Triggering versus posting functions and events


In PowerBuilder, when you trigger a function or event, it is called immediately. Its return value is available for use in the script.


When you post a function or event, it is added to the object's queue and executed in its turn. In most cases, it is executed when the current script is finished; however, if other system events have occurred in the meantime, its position in the queue might be after other scripts. Its return value is not available to the calling script.

Because POST makes the return value unavailable to the caller, you can think of it as turning the function or event call into a statement.

Use posting when activities need to be finished before the code checks state information or does further processing (see Example 2 below).

PowerBuilder messages processed first

All events posted by PowerBuilder are processed by a separate queue from the Windows system queue. PowerBuilder posted messages are processed before Windows posted messages, so PowerBuilder events that are posted in an event that posts a Windows message are processed before the Windows message.

For example, when a character is typed into an EditMask control, the PowerBuilder pdm_keydown event posts the Windows message WM_CHAR to enter the character. If you want to copy the characters as they are entered from the EditMask control to another control, do not place the code in an event posted in the pdm_keydown event. The processing must take place in an event that occurs after the WM_CHAR message is processed, such as in an event mapped to pdm_keyup.

Restrictions for POST

Because no value is returned, you:

  • Cannot use a posted function or event as an operand in an expression

  • Cannot use a posted function or event as the argument for another function

  • Can only use POST on the last call in a cascaded sequence of calls

These statements cause a compiler error. Both uses require a return value:

IF POST IsNull( ) THEN ...
w_1.uf_getresult(dw_1.POST GetBorderStyle(2))

TriggerEvent and PostEvent functions

For backward compatibility, the TriggerEvent and PostEvent functions are still available, but you cannot pass arguments to the called event. You must pass data to the event in PowerBuilder's Message object.

Examples of posting

The following examples illustrate how to post events.

Example 1

In a sample application, the Open event of the w_activity_manager window calls the functions uf_setup and uf_set_tabpgsystem. (The functions belong to the user object u_app_actman.) Because the functions are posted, the Open event is allowed to finish before the functions are called. The result is that the window is visible while setup processing takes place, giving the user something to look at:

guo_global_vars.iuo_app_actman.POST uf_setup()
guo_global_vars.iuo_com_actman.POST uf_set_tabpgsystem(0)

Example 2

In a sample application, the DoubleClicked event of the tv_roadmap TreeView control in the u_tabpg_amroadmap user object posts a function that processes the TreeView item. If the event is not posted, the code that checks whether to change the item's picture runs before the item's expanded flag is set:

parent.POST uf_process_item ()