyViewController *myViewController; @end

You can now create an instance of the view controller.

Creating the View Controller Instance

Now that you’ve added the view controller property to the application delegate, you need to actually create an instance of the view controller and set it as the value for the property. To make code-completion work for your new class in Xcode, you also need to import the relevant header file for the view controller. Perform the following tasks in the implementation file for the application delegate class

(HelloWorldAppDelegate.m).

To import the view controller’s header file . . .

● At the top of file, add:

#import "MyViewController.h"

To add a view controller instance . . .

● Add the following code as the first statements in the implementation of the

application:didFinishLaunchingWithOptions:method:

MyViewController *aViewController = [[MyViewController alloc] initWithNibName:@"MyViewController" bundle:nil]; [self setMyViewController:aViewController];

[aViewController release];

There’s quite a lot in just these three lines. What they do is:

● Create and initialize an instance of the view controller class.

● Set the new view controller to be the value of themyViewControllerproperty using an accessor method.

Remember that you didn’t separately declaresetMyViewController:, it was implied as part of the property declaration—see“Adding a View Controller Property” (page 22).

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● Adhere to memory management rules by releasing the view controller.

You create the view controller object usingalloc, then initialize it usinginitWithNibName:bundle:. The

initmethod specifies first the name of the nib file the controller should load and second the bundle in which it should find it. A bundle is an abstraction of a location in the file system that groups code and resources that can be used in an application. The advantages of using bundles over locating resources yourself in the file-system are that: bundles provide a convenient and simple API—the bundle object can locate a resource just by name; and they take account of localization for you. To learn more about bundles, see

Resource Programming Guide.

By convention, you own any objects you create using anallocmethod (amongst others, see “Memory Management Rules”). You should also:

● Relinquish ownership of any objects you create.

● Typically use accessor methods to set property values anywhere other than in an initializer method. The second line in the implementation uses an accessor method to set the property value, and then the third line usesreleaseto relinquish ownership.

There are other ways to implement the above. You could, for example, replace the three lines with just two:

MyViewController *aViewController = [[[MyViewController alloc]

initWithNibName:@"MyViewController" bundle:nil] autorelease]; [self setMyViewController:aViewController];

In this version, you useautoreleaseas a way to relinquish ownership of the new view controller but at some point in the future. To understand this, read “Autorelease Pools” in the Memory Management

Programming Guide. In general, however, you should try to avoid usingautoreleasewherever possible since it means memory may be reclaimed later than it would be if you usedrelease.

You could also replace[self setMyViewController:aViewController];with:

self.myViewController = aViewController;

The dot notation invokes exactly the same accessor method (setMyViewController:) as in the original implementation. The dot notation simply provides a more compact syntax for accessor methods—especially when you use nested expressions. Which syntax you choose is largely personal preference, although using the dot syntax does have some additional benefits when used in conjunction with properties—see “Declared Properties” in The Objective-C Programming Language. For more about the dot syntax, see “Dot Syntax” in

The Objective-C Programming Language in “Objects, Classes, and Messaging” in The Objective-C Programming Language.

Setting Up the View

View controllers are responsible for configuring and managing each “screenful” you see in an application. Each screenful should be managed by a view controller. To codify this principle, a window has a root view controller—the view controller responsible for configuring the view first displayed when the window appears. To get your view controller’s content into the window, you set your view controller to be the window’s root view controller.

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To set your view controller as the window’s root view controller . . .

● Add the following line of code just before the call tomakeKeyAndVisible.

self.window.rootViewController = self.myViewController;

Instead of using the dot syntax, you could also use square brackets:

[[self window] setRootViewController:[self myViewController]];

Both lines of code would compile to the same thing; again you can choose which you feel is more readable. A further important point about this line, though, is that it uses the accessor method to retrieve

myViewController.

Tip In addition to using accessor methods to set property values (except in the implementation of an

initmethod), you should use accessor methods to get property values, even those ofself(except in the implementation of adeallocmethod). This helps to preserve encapsulation, and in some cases ensures that a property value is properly initialized before you try to use it. This is discussed in more detail in“Housekeeping” (page 25).

