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How to snap a window to the screen edge


WinAMP has this very useful feature. If you drag it to the edge of the screen it will automatically position itself. Sometimes you want to make room on the screen but you don't want to minimize your program and again you want to see it in full. The only way is the put it in a corner and set it exactly to the screen border. Well, this can be quite a challenge :-)

But there's no need to panic. All we have to do is intercept when the window is being moved and adjust it as needed.

This code adjusts the window within 30 pixels.

  Form1 = class(TForm)
    procedure WMExitSizeMove(var Msg: TMessage); message WM_EXITSIZEMOVE;

procedure TForm1.WMExitSizeMove(var Msg: TMessage);
  iScreenWidth, iScreenHeight: Integer;
  ScreenRect, TaskBarRect: TRect;
  if Msg.Msg = WM_EXITSIZEMOVE then
    Pabd.cbSize := SizeOf(APPBARDATA);
    SHAppBarMessage(ABM_GETTASKBARPOS, Pabd);

    iScreenWidth := GetSystemMetrics(SM_CXSCREEN);
    iScreenHeight := GetSystemMetrics(SM_CYSCREEN);
    ScreenRect := Rect(0, 0, iScreenWidth, iScreenHeight);
    TaskBarRect := Pabd.rc;

    if (TaskBarRect.Left = -2) and (TaskBarRect.Bottom = iScreenHeight + 2)
      and (TaskBarRect.Right = iScreenWidth + 2) then
      ScreenRect.Bottom := TaskBarRect.Top
    else if (TaskBarRect.Top = -2) and (TaskBarRect.Left = -2)
      and (TaskBarRect.Right = iScreenWidth + 2) then
      ScreenRect.Top := TaskBarRect.Bottom
    else if (TaskBarRect.Left = -2) and (TaskBarRect.Top = -2)
      and (TaskBarRect.Bottom = iScreenHeight + 2) then
      ScreenRect.Left := TaskBarRect.Right
    else if (TaskBarRect.Right = iScreenWidth + 2) and (TaskBarRect.Top = -2)
      and (TaskBarRect.Bottom = iScreenHeight + 2) then
      ScreenRect.Right := TaskBarRect.Left;

    if Left < ScreenRect.Left + 30 then
      Left := ScreenRect.Left;
    if Top < ScreenRect.Top + 30 then
      Top := ScreenRect.Top;
    if Left + Width > ScreenRect.Right - 30 then
      Left := ScreenRect.Right - Width;
    if Top + Height > ScreenRect.Bottom - 30 then
      Top := ScreenRect.Bottom - Height;
  Msg.Result := 1;

Thanks to Thomas Jones for fixing a bug in the code that calculates which side of the screen the window should snap to.

Author: Unknown
Added: 2007-06-02
Last updated: 2008-08-14

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