WM_SIZE message

Sent to a window after its size has changed.

A window receives this message through its WindowProc function.

#define WM_SIZE                         0x0005

wParam

The type of resizing requested. This parameter can be one of the following values.

Expand table

Value	Meaning
SIZE_MAXHIDE 4	Message is sent to all pop-up windows when some other window is maximized.
SIZE_MAXIMIZED 2	The window has been maximized.
SIZE_MAXSHOW 3	Message is sent to all pop-up windows when some other window has been restored to its former size.
SIZE_MINIMIZED 1	The window has been minimized.
SIZE_RESTORED 0	The window has been resized, but neither the SIZE_MINIMIZED nor SIZE_MAXIMIZED value applies.

 

lParam

The low-order word of lParam specifies the new width of the client area.

The high-order word of lParam specifies the new height of the client area.

Type: LRESULT

If an application processes this message, it should return zero.

/******************************************************************
*                                                                 *
*  SimpleText::OnResize                                           *
*                                                                 *
*  If the application receives a WM_SIZE message, this method     *
*  resize the render target appropriately.                        *
*                                                                 *
******************************************************************/

void SimpleText::OnResize(UINT width, UINT height)
{
    if (pRT_)
    {
        D2D1_SIZE_U size;
        size.width = width;
        size.height = height;
        pRT_->Resize(size);
    }
}

LRESULT CALLBACK SimpleText::WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
   
    SimpleText *pSimpleText = reinterpret_cast<SimpleText *>(
                ::GetWindowLongPtr(hwnd, GWLP_USERDATA));

    if (pSimpleText)
    {
        switch(message)
        {
        case WM_SIZE:
            {
                UINT width = LOWORD(lParam);
                UINT height = HIWORD(lParam);
                pSimpleText->OnResize(width, height);
            }
            return 0;

// ...

Example from Windows classic samples on GitHub.

If the SetScrollPos or MoveWindow function is called for a child window as a result of the WM_SIZE message, the bRedraw or bRepaint parameter should be nonzero to cause the window to be repainted.

Although the width and height of a window are 32-bit values, the lParam parameter contains only the low-order 16 bits of each.

The DefWindowProc function sends the WM_SIZE and WM_MOVE messages when it processes the WM_WINDOWPOSCHANGED message. The WM_SIZE and WM_MOVE messages are not sent if an application handles the WM_WINDOWPOSCHANGED message without calling DefWindowProc.

Reference

HIWORD

LOWORD

MoveWindow

WM_WINDOWPOSCHANGED

Conceptual

Windows

Other Resources

SetScrollPos