The final line in the implementation of theapplication:didFinishLaunchingWithOptions:method from the template:

[self.window makeKeyAndVisible];

causes the window—now complete with your view—to be displayed on screen.

Housekeeping

There are a couple of unfinished tasks to complete: You need to synthesize the accessor methods, and—to conform to the rules of memory management—make sure the view controller is released in thedealloc

method.

You perform the following tasks in the@implementationblock of the class (HelloWorldAppDelegate).

To synthesize the view controller property . . .

● Tell the compiler to synthesize the accessor methods for the view controller, and to use a custom name for the corresponding instance variable.

@synthesize myViewController=_myViewController;

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This line of code tells the compiler to do several things:

● Create accessor methods for themyViewControllerproperty.

Strictly speaking, it means “create accessor methods, according to the property declaration, for which the developer hasn’t provided an implementation.” Since you haven’t created custom implementations, the compiler will generate a get and set accessor. (If the property were declared asreadonly, the compiler would only generate a get accessor.)

● Use_myViewControlleras the name of the instance variable for themyViewControllerproperty.

● Because you didn’t declare a_myViewControllerinstance variable, it also tells the compiler to add that to the class as well.

You use the “_” prefix for the instance variable to serve as a reminder that you shouldn’t access an instance variable directly. From an academic perspective, this helps to preserve encapsulation, but there are two important practical benefits in Cocoa:

● Some Cocoa technologies (notably key-value coding) depend on use of accessor methods, and in the appropriate naming of the accessor methods. If you don’t use accessor methods, your application may be less able to take advantage of standard Cocoa features.

● Some property values are created on-demand. If you try to use the instance variable directly, you may get nil or an uninitialized value. (A view controller’sviewis a good example.)

To complete the implementation of the dealloc method . . .

● In thedeallocmethod, release the view controller as the first statement.

[_myViewController release];

Implementation Source Listing

To make sure you’re still on track, confirm that yourHelloWorldAppDelegateclass implementation (HelloWorldAppDelegate.m) looks like this (the methods you didn’t edit are omitted):

#import "HelloWorldAppDelegate.h" #import "MyViewController.h" @implementation HelloWorldAppDelegate @synthesize window=_window; @synthesize myViewController=_myViewController; - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions

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{

MyViewController *aViewController = [[MyViewController alloc] initWithNibName:@"MyViewController" bundle:nil];

self.myViewController = aViewController; // Or, instead of the line above:

// [self setMyViewController:aViewController]; [aViewController release]; self.window.rootViewController = self.myViewController; [self.window makeKeyAndVisible]; return YES; }

/* Other methods omitted from the listing. */ - (void)dealloc { [_myViewController release]; [_window release]; [super dealloc]; } @end

Test the Application

You can now test your application.

To test the application . . .

● Compile and run the project.

Choose Product > Run (or click the Run button in Xcode’s toolbar).

Your application should compile without errors and you should again see a white screen in Simulator.

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Recap

In this section you added a new view controller class and its accompanying nib file. In the application delegate, you declared an instance variable and accessor methods for a view controller instance. You also synthesized the accessor methods and performed a few other housekeeping tasks. Most importantly, though, you created an instance of the view controller and passed it to the window as the root view controller. In the next chapter you’ll configure the nib file the controller uses to load its view.

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This chapter introduces two important concepts: outlets and the File’s Owner proxy object.

A View Controller’s View Often Comes from a Nib File

As you learned earlier, a view controller is responsible for configuring and managing each “screenful” you see in an application. Each screenful is defined by the view controller’s view; often you configure that view in a nib file (a common exception is a table view). Typically you specify the name of the nib file to load as the first argument toinitWithNibName:bundle:(see“Creating the View Controller Instance” (page 23)). A view controller loads its nib file in itsloadViewmethod. (You can load nib files yourself using an instance ofNSBundle. You can learn more about loading nib files in Resource Programming Guide.) If you initialize a view controller usinginitWithNibName:bundle:but you want to perform additional configuration after the view is loaded, you override the controller’sviewDidLoadmethod.

The view controller’s nib file contains three objects, the File’s Owner proxy, the First Responder proxy, and a view. You can see them by inspecting the nib file.

