When you specify a dynamic call in PowerBuilder, the function or event does not have to exist when you compile the code. You are indicating to the compiler that there will be a suitable function or event available at execution time.
For a dynamic call, PowerBuilder waits until it is time to execute the function or event to look for it. This gives you flexibility and allows you to call functions or events in descendants that do not exist in the ancestor.
Results of dynamic
calls To illustrate the results of dynamic calls, consider these objects:
• Ancestor window w_a with a function Set(integer).
• Descendent window w_a_desc with two functions: Set(integer) overrides the ancestor function, and Set(string) is an overload of the function.
Situation 1 Suppose you open the window mywindow of the ancestor window class w_a:
w_a mywindow Open(mywindow)
This is what happens when you call the Set function statically or dynamically:
Situation 2 Now suppose you open mywindow as the descendant window class w_a_desc:
w_a mywindow
Open(mywindow, "w_a_desc")
This statement Has this result
mywindow.Set(1) Compiles correctly because function is found in the ancestor w_a.
At runtime, Set(integer) in the ancestor is executed.
mywindow.Set("hello") Fails to compile; no function prototype in w_a matches the call.
mywindow.DYNAMIC Set("hello") Compiles successfully because of the DYNAMIC keyword.
An error occurs at runtime because no matching function is found.
Static versus dynamic calls
This is what happens when you call the Set function statically or dynamically in the descendant window class:
Disadvantages of dynamic calls
Slower performance Because dynamic calls are resolved at runtime, they are slower than static calls. If you need the fastest performance, design your application to avoid dynamic calls.
Less error checking When you use dynamic calls, you are foregoing error checking provided by the compiler. Your application is more open to
application errors, because functions that are called dynamically might be unavailable at execution time. Do not use a dynamic call when a static call will suffice.
Example using
dynamic call A sample application has an ancestor window w_datareview_frame that defines several functions called by the menu items of m_datareview_framemenu. They are empty stubs with empty scripts so that static calls to the functions will compile. Other windows that are descendants of w_datareview_frame have scripts for these functions, overriding the ancestor version.
The wf_print function is one of these—it has an empty script in the ancestor and appropriate code in each descendent window:
guo_global_vars.ish_currentsheet.wf_print ()
The wf_export function called by the m_export item on the m_file menu does not have a stubbed-out version in the ancestor window. This code for m_export uses the DYNAMIC keyword to call wf_export. When the program runs, the value of variable ish_currentsheet is a descendent window that does have a definition for wf_export:
guo_global_vars.ish_currentsheet.DYNAMIC wf_export()
This statement Has this result
mywindow.Set(1) Compiles correctly because function is found in the ancestor w_a.
At runtime, Set(integer) in the descendant is executed.
mywindow.Set("hello") Fails to compile; no function prototype in the ancestor matches the call.
mywindow.DYNAMIC Set("hello") Compiles successfully because of the DYNAMIC keyword.
At runtime, Set(string) in the descendant is executed.
Errors when calling functions and events dynamically
If you call a function or event dynamically, different conditions create different results, from no effect to an execution error. The tables in this section illustrate this.
Functions The rules for functions are similar to those for events, except functions must exist: if a function is not found, an error always occurs. Although events can exist without a script, if a function is defined it has to have code. Consider the following statements:
1 This statement calls a function without looking for a return value:
object.DYNAMIC funcname( ) 2 This statement looks for an integer return value:
int li_int
li_int = object.DYNAMIC funcname( ) 3 This statement looks for an Any return value:
any la_any
la_any = object.DYNAMIC funcname( ) The following table uses these statements as examples.
Table 6-3: Dynamic function calling errors
Events Consider these statements:
1 This statement calls an event without looking for a return value:
object.EVENT DYNAMIC eventname( ) 2 This example looks for an integer return value:
int li_int
li_int = object.EVENT DYNAMIC eventname( )
Condition 1 Condition 2 Result Example
The function does not exist.
None. Execution error 65:
Dynamic function not found.
All the
statements cause error 65.
The function is found and executed but is not defined with a return value.
The code is looking for a return value.
Execution error 63:
Function/event with no return value used in expression.
Statements 2 and 3 cause error 63.
Static versus dynamic calls
3 This example looks for an Any return value:
any la_any
la_any = object.EVENT DYNAMIC eventname( ) The following table uses these statements as examples.
Table 6-4: Dynamic event calling errors
Condition 1 Condition 2 Result Example
The event does not exist.
The code is not looking for a return value.
Nothing; the call fails silently.
Statement 1 fails but does not cause an error.
The code is looking for a return value.
A null of the Any datatype is returned.
La_any is set to null in statement 3.
If the expected datatype is not Any, execution error 19 occurs: Cannot convert Any in Any variable to datatype.
The assignment to li_int causes execution error 19 in statement 2.
If eventname is defined to return integer, li_int is set to null in statement 2.
The event does not have a defined return value.
A null of the Any datatype is returned.
La_any is set to null in statement 3.
If the expected datatype is not Any, execution error 19 occurs: Cannot convert Any in Any variable to datatype.
The assignment to li_int causes execution error 19 in statement 2.
The event is found and executed but is not defined with a return value.
The code is looking for a return value.
Execution error 63:
Function/event with no return value used in expression.
Statements 2 and 3 cause error 63.
When an error occurs You can surround a dynamic function call in a try-catch block to prevent the application from terminating when an execution error occurs. Although you can also handle the error in the SystemError event, you should not allow the application to continue once the SystemError event is invoked—the SystemError event should only clean up and halt the application.
For information on using try-catch blocks, see the chapter on exception handling in Application Techniques.
If the arguments do
not match Function arguments are part of the function’s definition. Therefore, if the arguments do not match (a compatible match, not an exact match), it is essentially a different function. The result is the same as if the function did not exist.
If you call an event dynamically and the arguments do not match, the call fails and control returns to the calling script. There is no error.
Error-proofing your
code Calling functions and events dynamically opens up your application to potential errors. The surest way to avoid these errors is to always make static calls to functions and events. When that is not possible, your design and testing can ensure that there is always an appropriate function or event with the correct return datatype.
One type of error you can check for and avoid is data conversion errors.
The preceding tables illustrated that a function or event can return a null value either as an Any variable or as a variable of the expected datatype when a function or event definition exists but is not implemented.
If you always assign return values to Any variables for dynamic calls, you can test for null (which indicates failure) before using the value in code.
This example illustrates the technique of checking for null before using the return value.
any la_any
integer li_gotvalue
la_any = object.DYNAMIC uf_getaninteger( ) IF IsNull(la_any) THEN
... // Error handling ELSE
li_gotvalue = la_any END IF
Overloading, overriding, and extending functions and events