10% CODE - 100% APP. PDF - light version

// 10% CODE - 100% APP // 2

Although the Qt framework is C++ based, you can also code with QML and JavaScript. In fact, you can create full apps without even touching C++. This is going to save you a ton of time and make it easier to develop your projects.

This guide shows you how you can save up to 90% of code by choosing QML as your main language.

It holds many best practices and tips for creating QML-driven apps that offer the same performance as pure Qt C++ applications. This comprehensive QML development guide summarizes all the knowledge we gathered from creating more than 100 QML-based mobile apps for enterprises, startups and own projects.

How to rule Qt with QML & JS

Here’s a case study from the 2019 Qt Summit in Berlin & Tokyo. Naturally, we tackled this project using the Qt framework, specifically the Felgo SDK. We ended up using 90% less code compared to a plain Qt proj-ect.

Felgo developed the conference app within a week. This was possible thanks to the “Felgo Conference” app template - ready to use for all kinds of confer-ences and events.

It comes with ready-made connections to a variety of APIs like Felgo SocialView, Push Notifications or Of-fline Mode. Participants also enjoyed a cutting-edge, custom UI & UX when browsing the schedule or look-ing at detailed information for all talks and speakers. Make sure you check out the open-source code on GitHub, which will show you just how useful QML is.

Case Study Summit Conference App:

90% less code vs. Qt:

QML

QML

C++

C++

LINES OF CODE 1 910

LINES OF CODE 23 200

In case you have never heard of Qt: It is a powerful and feature-rich C++ framework used in many differ-ent industries. It supports developmdiffer-ent of apps for desktop, mobile platforms and embedded systems – all from a single code base.

One of the best things about Qt is QML, a reactive programming language (sometimes also referred to as a declarative language) that extends JavaScript. It allows developers to create stunning apps that save up to 90% of code compared to native development for iOS or Android. With less code and a single code base for iOS, Android and desktop, your maintenance costs decrease dramatically. You can release updates to different platforms faster and have a single team sharing the knowledge.

Why use QML for iOS or Android Apps?

Try it now!

title: “My First App”

AppText {

text: “Hello World!” anchors.centerIn: parent } } } } https://felgo.com/web-editor?snippet=d0846f61

// 10% CODE - 100% APP // 4

Custom Rendering and QML Item Trees

In contrast to many other cross-platform solutions, Qt offers an architecture that does not rely on native platform rendering or a web view. It moves all con-trols and the rendering into the app itself. To show items on the device screen, it only requires a native view with a canvas to draw onto.

This architecture breaks away from the so-called “cookie cutter” apps that have been common in the past. Instead of the native platform renderer, Qt uses app-driven custom rendering.

This allows for a unified renderer and app experience across all supported platforms.

But don’t let the fact that Qt relies on C++ mislead you. Since the introduction of QML and JavaScript for rapid UI development, you can create item trees in a customizable and extensible way. You can combine and nest individual QML Items to form a tree that de-scribes your UI. There’s no need to dive into complex C++ code!

Native Performance and Compilation

of View Code

Native Performance and Compilation of View Code In the background, each QML Item uses a performant native C++ implementation. When the Qt runtime parses your QML code, it creates all QML Items as C++ objects. With your QML markup and JavaScript logic, you can control and combine them. This makes it easy to build your UI.

There’s no need to create your UI with C++ widgets. Apps created with QML offer the same performance.

Qt compiles your QML tree as soon as it parses the code the first time. You get this Just-in-Time (JIT) compilation of QML apps out-of-the-box.

With the Qt Quick Compiler, you can also compile your QML code Ahead-of-Time (AOT). It transforms your QML and JavaScript directly into bytecode. This architecture makes apps powered by QML super fast and performant.

// 10% CODE - 100% APP // 6

Qt MVC: Models and Views in QML

import Felgo 3.0 import QtQuick 2.0 App { // data model ListModel { id: fruitModel ListElement { name: “Banana” cost: 1.95 } ListElement { name: “Apple” cost: 2.45 } ListElement { name: “Orange” cost: 3.25 } } // list page NavigationStack { Page {

title: “List Page” AppListView { anchors.fill: parent model: fruitModel delegate: SimpleRow { text: name

detailText: “cost: “+cost } } // AppListView } // Page } // NavigationStack } // App https://felgo.com/web-editor?snippet=4c2ba057

For most apps, it is a basic task to populate UI views based on some application data. Qt offers a built-in separation into model, view and delegate compo-nents. The following example visualizes a set of data records in a list.

