Windows Message Handling

Introduction

The messages for your program will be sent by Windows into a queue, which is worked off only if you want this. Windows sends thousand messages to your program without noticing this directly.

For example if you change the status of a gadget (identical whether you add a new entry or change the image of an ImageGadget), a message is sent to the queue of your program.

There are two possibilities to receive and to process the Windows messages in PureBasic: WaitWindowEvent() and WindowEvent(). The difference is, that WaitWindowEvent() waits till a message arrives and WindowEvent() continues to work also so. The messages in the queue will be processed however only, after you have called WindowEvent() or WaitWindowEvent().

Specials of WindowEvent()

The command WindowEvent() don't wait, until a message arrives, but checks only whether one is in the queue. If yes, the message is processed and WindowEvent() gives back the number of the message. If no message lines up, then zero (0) is given back.

Example

  While WindowEvent() : Wend

cause, that WindowEvent() becomes called as long till it gives back 0, i.e. until all messages of the queue are processed.

It doesn't reach, if you insert a simple 'WindowEvent()' after a SetGadgetState() to process this 1 message. Firstly there still can be other messages in the queue, which have arrived before, and secondly Windows also sends quite a number of other messages, we don't have to take care of... which nevertheless are in the queue.

A simple call of

  WindowEvent()

doesn't reach, the code then runs correct under circumstances on one Windows version, but on another version not. The different Windows versions are internally very different, so that one version only sends 1 message but another Windows version sends 5 messages for the same circumstance.

Because of this one always uses for updating:

  While WindowEvent() : Wend

Of course there is also the alternative

  Repeat : Until WindowEvent() = 0

possible, what isn't rather usual however.

The described method While : WindowEvent() : Wend is frequently useful in connection with the command Delay(), where the loop is inserted BEFORE the Delay(), e.g. to firstly wait for the update of an ImageGadget after changing an image with SetGadgetState().