To inspect the nib file . . .

● In Xcode, click the view controller’s nib file (MyViewController.xib). You should see the file displayed like this:

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The view is the main view for the view controller. The view controller is represented by the File’s Owner proxy. It’s important to understand the role of File’s Owner, and how to create connections to and from both File’s Owner and other objects.

File’s Owner

In a nib file, in contrast to the other objects you add to the interface, the File’s Owner object is not created when the nib file is loaded. It represents the object set to be the owner in the method that loads the nib file—in this case,loadNibNamed:owner:options:. This is discussed in more detail in Resource Programming

Guide. In your application, the File’s Owner will be the instance ofMyViewController.

To allow you to make appropriate connections to and from the File’s Owner, Xcode has to know what sort of object File’s Owner is. You tell Xcode the class of the object using the Identity Inspector. It was actually set when you created the nib file together with the view controller classes, but it’s useful to look at the inspector now.

To show the inspector . . .

1. With the nib file selected, in Xcode display the Utilities view.

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2. In the Inspector pane, select the Identity inspector.

In the Class field of the Custom Class section, you should seeMyViewController. It’s important to understand that this is just a promise to Interface Builder that the File’s Owner will be an instance ofMyViewController. Setting the class doesn’t ensure that the File’s Owner will be an instance of that class. Remember, the File’s Owner is simply whatever object is passed as theownerargument to theloadNibNamed:owner:options:

method. If it’s an instance of a different class, the connections you make in the nib file will not be properly established.

The View Outlet

You can look at—and make and break—an object’s connections using the Inspector pane, or in a connections panel.

To look at the view controller’s connections . . .

1. In the Interface Builder editor window, click File’s Owner.

2. In the Inspector pane, select the Connections inspector.

This displays File’s Owner’s outlet connections in the utility area.

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3. In the Interface Builder editor window, Control-click File’s Owner. This displays a translucent panel showing File’s Owner’s connections.

The utility area and the connections pane both show the same information. You can use whichever is more convenient at any particular time.

The only connection at the moment is the view controller’sviewoutlet. An outlet is just a property that happens to connect to an item in a nib file. The outlet connection means that when the nib file is loaded and theUIViewinstance is unarchived, the view controller’s (strictly File’s Owner’s)viewproperty is set to that view.

Test the Application

To make sure your application’s working correctly, you could set the background color of the view to something other than white and verify that the new color is displayed after the application launches.

To set the background color of the view controller’s view . . .

1. In Xcode, select the view controller’s nib file (MyViewController.xib).

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2. In the utilities area, select the Attributes inspector.

3. In the editor area, select the view.

4. In the Attributes inspector for the view, click the frame of the Background color well to display the Colors panel.

5. In the Colors panel, select a different color.

6. Save the nib file.

7. Click the Run button in the toolbar to compile and run the project.

Your application should compile without errors and you should again see an appropriately-colored screen in Simulator.

To restore the view’s background color . . .

1. In Xcode, using the attributes inspector, set the view’s background color to white.

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Recap

In this section you inspected a nib file, learned about outlets, and set the background color of a view. You also learned more about resource loading and how the view controller loads the nib file.

In the next chapter you will add controls to the view.

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Interface Builder provides a library of objects that you can add to a nib file. Some of these are user interface elements such as buttons and text fields; others are controller objects such as view controllers. Your nib file already contains a view—now you just need to add the button and text fields.

Adding the User Interface Elements

You add user interface elements by dragging them from the object library.

To add objects to the nib file and lay them out appropriately . . .

1. In Xcode, select the view controller’s nib file.

2. In the utilities area, display the object library.

3. Add a button (UIButton), a text field (UITextField), and a label (UILabel) to the view.

You can drag view items from the library and drop them onto the view just as you might in a drawing application.

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4. Lay out the elements appropriately.

You can then resize the items using resize handles where appropriate, and reposition them by dragging. As you move items within the view, alignment guides are displayed as dashed blue lines.

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5. Finalize the view.

● Add a placeholder stringYour Nameto the text field by entering it in the Text Field Attributes inspector.

● Resize the label so that it extends for the width of the view.