The AppListView presents you with a scrollable list view. For each ListElement in your ListModel, the view instantiates the delegate Item to visualize each element in your list. In this case, a SimpleRow item represents each list element.

Why Choose a QML ListModel instead

of a Qt C++ Model?

The ListModel type is actually a C++ QAbstractList-Model implementation exposed to QML. The above code does not only create a UI tree of C++ objects. It also handles the list data in a C++ data structure – pretty nice!

It’s exceptionally fast to develop user interfaces this way, and since C++ is actually used under the

hood, there are no drawbacks in performance. Still, for long-running, heavyweight and data-intensive calculations it does make sense to move away from QML. In such cases, C++ will outperform JavaScript. For more examples on mixing both languages and when it’s worth the extra effort, read on as we’ve got you covered with details on combining C++ code with QML!

When using QML, many Qt developers tend to be-lieve they should at least code the model in C++. This is not the case. QML is optimized to integrate seam-lessly with C++.

All QML code gets compiled to native code with native performance. And when using the Qt Quick Compiler, this already happens during compilation of the app. The QML ListModel offers a simple API and perfectly works together with view types like ListView, GridView or Repeater.

QML Best Practice: Use REST Services

and JSON for your ListView

https://felgo.com/web-editor/?snippet=d5ad7af6

There is another big advantage of using QML and JavaScript as your main coding language: It is super easy to work with REST services and JSON data. The application logic for QML gets written in JavaS-cript. QML thus has built-in support to handle variant data types like JSON structures. With a simple HTTP request, you can fetch JSON data and use it in your views.

The following code snippet retrieves and shows a list of todo entries from a REST service:

import Felgo 3.0 import QtQuick 2.0 App {

// on app start: fetch data from REST api Component.onCompleted: { HttpRequest.get(“https://json-placeholder.typicode.com/todos”) .timeout(5000) .then(function(res) { dataFetched(res.body) }) .catch(function(err) { console. err(“Fetch failed:”+err) }); }

// dataFetched gets called on suc-cess of http request

function dataFetched(jsonData) { listView.model = jsonData // set retrieved json data as model for list

}

// list page NavigationStack { Page {

title: “Todo List” AppListView { id: listView anchors.fill: parent delegate: SimpleRow { text: modelData.title } } } } }

Instead of a ListModel, QML also allows you to assign JSON data structures as the model. This is very con-venient when working with REST services. You can directly use the retrieved JSON result to display your data.

Drawbacks of Using JSON data for the

ListView Model

A JSON structure is no ListModel and thus no QAb-stractListModel implementation. You do not have the benefits of a performant C++ model in this case. Also, a JSON array is a variant type. The list can thus not expect a defined data structure to create its ele-ments.

This results in performance and usability drawbacks. For example, when you change or replace the JSON data. The view then parses the whole model again and redraws all items from scratch. This full redraw can take a while and is noticeable by the user. You also lose the current scroll position, and advanced features like transition animations are not supported. At this point, you might think: Let’s create a ListMod-el and fill it with the JSON data. While this solves the performance problem, another issue quickly pops up: How do you update the model when you fetch new data from the API at a later point?

If you re-create the ListModel from scratch, the view again requires a full redraw. To avoid this, we could compare each entry of the latest JSON data with the current state of the ListModel.

This allows us to only synchronize changed or new data entries to the model.

However, such manual synchronization requires a lot of extra effort. It also comes with some overhead, es-pecially when you start to compare large data sets. JavaScript and QML are not well suited to perform such data-intense operations.

But here’s the good news: the next section introduces a QML component that covers exactly this synchroni-zation. The JsonListModel is a special ListModel im-plementation that can handle JSON data and there-fore make QML the right choice, even in the case of data-intensive applications.

// 10% CODE - 100% APP // 10

How Does the JsonListModel Work?

The JsonListModel offers an easy way to transform JSON data into a QML ListModel for usage with e.g. an AppListView.

