CODEWITHSUNDEEP
c++cwindowswinapi
Marko29
Marko29

Reputation: 1005

SendMessageCallback usage example

http://msdn.microsoft.com/en-us/library/ms644951%28v=VS.85%29.aspx

I v been searching everywhere but cant find a single working thing in c on how this could be done. There seems to be several calls which totally confused me. Perhaps someone can put just a small example on how to declare this callback then post message to it?

Thank you

Upvotes: 5

Views: 5399

Answers (1)

Johann Gerell
Johann Gerell

Reputation: 25581

I assume that you are perfectly comfortable with plain ol' SendMessage. The step from there to SendMessageCallback is not that long.

First, look at

LRESULT WINAPI SendMessage(__in  HWND hWnd,
                           __in  UINT Msg,
                           __in  WPARAM wParam,
                           __in  LPARAM lParam);

Then look at

BOOL WINAPI SendMessageCallback(__in  HWND hWnd,
                                __in  UINT Msg,
                                __in  WPARAM wParam,
                                __in  LPARAM lParam,
                                __in  SENDASYNCPROC lpCallBack,
                                __in  ULONG_PTR dwData);

It's glaringly obvious that the differing parts are the SENDASYNCPROC and ULONG_PTR parameters of SendMessageCallback.

The lpCallBack up there is the name of a callback of yours that will be called by the OS when the hWnd window procedure returns after handling the message Msg you sent to it.

The type of lpCallBack is SENDASYNCPROC, which is declared as

VOID CALLBACK SendAsyncProc(__in  HWND hwnd,
                            __in  UINT uMsg,
                            __in  ULONG_PTR dwData,
                            __in  LRESULT lResult);

The dwData up there is any kind of data you want to use inside your callback, whenever it gets called. This is usually a pointer to complex data, like a struct or C++ class. In that case the lifetime of the memory must be considered carefully, so that it's valid when the callback is called. The dwData can also be just the simple integer data it looks like.

Bringing it all together, then. In your code you call SendMessageCallback like this (error checking omitted for readability):

SendMessageCallback(hWnd, WM_FOO, 0, 0, MySendAsyncProc, (ULONG_PTR)myData);

But, hmmm, since this is an exercise, let's assume that myData is just 0:

SendMessageCallback(hWnd, WM_FOO, 0, 0, MySendAsyncProc, 0);

That means you have declared a callback MySendAsyncProc:

VOID CALLBACK MySendAsyncProc(__in  HWND hwnd,
                              __in  UINT uMsg,
                              __in  ULONG_PTR dwData,  // This is *the* 0
                              __in  LRESULT lResult)   // The result from the callee
{
    // Whohoo! It called me back!
}

And that callback will be called when your WM_FOO message has been handled.

That pretty much sums it up.

Upvotes: 9

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