How to catch the ending resize window?

Question

I need catch the event endresize in WPF.

Answer 1

You can detect exactly when a WPF window resize ended, and you don't need a timer. A native window receive the WM_EXITSIZEMOVE message when the user release the left mouse button at the end of a window resize or move operation. A WPF window doesn't receive this message, so we need to hook up a WndProc function which will receive it. We can use HwndSource with WindowInteropHelper to get our window handle. Then we will add the hook to our WndProc function. We will do all that in the window Loaded event (vb.net code):

Dim WinSource As HwndSource    

Private Sub WindowLoaded_(sender As Object, e As RoutedEventArgs)

    WinSource = HwndSource.FromHwnd(New WindowInteropHelper(Me).Handle)
    WinSource.AddHook(New HwndSourceHook(AddressOf WndProc))
End Sub

Now, in our WndProc, we will listen to the WM_EXITSIZEMOVE message:

Const WM_EXITSIZEMOVE As Integer = &H232

Private Function WndProc(hwnd As IntPtr, msg As Integer, wParam As IntPtr, lParam As IntPtr, ByRef handled As Boolean) As IntPtr

    If msg = WM_EXITSIZEMOVE Then

        DoWhatYouNeed()
    End If

    Return IntPtr.Zero
End Function

This and a similar technique is explained here and here.

Notice that the function should return IntPtr.Zero. Also, don't do in this func anything except handling the specific messages you are interested in.

Now, WM_EXITSIZEMOVE is also sent at the end of a move operation, and we are interested only in resize. There are several ways to determine that this was the end of resize operation. I did it by listening to the WM_SIZING message (which sent many times during resize), combined with a flag. The whole solution looks like this:

(Note: Don't get confused with the code highlighting here, cause its wrong for vb.net)

Dim WinSource As HwndSource
Const WM_SIZING As Integer = &H214
Const WM_EXITSIZEMOVE As Integer = &H232

Dim WindowWasResized As Boolean = False

Private Sub WindowLoaded_(sender As Object, e As RoutedEventArgs)

    WinSource = HwndSource.FromHwnd(New WindowInteropHelper(Me).Handle)
    WinSource.AddHook(New HwndSourceHook(AddressOf WndProc))
End Sub

Private Function WndProc(hwnd As IntPtr, msg As Integer, wParam As IntPtr, lParam As IntPtr, ByRef handled As Boolean) As IntPtr

    If msg = WM_SIZING Then

        If WindowWasResized = False Then

            'indicate the the user is resizing and not moving the window
            WindowWasResized = True
        End If
    End If

    If msg = WM_EXITSIZEMOVE Then

        'check that this is the end of resize and not move operation          
        If WindowWasResized = True Then

             DoWhatYouNeed()

             'set it back to false for the next resize/move
             WindowWasResized = False
        End If            
    End If

    Return IntPtr.Zero
End Function

That's it.