CODEWITHSUNDEEP
cwinapi
user4129935
user4129935

Reputation:

When does Windows stops sending WM_PAINT messages?

If a specified rectangle on my Window is invalid (i.e. it needs to be painted), a WM_PAINT message is posted to the message queue, and if I did not validate the rectangle in the WM_PAINT event handler, then a new WM_PAINT message is posted to the message queue.

Is this correct?

If it is correct, then how does Windows knows not to keep sending WM_PAINT messages, what I mean is how does Windows knows to wait until I finish handling the WM_PAINT event and then send a new WM_PAINT message if I did not validate the rectangle, and not to send a new WM_PAINT message directly after I dequeue the WM_PAINT message from the message queue?

Upvotes: 1

Views: 1418

Answers (1)

David Heffernan
David Heffernan

Reputation: 612993

How does Windows know to wait until I finish handling the WM_PAINT event and then send a new WM_PAINT message if I did not validate the rectangle?

You handle WM_PAINT by calling BeginPaint, then painting, and then calling EndPaint. The region that you paint is marked as valid when you call EndPaint.

... and not to send a new WM_PAINT message directly after I dequeue the WM_PAINT message from the message queue?

Windows generates WM_PAINT messages when you process the message queue. It will only generate WM_PAINT messages when there are invalid regions. Once you have painted, there are no more invalid regions, and so no WM_PAINT messages are generated.

Note that Windows, typically, does not send WM_PAINT messages. These are asynchronous messages that are retrieved when your application's message loop pulls messages from the message queue. For instance, when you call GetMessage, and the message queue is empty, and there are invalid regions, then WM_PAINT messages are generated.

The exception to the statement above is when calls to UpdateWindow and RedrawWindow are made. In those scenarios, WM_PAINT messages are sent synchronously. From the documentation:

The WM_PAINT message is sent when the system or another application makes a request to paint a portion of an application's window. The message is sent when the UpdateWindow or RedrawWindow function is called, or by the DispatchMessage function when the application obtains a WM_PAINT message by using the GetMessage or PeekMessage function.

Upvotes: 3

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