This is how you can use the model for our todo list example:

import Felgo 3.0 import QtQuick 2.0 App {

// on app start: fetch data from REST api Component.onCompleted: { HttpRequest.get(“https://json-placeholder.typicode.com/todos”) .timeout(5000) .then(function(res) { dataFetched(res.body) }) .catch(function(err) { console. err(“Fetch failed:”+err) }); }

// dataFetched gets called on suc-cess of http request

function dataFetched(jsonData) { listView.jsonData = jsonData // set retrieved json data as model for list } } } // list page NavigationStack { Page {

title: “List Page” AppListView { anchors.fill: parent model: fruitModel delegate: SimpleRow { text: name

detailText: “cost: “+cost } } // AppListView } // Page } // NavigationStack } // App https://felgo.com/web-editor?snippet=0427bb7e

JsonListModel

The JsonListModel holds a local copy of the specified JSON data. Whenever the JSON source changes, the data gets compared to the local copy of the list mod-el. To identify each unique data record, it is import-ant to specify the keyField of your data objects. After diffing the old and new data sets, the JsonListModel applies all detected changes individually. It thus syn-chronizes the model with the changed JSON data step by step.

The JsonListModel type implements the full QML ListModel API and fires separate events for all chang-es. The list view can thus only update relevant entries or apply transition animations. This is super useful, as you can e.g. fetch new data and simply replace the old JSON. The JsonListModel will detect all changes, and the ListView updates changed items accordingly – without a full redraw.

// 10% CODE - 100% APP // 12

Benefits of JsonListModel

Json Model: The list jumps to the top

after a model update.

JsonListModel: The list keeps its scroll

position.

You thus get a much better performance and

Summary

Fetch JSON data from REST APIs with QML and JavaScript.

Pass your JSON to the model, which synchronizes the data to a ListModel and

pre-pares it for usage in your view.

Show the model data in your QML view, which now only updates items that changed.

You do not require to create a custom model in C++ when working with REST APIs and JSON data.

The JsonListModel itself is your C++ model. It is fully usable from QML and can work with JSON objects of any format

JsonListModel

sup-ports the GridView and Repeater types to display model data. It is based on QSyncable, a syncable C++ model implementation by Ben Lau. You can find his full project on GitHub.

The JsonListModel is also available with the free Fel-go SDK. The FelFel-go Apps module used in the above

example holds many such useful components. Felgo focuses on making mobile app development with Qt as easy as possible. For example, the HttpRequest type for networking or UI types like NavigationStack are also part of the SDK.

Dynamically Sort or Filter Data with

SortFilterProxyModel

To sort and filter list models, C++ Qt offers the QSort-FilterProxyModel class. In the same way, you can use the SortFilterProxyModel QML type with any list model in QML. It also works in conjunction with the JsonListModel:

This example shows a list of different fruit and meat entries. The list entries get sorted by type with the SortFilterProxyModel. You can also change the sort-ing order by presssort-ing the AppButton.

Similar to JsonListModel, the SortFilterProxyModel is a C++ type exposed to QML. So for this simple QML example, all data and model-related tasks are actual-ly performed by C++ types. When working with REST services and JSON data, you can fully handle all mod-el and view code in QML!

import Felgo 3.0 import QtQuick 2.0 App {

Page { id: page

// property with json data property var jsonData: [ { “id”: 1, “title”: “Apple”, “type”: “Fruit” }, { “id”: 2, “title”: “Ham”, “type”: “Meat” }, { “id”: 3, “title”: “Bacon”, “type”: “Meat” }, { “id”: 4, “title”: “Banana”, “type”: “Fruit” } ]

// list model for json data JsonListModel {

id: jsonModel

source: page.jsonData keyField: “id”

fields: [“id”, “title”, “type”] }

// SortFilterProxyModel for sorting or filtering lists

SortFilterProxyModel { id: sortedModel

// use the Component.onCom-pleted handler to configure SortFil-terProxyModel for JsonListModel Component.onCompleted: source-Model = jsonsource-Model

// add a sorter to sort the list by type

sorters: StringSorter { id: typeSorter; roleName: “type”; as-cendingOrder: true } } // list view AppListView { anchors.fill: parent model: sortedModel delegate: SimpleRow { text: model.title } section.property: “type” section.delegate: SimpleSec-tion { } }

// Button change the sorting or-der

AppButton {

anchors.horizontalCenter: par-ent.horizontalCenter

anchors.bottom: parent.bottom text: “Change Order”

onClicked: typeSorter.ascend-ingOrder = !typeSorter.ascendin-gOrder } } // Page }

Dynamically Sort or Filter Data with

SortFilterProxyModel

// 10% CODE - 100% APP // 16

Offline Caching, File Downloads and

Native Integrations for your Mobile

App

As QML and C++ integrate seamlessly, it is possible to expose any native feature for usage with QML. Avail-able Qt components already allow to work with sen-sors, SQLite databases and much more. And the Fel-go SDK offers lots of additional features. For example to fetch assets at runtime with the DownloadableRe-source item. It allows you to keep the initial app size small and dynamically load content if required – all with QML.