● Delete the text (“Label”) from the label either using the Label Attributes inspector or by selecting the text in the label itself (double-click to make a selection) and press Delete.

● Add a title to the button by double-clicking inside the button and typingHello.

● Use the inspector to set the text alignment for the text field and label to centered. Your view should look like this:

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6. In the view section of the label’s Label Attributes inspector, select Clear Context Before Drawing. This ensures that when you update the greeting the previous string is removed before the new one is drawn. If you don’t do this, the strings are drawn on top of each other.

There are several other changes to make to the text field—it might be fairly obvious that the first change applies to the text field, but the others are less obvious. First, you might want to ensure that names are automatically capitalized. Second, you might want to make sure that the keyboard associated with the text field is configured for entering names, and that the keyboard displays a Done button.

The guiding principle here is this: You know when you’re putting it on screen what a text field will contain. You therefore design the text field to make sure that at runtime the keyboard can configure itself to best suit the user’s task. You make all of these settings using text input traits.

To configure the text field . . .

1. Select the text field.

2. In the utilities area, display the attributes inspector.

3. Configure the text field using the Text Input Traits.

● In the Capitalize pop-up menu, select Words.

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● In the Keyboard Type pop-up menu, select Default.

● In the Keyboard Return Key pop-up menu, select Done.

If you build and run your application in Xcode, when it launches you should see the user interface elements as you positioned them. If you press the button, it should highlight, and if you tap inside the text field, the keyboard should appear. At the moment, though, after the keyboard appears, there’s no way to dismiss it. To remedy this, and add other functionality, you need to make appropriate connections to and from the view controller. These are described in the next section.

Make Connections

You can create and connect outlets and actions in Xcode by dragging from a nib file to the appropriate source file. Whereas this makes it easy to add code to support outlets and actions, it adds a conceptual level of indirection. Although you’re dragging to the source file, the actual connections are made in the nib file itself, to the object for which the header file is the declaration—in this case, the view controller represented by File’s Owner.

You want to add an action method to the view controller; when the button is “activated” it should send a

changeGreeting:message to the view controller. For the button, you want to create an action called

changeGreeting::

To add an action for the button . . .

2. Display the Assistant editor.

3. Make sure Assistant displays the view controller’s header file (MyViewController.h).

4. Control drag from the button in the nib file to the method declaration area in the header file.

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5. Configure the button’s connection as an action.

● Set the connection type to Action.

● Set the name tochangeGreeting:.

● Set the type toid.

It doesn’t matter what sort of object sends the message.

● Set the event to touch up inside.

The message should be sent if the user lifts their finger inside the button—this corresponds to

UIControlEventTouchUpInside.

● Set the arguments to sender.

6. Click Connect.

You accomplished two things by adding an action for a button:

1. Added appropriate code to the view controller class. You added the action method declaration to the header file:

- (IBAction)changeGreeting:(id)sender;

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and a stub implementation to the implementation file:

- (IBAction)changeGreeting:(id)sender { }

IBActionis a special keyword that is used only to tell Interface Builder to treat a method as an action for target/action connections. It’s defined tovoid.

2. Established a connection from the button to the view controller, as represented by the File’s Owner. Establishing the connection is the equivalent of invokingaddTarget:action:forControlEvents:

on the button, with the target being Fie’s Owner, the action being thechangeGreeting:selector, and the control events containing justUIControlEventTouchUpInside.

You follow similar operations to add outlets for the text field and label.

To add an outlet for the text field . . .

2. Display the Assistant editor.

3. Make sure Assistant is displaying the view controller’s header file (MyViewController.h).

4. Control drag from the text field in the nib file to the method declaration area in the header file.

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5. Configure the text field’s connection as an outlet.

Add an outlet from the view controller connected to the text field.

● Set the connection type to Outlet.

● Set the name totextField.

You can call the outlet whatever you want, but it makes your code more understandable if the name bears some relationship with the thing it represents.

● Set the type toUITextField.

You want to make sure the outlet can only connect to a text field.

