如何使用DrawThemeBackground绘制时，从右到左（RTL）方向绘制元素部分？

Question

我正在尝试从右向左绘制ttGlyphClosed类的Explorer::Treeview元素（就像BiDiMode为bdLeftToRight时）。我有一个问题，我不知道如何使我的屏幕外位图透明。位图的背景始终为白色。

我正在使用以下代码来镜像图像：

procedure TForm5.FormPaint(Sender: TObject);
var
  bm: TBitmap;
  ARect: TRect;
  Details: TThemedElementDetails;
begin    
  if ExplorerTreeviewhTheme = 0 then
    ExplorerTreeviewhTheme := OpenThemeData(0, 'Explorer::Treeview');

  ARect := Rect(20, 20, 40, 40);
  Details := ThemeServices.GetElementDetails(ttGlyphClosed);
  DrawThemeBackground(ExplorerTreeviewhTheme, Canvas.Handle,
    Details.Part, Details.State, ARect, nil); //Ok

  bm := TBitmap.Create;
  try
    bm.Width := 20;
    bm.Height := 20;

    ARect := Rect(00, 00, 20, 20);
    DrawThemeBackground(ExplorerTreeviewhTheme, bm.Canvas.Handle,
      Details.Part, Details.State, ARect, nil);

    // rendered result has white background
    Canvas.Draw(60, 10, bm);    
    // rendered result is mirrored but has also white background
    StretchBlt(Canvas.Handle, 100, 10, -20, 20, bm.Canvas.Handle, 0, 0, 20, 20, SRCCOPY);
  finally
    bm.Free;
  end;    
end;

问题是如何镜像由DrawThemeBackground函数绘制的元素（用于RTL读取）或如何将此函数用于RTL（从右到左）渲染？

Answer 1

使用SetLayout，因为TLama在他现在删除的答案中显示，在您绘制之前切换画布的布局。

function SetLayout(hdc: HDC; dwLayout: DWORD): DWORD; stdcall;
  external 'gdi32' name 'SetLayout';

const
  LAYOUT_RTL = $00000001;

procedure TForm1.FormPaint(Sender: TObject);
var
  ExplorerTreeviewhTheme: HTHEME;
  Details: TThemedElementDetails;
  ARect: TRect;
  Size: TSize;
begin
  ExplorerTreeviewhTheme := OpenThemeData(Handle, 'Explorer::Treeview');
  Details := ThemeServices.GetElementDetails(ttGlyphClosed);
  GetThemePartSize(ExplorerTreeviewhTheme, Canvas.Handle, Details.Part,
      Details.State, nil, TS_DRAW, Size);

  ARect := Rect(20, 30, 20 + Size.cx, 30 + Size.cy);

  // normal layout
  DrawThemeBackground(ExplorerTreeviewhTheme, Canvas.Handle,
                      Details.Part, Details.State, ARect, nil);

  // switched layout
  SetLayout(Canvas.Handle, LAYOUT_RTL);

  // calculate the rectangle for RTL as if it's in LTR
  OffsetRect(ARect, 0, Size.cy); // align to the bottom of the first image so that we can see
  ARect.Left := ClientWidth - ARect.Left - Size.cx;
  ARect.Right := ARect.Left + Size.cx;

  DrawThemeBackground(ExplorerTreeviewhTheme, Canvas.Handle,
                      Details.Part, Details.State, ARect, nil);

  // restore layout
  SetLayout(Canvas.Handle, 0);
  CloseThemeData(ExplorerTreeviewhTheme);
end;

输出：

主题api绘制一个16px零件尺寸（W7-aero）的6px宽三角形。由于您无法知道图像在零件中的位置，因此您无法更好地对齐它。

Answer 2

你在你的stretchblt调用中使用srccopy，但我认为你可能需要研究替代方案，包括可能使用掩码

我没有做过这样的事情，所以我记不起来了