You can create your own C++ components and regis-ter them with QML as well. For iOS and Android apps, you can even add components that weave-in native iOS code with Obj-C or run Java Android code over JNI.

Please note that such native code requires custom implementations for each platform then. Felgo al-ready has you covered with the NativeUtils compo-nent. It offers lots of features like native dialogs, cam-era and image picker or sharing.

The SDK also comes with many Felgo Plugins that in-tegrate 3rd party frameworks for:

- server-triggered or local notifications - in-app purchases and monetization - analytics and crash reporting

- Firebase authentication and database - ...and much more.

There is no limit to what you can do – it’s possible to manage your full app logic purely with QML. You can even perform calculations in different threads with a WorkerScript.

It is very simple to create stunning views and handle application data with QML. But

why stop there? Let’s take it one step further!

App Example with JsonListModel

and Offline Caching

https://felgo.com/web-editor?snippet=39223058

Mobile apps have to work properly even without in-ternet access. The Storage component allows cach-ing data in a simple key-value store. It can save full JSON structures as well. The data gets serialized and converted to string format automatically.

You can thus use the Storage to initialize the JsonList-Model with previously cached JSON data. This way, your app can show the latest state of the data even if a fetch request fails.

In addition to fetched data, you can locally cache new entries as well. As soon as the app has internet connection again, save them to your server and re-move the local entries.

We prepared a full demo app for you, which allows browsing and creating todo list entries. You can find the full example with caching, local draft entries, list filtering and more on GitHub.

// 10% CODE - 100% APP // 18

App Example with JsonListModel

and Offline Caching

https://felgo.com/web-editor?snippet=39223058

import Felgo 3.0 import QtQuick 2.0 App {

// on app start: fetch data Component.onCompleted: { fetchData() } // list page NavigationStack { Page {

title: “Todo List”

AppListView { id: listView

anchors.fill: parent

// property for json data, used as source for JsonListModel property var jsonData: []

// use JsonListModel as mod-el

model: JsonListModel { source: listView.jsonData keyField: “id”

fields: [“id”, “title”] } // delegate delegate: SimpleRow { text: title } } } }

// storage for caching Storage {

id: cache }

// fetch data from cache or from api

function fetchData() {

// check cached value first var cachedData = cache.getVal-ue(“todos”)

if(cachedData)

listView.jsonData = cachedData

// load new data from api HttpRequest.get(“https://json-placeholder.typicode.com/todos”) .timeout(5000) .then(function(res) { dataFetched(res.body) }) .catch(function(err) { console. err(“Fetch failed:”+err) }); }

// dataFetched gets called on suc-cess of http request

function dataFetched(jsonData) { // cache new data before updat-ing the model

cache.setValue(“todos”, jsonDa-ta)

listView.jsonData = jsonData // set retrieved json data as model for list

} }

When Should You Use C++

and When QML

Coding in QML has several advantages over develop-ment with C++:

Coding with QML + JavaScript is very easy to learn and makes a great reduction in the required amount of code possible.

Language concepts like states, signals or property bindings are a huge time-saver.

QML makes adding animations simple. You can ani-mate every property of your QML types with simple Animation components.

QML is extensible and flexible. For example, you can extend objects with new properties and features in-line. No need to create a new reusable type for small extensions.

The QML Rendering Engine offers great performance. The renderer uses C++ Qt and relies on a hardware accelerated scene graph. This makes it fast enough to power even high-performance games.

Qt app development with C++ has advantages as well. For some scenarios, you might need features that are only available with Qt C++. Also, C++ is fast and type-safe. This provides the best possible perfor-mance for long-running and data-intensive calcula-tions.

For the following examples, you would choose C++ over QML:

Native C++ code is the right choice for data-inten-sive operations. It will outperform interpreted QML/ JavaScript code.

C++ code is type-safe and compiled into object code. For parts where stability and security are important, using C++ helps to make your app less error-prone. The Qt C++ components offer different, and in some cases more features than the QML types. For exam-ple, advanced networking features.