6. Click Connect.

You accomplished two things by adding an outlet for the text field:

1. You added appropriate code to the view controller class. You added a property declaration to the header file:

@property (nonatomic, retain) IBOutlet UITextField *textField;

In the implementation file, you added:

@synthesize textField=_textField;

to the top of the@implementationblock;

[_textField release];

to thedeallocmethod, and

[self setTextField:nil];

to theviewDidUnloadmethod.

Note: Depending on which version of Xcode you use, the instance variable name may not have an

underscore prefix, so you may see:

@synthesize textField; [textField release];

Property declarations and the synthesize keyword are explained in more detail in“The User’s Name Property” (page 47).

IBOutletis a special keyword that is used only to tell Interface Builder to treat a property as an outlet. It’s actually defined as nothing so it has no effect at compile time.

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2. You established a connection from the the view controller (as represented by the File’s Owner) to the text field.

Establishing a connection from the the view controller does the equivalent of invokingsetTextField:

on the view controller, passing the text field as the argument.

Now configure the label. Follow the same steps as for the text field, but with appropriate changes to the configuration.

To add an outlet from the view controller to the label . . .

● Follow the same steps as for the text field. In the connection configuration dialog:

● Set the connection type to Outlet.

● Set the name tolabel.

● Set the type toUILabel.

Make the Text Field’s Delegate Connection

The text field sends a message to its delegate when the user taps the Return button in the keyboard. You can use this callback to dismiss the keyboard (see“Configure the View Controller as the Text Field’s Delegate” (page 50)).

To set the text field’s delegate . . .

2. Control-click in the text field.

3. Using the translucent panel that appears, select the open circle afterdelegateand drag to the File’s Owner.

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Testing

You can now test the application.

To test the application . . .

● Click Run.

Make sure that all the files are saved.

You should find that the button works (it highlights when you tap it). You should also find that if you touch in the text field, the keyboard appears and you enter text. There is, however, no way to dismiss the keyboard. To do that, you have to implement the relevant delegate method. You’ll do that in the next chapter.

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Recap

You configured the nib file with appropriate connections; in doing so, you also updated the header and implementation files to support the outlets and action.

There’s no need to use the Xcode feature to add the source code at the same time as establishing a connection; you can add the property and method declarations to the header file yourself, and then make the connections like you did with the text field’s delegate. Typically, though, you make fewer mistakes (and have less typing to do) if you let the tools do as much of the work as they can.

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There are several parts to implementing the view controller. You need to add a property for the user’s name including memory management, implement thechangeGreeting:method, and ensure that the keyboard is dismissed when the user taps Done.

The User’s Name Property

You then need to add a property for the user’s name, which is a string. You add the property declaration to the view controller’s header file (MyViewController.h).

To add a property declaration for the user’s name . . .

● Add a@propertystatement for a string calleduserName. The property declaration looks like this.

@property (nonatomic, copy) NSString *userName;

Your interface file should look similar to this:

#import <UIKit/UIKit.h>

@interface MyViewController : UIViewController <UITextFieldDelegate> { UITextField *textField;

UILabel *label; NSString *userName; }

@property (nonatomic, retain) IBOutlet UITextField *textField; @property (nonatomic, retain) IBOutlet UILabel *label;

@property (nonatomic, copy) NSString *userName; - (IBAction)changeGreeting:(id)sender;

@end

To complete the implementation, you also need to tell the compiler to synthesize the accessor methods, and update thedeallocmethod. You add the implementation parts to the view controller’s implementation file (MyViewController.m).

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To synthesize the accessor methods for user name property . . .

● Synthesize theuserNameproperty by adding the following after the@implementation MyViewControllerline:

@synthesize userName=_userName;

This tells the compiler to synthesize accessor methods for these property according to the specification you gave in the interface file. The declaration of theuserNameproperty is@property (nonatomic, copy) NSString *userName;), so the compiler generates two accessor methods:- (NSString *)userName

and- (void)setUserName:(NSString *)newUserName. In thesetUserName:method a copy is made of the string that’s passed in. This is useful to ensure encapsulation (the passed-in string might be

mutable—you want to make sure that the controller maintains its own copy). For more about encapsulation, see “Mechanisms Of Abstraction” in Object-Oriented Programming with Objective-C.

