delphi - 将屏幕对象句柄获取到 DrawFocusRect 时如何消除屏幕闪烁

Question

我正在尝试在多年前开发的 Apprehend 屏幕捕获组件中围绕选定的屏幕对象绘制聚焦矩形。我可以通过使用 Handles := WindowFromPoint( P ); 获取光标下对象的句柄来绘制焦点矩形。但这需要我隐藏然后显示 self 才能工作，否则会返回 self 的句柄。

不幸的是，当我隐藏和显示表单时，它会在隐藏和显示表单时导致闪烁。

我可以毫无问题地获取所选对象的位图，只是绘制所选对象就让我发疯了。

有人有什么建议可以在选定的对象周围绘制 FocusedRect 以避免闪烁吗？如果表单位于屏幕顶部，是否有 API 可以获取屏幕句柄？

我尝试使用 Handles := WindowFromDC(ScreenDC) 因此我不必隐藏和显示表单，但 WindowFromDC 仍然返回表单而不是屏幕。

TCaptureObjectForm 是透明的并且位于屏幕上方。我需要组件中的 TCaptureObjectForm。

//FormMouseMove 事件 - 添加于 2011 年 8 月 2 日

procedure TCaptureObjectForm.FormMouseMove( Sender: TObject; Shift: TShiftState; X, Y: Integer );
const
  crHand = -18;
var
  P: TPoint;
  Handles: HWND;
  Rect: TRect;
  ScreenDC: HDC;
begin
  // hide the TCaptureObjectForm form so the screen is found by WindowFromPoint
  Self.Hide;
  // get the object on the screen
  GetCursorPos( P );
  Handles := WindowFromPoint( P );
  // tried this but it returns self.handle rather than the screen handle
  //ScreenDC := GetDC( 0 );
  //Handles := WindowFromDC(ScreenDC);
  //ReleaseDC( 0, ScreenDC );
  // restore the TCaptureObjectForm
  Self.Show;
  // get object rect
  GetWindowRect( Handles, Rect );
  // draw a rect to show it is focused
  Self.Canvas.DrawFocusRect( Rect );
end;

Answer 1

This article是 Microsoft 的 Visual Basic 示例，其功能与您的需求非常相似。

他们采取以下方法:

1. 在 Form_MouseDown 中捕获鼠标。
2. 当鼠标移动时，在鼠标指向的窗口周围绘制一个矩形:Form_MouseMove。
3. 在 Form_MouseUp 中释放鼠标，并使整个屏幕无效以删除最后绘制的矩形。

他们直接在他们选择的窗口中绘制。我不认为使用透明窗口方法可以避免所有闪烁。

该代码示例似乎不完整并且运行效果不佳，因此我对其进行了修改(并翻译为 Delphi):

// Not global variables, but private form ones
var
  HwndLastTracked: HWND;
  CapturedMouse: boolean;

procedure InvertTracker(hwndWindow: HWND);
var
  rc: TRect;
  dc: HDC;
  pen, oldPen: HPEN;
  oldBrush: HBRUSH;
  style, exStyle: longint;
  cx, cy: integer;
begin
  GetWindowRect(hwndWindow, rc);

  // Window coordinates of the origin (top-left corner) of a window is (0, 0)
  OffsetRect(rc, -rc.Left, -rc.Top);

  // DC returned by GetWindowDC covers the full window area, but in Windows
  // Vista/7 it seems to be clipped excluding the nonclient region, due to
  // DWM handling nonclient drawing, so it doesn't allow painting over it.
  // Thus we need to skip this nonclient area and that is why I adjust the
  // window rect to match the client area. Using GetClientRect instead of
  // GetWindowRect is not suitable as excludes scroll bars and child 
  // parts drawed in WM_NCPAINT, such as Windows' WS_EXEDGEs and Delphi's
  // bevels.

  style := GetWindowLong(hwndWindow, GWL_STYLE);
  exStyle := GetWindowLong(hwndWindow, GWL_EXSTYLE);

  if style and WS_CAPTION <> 0 then begin
    if exStyle and WS_EX_TOOLWINDOW <> 0 then
      cy := GetSystemMetrics(SM_CYSMCAPTION)
    else
      cy := GetSystemMetrics(SM_CYCAPTION);

    // discard area covered by caption 
    Inc(rc.Top, cy);
  end;

  if style and WS_THICKFRAME <> 0 then begin
    cx := GetSystemMetrics(SM_CXFRAME);
    cy := GetSystemMetrics(SM_CYFRAME);
  end
  else if style and WS_DLGFRAME <> 0 then begin
    cx := GetSystemMetrics(SM_CXDLGFRAME);
    cy := GetSystemMetrics(SM_CYDLGFRAME);
  end
  else if style and WS_BORDER <> 0 then begin
    cx := GetSystemMetrics(SM_CXBORDER);
    cy := GetSystemMetrics(SM_CYBORDER);
  end
  else begin
    cx := 0;
    cy := 0;
  end;

  if (cx <> 0) or (cy <> 0) then begin
    // discard area covered by borders
    OffsetRect(rc, cx, cy);
    Dec(rc.Right, cx*2);
    Dec(rc.Bottom, cy*2);
  end;

  // Windows API functions don't raise exceptions, so I don't use try-finally

  dc := GetWindowDC(hwndWindow);

  // Option 1: focused rect
  //DrawFocusRect(dc, rc);

  // Option 2: inverted thick border
  SetROP2(dc, R2_NOT);
  pen := CreatePen(PS_INSIDEFRAME, 3 * GetSystemMetrics(SM_CXBORDER), 0);
  oldPen := SelectObject(dc, pen);
  oldBrush := SelectObject(dc, GetStockObject(NULL_BRUSH));

  Rectangle(dc, rc.Left, rc.Top, rc.Right, rc.Bottom);

  SelectObject(dc, oldBrush);
  SelectObject(dc, oldPen);
  DeleteObject(pen);
  // End option 2

  ReleaseDC(hwndWindow, dc);
end;

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if SetCapture(Handle) <> 0 then begin
    CapturedMouse := true;
    HwndLastTracked := 0;
    Screen.Cursor := crCross;
  end;
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState;
  X, Y: Integer);
var
  hwndCaptured: HWND;
begin
  if CapturedMouse then begin
    hwndCaptured := WindowFromPoint(ClientToScreen(Point(X, Y)));

    // Uncomment this for track root windows instead of childs
    //hwndCaptured := GetAncestor(hwndCaptured, GA_ROOT);

    if hwndCaptured <> HwndLastTracked then begin
      if HwndLastTracked <> 0 then
        InvertTracker(HwndLastTracked);
      InvertTracker(hwndCaptured);
      HwndLastTracked := hwndCaptured;
    end;
  end;
end;

procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if CapturedMouse then begin
    ReleaseCapture;
    CapturedMouse := false;

    if HwndLastTracked <> 0 then begin
      InvertTracker(HwndLastTracked);
      HwndLastTracked := 0;
    end;

    Screen.Cursor := crDefault;
  end;
end;

下面是 Microsoft 如何在 Visual Studio 的 Spy++ 中使用此技术的屏幕截图。红色气球和文字是我的!