It is also possible to mix C++ with native code for An-droid (over JNI) or iOS (Obj-C or Swift). This provides native functionality for QML as well.

Occasionally, you might need a 3rd party library that is only available in C++, or have existing code in C++ which you’d like to reuse.

It is possible to code most parts of the application without the need to touch C++. The Felgo SDK and Qt have many QML components available. For a start-ing point to integrate Qt C++ or make native features available for QML, you can see the guide How to Ex-pose a Qt C++ Class with Signals and Slots to QML.

Application Development with QML is simple and powerful. But Qt C++ can be more

performant, offers many features and is less error-prone. So it’s important to know

QML MVC: App Architecture Best

Practices and Design Patterns

With all this knowledge and QML benefits in mind, we will use QML for more than only view code. We handle data-related tasks and application logic as well. It thus becomes very important to think about a clean app architecture and component structure for such QML-driven apps. We want to keep our code clean, maintainable and extensible.

The ease and flexibility of QML can lead to certain problems. When using signals and property

bind-ings a simple value change can affect many com-ponents. Properties that rely on other properties update automatically. They handle the signal and update their value as well. This is even possible across multiple QML Items:

This doesn’t look complex now. But imagine that we add a few new components with different proper-ties and cross-dependencies.

Practices and Design Patterns

Different connections between all the items start to form. You can no longer say for sure what effect a single property change may have for directly or indi-rectly connected components. In the worst case, this can result in circular dependencies or loops. If you face such issues, this is an indicator for bad compo-nent architecture.

This problem gets worse and worse the more your project grows. Each component should thus have a clean interface and only manage its own data. This is what separation of concerns means.

A bad component architecture can quickly lead to unwanted side-effects or corrupted data. Imagine lots of signals firing and event handlers running in unknown order. Your code at different parts of the app changes data seemingly random or with

dupli-cated code. This is a nightmare for debugging, main-tenance or refactoring.

For most design patterns, the golden rule is: Keep the code that displays data (view) separate from the code that reads or modifies data (model).

Design Patterns like MVC, MVVM

or Flux (React Native)

The DataModel is your central storage for application data. Pages can only access application data with the DataModel. It manages your data in a way that makes sense for your use-case and views. For different views and use-cases, the usage of several DataModel com-ponents is possible.

For the app logic, you will also add many functions and signal handlers to your view or model. If not done properly, this results in a lot of fragmented code spread across view items. Once your application gets more complex, it also gets more difficult to maintain

your code clean and testable. It gets hard to decide where to perform which action, and duplicate code spread across pages is inevitable.

With the above architecture, the data flow between model and page is bi-directional. This means, pages do not only show model data, but can also write to the model directly. Modern application frameworks like React Native use a different approach. For exam-ple, the Flux architecture designed by Facebook fa-vors an unidirectional data flow.

Each content view of a Felgo app is usually com-posed as a Page. The Page type is a view controller, which provides and displays data for the user. By

introducing a DataModel component, we can move data handling away from the view:

With Flux, each user interaction propagates the action through a central dispatcher. The dispatcher

forwards the action to various stores that hold appli-cation data and business logic:

QML Architecture inspired by Flux

You can read more about the quite sophisticated im-plementation of such a pattern for QML in this post by Ben Lau: Revised QML Application Architecture Guide with Flux

It’s hard to say if the overhead of a complex solution is worth the effort. For most mobile app projects, a more relaxed implementation with the same advan-tages is sufficient.

Doing a lot of work in QML can quickly result in un-readable and fragmented code. With a clean compo-nent architecture and clear responsibilities, you can avoid this. Using an architecture like above helps to provide a consistent app structure that simplifies testing and refactoring.

With the principles shown in this guide, you can cre-ate well-structured apps that handle app logic, da-ta-related tasks and view code purely with QML.

// 10% CODE - 100% APP // 24

Felgo offers many demos and examples you can use to

build a first prototype of your project

Not sure how to get started?

We will help you out with Qt Trainings and App

Development Workshops to get your team started!

Qt Trainings & Workshops

Qt Software Development

Services

Qt Consulting Services

Qt for iOS & Android Services

The Felgo team is here to support you! We can cover the whole process from the first idea up to the release of your app in the stores. Take advantage of our Mobile App Development Services and join

the ranks of satisfied Felgo customers:

Do you already have a project or app

in mind?

The Felgo SDK is free to use, so make sure to check it out!

Join more than 50.000 Felgo Users