In addition,userName=_userNametells the compiler to use an instance variable called_userNamein its implementation of the accessor methods. Because you didn’t add an instance variable called_userNameto the class, it also asks the compiler to synthesize that as well. You call the instance variable_userNamerather thanuserNameto help to ensure that you don’t access it directly in your code. You should only get and set instance variables directly ininitanddeallocmethods.

Because the property declaration specifies that the view controller owns the instance variables (retainand

copyimply ownership, you must relinquish ownership of the string variable in thedeallocmethod (see “Memory Management Rules” in Memory Management Programming Guide).

To correctly handle memory management for the user name property in dealloc . . .

● In thedeallocmethod, releaseuserNameby adding the following before the invocation ofsuper:

[_userName release];

The completeddeallocmethod to should look similar to this:

- (void)dealloc { [_textField release]; [_label release]; [_userName release]; [super dealloc]; }

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The changeGreeting: Method

When a button is tapped, it sends achangeGreeting:message to the view controller. The view controller then should retrieve the string from the text field and update the label appropriately. Thus, the

changeGreeting:method should:

1. Retrieve the text from the text field and set the controller’suserNameproperty to the result.

2. Create a new string, based on theuserNameproperty, to display in the label.

To implement the changeGreeting: method . . .

1. In Xcode, displayMyViewController.min the source editor.

2. Complete the implementation of thechangeGreeting:method.

- (IBAction)changeGreeting:(id)sender { self.userName = self.textField.text; NSString *nameString = self.userName; if ([nameString length] == 0) { nameString = @"World"; }

NSString *greeting = [[NSString alloc] initWithFormat:@"Hello, %@!", nameString];

self.label.text = greeting; [greeting release];

}

There are several new pieces to this method:

● self.userName = self.textField.text;retrieves the text from the text field and sets the controller’suserNameproperty to the result.

In this case, you don’t actually use the user name anywhere else, but it’s important to understand its role. It’s the very simple model object that the view controller is managing. In general, the controller should maintain information about application data in its own model objects—application data shouldn’t be stored in user interface elements.

● @"World"is a string constant represented by an instance ofNSString.

● TheinitWithFormat:method creates a new string that follows the format specified by the format string, like theprintffunction.

%@indicates that a string object should be substituted into the format string. To learn more about strings, see String Programming Guide.

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Configure the View Controller as the Text Field’s Delegate

If you build and run the application, you should find that if you tap the button, the label shows “Hello, World!” If you select the text field and start typing, though, you should find that you have no way to indicate that you’ve finished entering text and dismiss the keyboard.

In an iOS application, the keyboard is shown automatically when an element that allows text entry becomes the first responder; it is dismissed automatically when the element loses first responder status. (You can learn more about first responder by reading “The Event-Handling System” in iOS Application Programming Guide.) There’s no way to directly message the keyboard; you can, however, make it appear or disappear as a side-effect of toggling the first responder status of a text-entry element.

TheUITextFieldDelegateprotocol includes a method,textFieldShouldReturn:, that the text field calls whenever the user taps the Return button (whatever text the button shows). Because you set the view controller as the text field’s delegate, you can implement this method to force the text field to lose first responder status by sending it theresignFirstRespondermessage—which has the side-effect of dismissing the keyboard.

The view controller must adopt theUITextFieldDelegateprotocol. To specify that a class adopts a protocol, in the interface add the name of the protocol in angle brackets (<>) after the name of the class from which your class inherits.

To implement MyViewController as the text field’s delegate . . .

1. In the view controller’s header file (MyViewController.h), add<UITextFieldDelegate>after

UIViewController.

The interface declaration should look like this:

@interface MyViewController : UIViewController <UITextFieldDelegate> {

This declaration specifies that the view controller class adopts theUITextFieldDelegateprotocol.

2. In the view controller’s implementation file (MyViewController.m), implement the

textFieldShouldReturn:method.

The method should tell the text field to resign first responder status. The implementation might look like this: - (BOOL)textFieldShouldReturn:(UITextField *)theTextField { if (theTextField == self.textField) { [theTextField resignFirstResponder]; } return YES; }

In this application, it’s not really necessary to include thetheTextField == textFieldtest since there’s only one text field. It’s worth pointing out the pattern, though, since there may be occasions when your object is the delegate of more than one object of the same type and you may need to differentiate between them.

