delphi - 在 Delphi 窗体中绘制控件

Question

如何在表单 Canvas 上以及表单上的控件上绘制内容？

我尝试以下操作:

procedure TForm1.FormPaint(Sender: TObject);
var x,y: Integer;
begin
  x := Mouse.CursorPos.X - 10;
  y := Mouse.CursorPos.Y - 10;
  x := ScreentoClient(point(x,y)).X - 10;
  y := ScreenToClient(point(x,y)).Y - 10;
  Canvas.Brush.Color := clRed;
  Canvas.FillRect(rect(x, y, x + 10, y + 10));
  Invalidate;
end;

矩形是在绘制其他控件之前绘制的，因此它隐藏在控件后面(根据 Delphi 文档，这是预期行为)。

我的问题是如何绘制控件？

Answer 1

不要在绘画处理程序中“无效”。 Invalidating导致发送WM_PAINT，这当然会重新开始绘画处理。即使您不移动鼠标，您发布的代码示例也会导致“OnPaint”事件一次又一次运行。由于您的绘图取决于光标的位置，因此您可以为此使用“OnMouseMove”事件。但是您还需要拦截其他窗口控件的鼠标消息。因此，下面的示例使用“ApplicationEvents”组件。如果您的应用程序有多个表单，您需要设计一种机制来区分您正在绘制的表单。

另请参阅文档，VCL 的 Invalidate使整个窗口无效。您不需要这样做，您正在绘制一个小矩形，并且您确切地知道您在绘制的位置。只需使您要绘制的位置和已绘制的位置无效即可。

至于在控件上绘图，实际上绘图部分很容易，但你不能使用提供的 Canvas 来做到这一点。表格有 WS_CLIPCHILDREN样式，子窗口的表面将被排除在更新区域之外，因此您必须使用 GetDCEx或GetWindowDC 。正如评论中提到的“user205376”，删除您绘制的内容有点棘手，因为您实际上可以在多个控件上绘制一个矩形。但 API 也有一个快捷方式，正如您将在代码中看到的那样。

我尝试对代码进行一些注释以便能够遵循，但跳过了错误处理。实际的绘制可以在“OnPaint”事件处理程序中进行，但不是从“TWinControl”派生的控件将在处理程序之后进行绘制。所以它位于 WM_PAINT 处理程序中。

type
  TForm1 = class(TForm)
    [..]
    ApplicationEvents1: TApplicationEvents;
    procedure FormCreate(Sender: TObject);
    procedure ApplicationEvents1Message(var Msg: tagMSG; var Handled: Boolean);
  private
    FMousePt, FOldPt: TPoint;
    procedure WM_PAINT(var Msg: TWmPaint); message WM_PAINT;
  public
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  // no rectangle drawn at form creation
  FOldPt := Point(-1, -1);
end;

procedure TForm1.ApplicationEvents1Message(var Msg: tagMSG;
  var Handled: Boolean);
var
  R: TRect;
  Pt: TPoint;
begin
  if Msg.message = WM_MOUSEMOVE then begin

    // assume no drawing (will test later against the point).
    // also, below RedrawWindow will cause an immediate WM_PAINT, this will
    // provide a hint to the paint handler to not to draw anything yet.
    FMousePt := Point(-1, -1);


    // first, if there's already a previous rectangle, invalidate it to clear
    if (FOldPt.X > 0) and (FOldPt.Y > 0) then begin
      R := Rect(FOldPt.X - 10, FOldPt.Y - 10, FOldPt.X, FOldPt.Y);
      InvalidateRect(Handle, @R, True);

      // invalidate childs
      // the pointer could be on one window yet parts of the rectangle could be
      // on a child or/and a parent, better let Windows handle it all
      RedrawWindow(Handle, @R, 0,
                     RDW_INVALIDATE or RDW_UPDATENOW or RDW_ALLCHILDREN);
    end;


    // is the message window our form?
    if Msg.hwnd = Handle then
      // then save the bottom-right coordinates
      FMousePt := SmallPointToPoint(TSmallPoint(Msg.lParam))
    else begin
      // is the message window one of our child windows?
      if GetAncestor(Msg.hwnd, GA_ROOT) = Handle then begin
        // then convert to form's client coordinates
        Pt := SmallPointToPoint(TSmallPoint(Msg.lParam));
        windows.ClientToScreen(Msg.hwnd, Pt);
        FMousePt := ScreenToClient(Pt);
      end;
    end;

    // will we draw?  (test against the point)
    if PtInRect(ClientRect, FMousePt) then begin
      R := Rect(FMousePt.X - 10, FMousePt.Y - 10, FMousePt.X, FMousePt.Y);
      InvalidateRect(Handle, @R, False);
    end;
  end;
end;

procedure TForm1.WM_PAINT(var Msg: TWmPaint);
var
  DC: HDC;
  Rgn: HRGN;
begin
  inherited;

  if (FMousePt.X > 0) and (FMousePt.Y > 0) then begin
    // save where we draw, we'll need to erase before we draw an other one
    FOldPt := FMousePt;

    // get a dc that could draw on child windows
    DC := GetDCEx(Handle, 0, DCX_PARENTCLIP);

    // don't draw on borders & caption
    Rgn := CreateRectRgn(ClientRect.Left, ClientRect.Top,
                          ClientRect.Right, ClientRect.Bottom);
    SelectClipRgn(DC, Rgn);
    DeleteObject(Rgn);

    // draw a red rectangle
    SelectObject(DC, GetStockObject(DC_BRUSH));
    SetDCBrushColor(DC, ColorToRGB(clRed));
    FillRect(DC, Rect(FMousePt.X - 10, FMousePt.Y - 10, FMousePt.X, FMousePt.Y), 0);

    ReleaseDC(Handle, DC);
  end;
end;