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Test the Application

Build and run the application; it should behave as you expect. (Tap Done to dismiss the keyboard when you have entered your name, then tap the Hello button to display “Hello, <Your Name>!” in the label.)

If the application doesn’t behave as you expect, you need to troubleshoot (see“Troubleshooting” (page 53)).

Recap

You finished the implementation of the view controller and so completed your first iOS application. Congratulations.

Take a moment to think about how the view controller fits into the overall application architecture. You will use view controllers in all iOS applications you write.

Then take a break, and start to think about what you should do next.

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This section describes some approaches to solving common problems that you may encounter.

Code and Compiler Warnings

If things aren’t working as they should, first compare your code with the complete listings as given in“Code Listings” (page 59).

Your code should compile without any warnings. Objective-C is a very flexible language, and so sometimes the most you get from the compiler is a warning. Typically you should treat warnings as very likely to be errors.

Check the Nib Files

As a developer, if things don’t work correctly, your natural instinct is to check your source for bugs. Cocoa adds another dimension. Much of your application’s configuration may be “encoded” in the nib files. And if you haven’t made the correct connections, then your application won’t behave as you expect.

● If your user interface doesn’t reflect changes you made to the nib file, make sure you saved the nib file before you built the project.

● If the text doesn’t update when you tap the button, it might be that you didn’t connect the button’s action to the view controller, or connect the view controller’s outlets to the text field or label.

● If the keyboard does not disappear when you tap Done, you may not have connected the text field’s delegate or connected the view controller’stextFieldoutlet to the text field (see“Make the Text Field’s Delegate Connection” (page 43)). If you have connected the delegate, there may be a more subtle problem (see“Delegate Method Names” (page 53)).

Delegate Method Names

A common mistake with delegates is to misspell the delegate method name. Even if you’ve set the delegate object correctly, if the delegate doesn’t implement the method with exactly the right name, it won’t be invoked. It’s usually best to copy and paste delegate method declarations from the documentation.

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This chapter suggests directions you might take next in learning about iOS development.

Improve the User Interface

In this tutorial, you created a very simple iOS application. Cocoa Touch offers a rich development environment, though, and you’ve only scratched the surface. From here, you should explore further. Start with this application. As noted in the first chapter, the user interface is critical to a successful iOS application. Try to improve the user interface. Add images and color to the elements. Add a background image and an icon for the application. Look at the inspectors in Interface Builder to see how else you can configure elements. Many iPhone applications support multiple orientations; applications for iPad should support all orientations. Make sure the view controller supports multiple orientations (see the

shouldAutorotateToInterfaceOrientation:method), then (in Interface Builder) adjust your user interface to make sure that all the user interface elements are properly positioned if the view is rotated.

Create User Interface Elements Programmatically

In the tutorial, you created the user interface using Interface Builder. Interface Builder allows you to assemble user interface components quickly and easily. Sometimes, however, you may want—or need—to create user interface elements in code (for example, if you create a custom table view cell you typically create and lay out the subviews programmatically).

First, open theMyViewControllernib file and remove the text field from view.

If you want to create the entire view hierarchy for a view controller in code, you overrideloadView. In this case, however, you want to load the nib file then perform additional configuration (add another view). You therefore overrideviewDidLoadinstead. (TheviewDidLoadmethod gives you a common override point you can use whether you load the main view using a nib file or by overridingloadView.)

InMyViewController.m, add the following implementation ofviewDidLoad:

- (void)viewDidLoad {

CGRect frame = CGRectMake(20.0, 68.0, 280.0, 31.0);

UITextField *aTextField = [[UITextField alloc] initWithFrame:frame]; aTextField.textAlignment = UITextAlignmentCenter; aTextField.borderStyle = UITextBorderStyleRoundedRect; aTextField.autocapitalizationType = UITextAutocapitalizationTypeWords; aTextField.keyboardType = UIKeyboardTypeDefault; aTextField.returnKeyType = UIReturnKeyDone; aTextField.delegate = self;

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Your First ios Application