delphi - 如何及时重新绘制 Canvas ？

Question

问题是:我在桌面上绘制一些矩形，当鼠标移动时(矩形大小增加)我没有滞后、伪影等，一切都很好:

但是当我将矩形大小调整为低于原来的大小时，我得到了以下结果:

红色矩形是真正的矩形，所有其他都是错误。

完美的解决方案是重新绘制 Canvas ，但我不能在鼠标移动时一直这样做。

鼠标移动后绝对停止时是否有解决方案？

更新

代码:

    unit Unit2;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls;

type
  TForm2 = class(TForm)
    Timer1: TTimer;
    procedure FormMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
    procedure FormCreate(Sender: TObject);
    procedure FormMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
    isDown: Boolean;
    downX, downY: Integer;
  public
    { Public declarations }
    Bild: TBitMap;
  end;

implementation

{表单 Prop :边框style= bsNoneAlphaBlend 真实，150透明颜色 = true, clBlack}

{$R *.dfm}

procedure TForm2.FormCreate(Sender: TObject);
begin
  Bild := TBitMap.Create;
end;

procedure TForm2.FormDestroy(Sender: TObject);
begin
  Bild.Free;
end;

procedure TForm2.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  isDown := true;
  downX := X;
  downY := Y;
end;

procedure TForm2.FormMouseMove(Sender: TObject; Shift: TShiftState;
  X, Y: Integer);
const
  cVal = 4;
begin
  if isDown then
  begin
    Self.Canvas.Lock;
    Self.Repaint;
    Self.Canvas.Pen.Color := clNone;
    Self.Canvas.Pen.Width := 1;

    Self.Canvas.Pen.Style := psDot;
    //Self.Canvas.Pen.Mode := pmNotCopy;
    Self.Canvas.Brush.Color := clGreen;
    Self.Canvas.Rectangle(downX, downY, X, Y);
    Self.Canvas.Pen.Style := psSolid;
    Self.Canvas.Brush.Color := clNone;
    Self.Canvas.Unlock;
    { Self.Canvas.Rectangle(downX - cVal, downY - cVal, downX + cVal, downY + cVal);
     Self.Canvas.Rectangle(X - cVal, Y - cVal, X + cVal, Y + cVal);
     Self.Canvas.Rectangle(X - cVal, downY - cVal, X + cVal, downY + cVal);
     Self.Canvas.Rectangle(downX - cVal, Y - cVal, downX + cVal, Y + cVal);

     Self.Canvas.Rectangle(downX - cVal, (downY + Y) div 2 - cVal, downX + cVal,
       (downY + Y) div 2 + cVal);
     Self.Canvas.Rectangle(X - cVal, (downY + Y) div 2 - cVal, X + cVal,
       (downY + Y) div 2 + cVal);

     Self.Canvas.Rectangle((downX + X) div 2 - cVal, downY - cVal,
       (downX + X) div 2 + cVal, downY + cVal);
     Self.Canvas.Rectangle((downX + X) div 2 - cVal, Y - cVal, (downX + X) div 2 + cVal,
       Y + cVal);   }
  end;
end;

function CaptureRect(aRect: TRect; out aBmp: TBitmap): Boolean;
var
  ScreenDC: HDC;
begin
  Result := False;
  try
    with aBmp, aRect do
    begin
      Width := Right - Left;
      Height := Bottom - Top;
      ScreenDC := GetDC(0);
      try
        BitBlt(Canvas.Handle, 0, 0, Width, Height, ScreenDC, Left, Top, SRCCOPY);
      finally
        ReleaseDC(0, ScreenDC);
      end;
    end;
  except
  end;
end;

procedure TForm2.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
var
  r: TRect;
begin
  isDown := false;
  r.Left := downX;
  r.Top := downY;
  r.Right := X;
  r.Bottom := Y;
  CaptureRect(r, Bild);
  Self.Close;
end;

end.

Answer 1

你的问题是你画错了地方。在 OnMouseMove 事件处理程序中停止绘画。将绘画代码移至绘画处理程序。例如表单的 OnPaint 处理程序。

然后，在 OnMouseMove 事件处理程序以及 OnMouseDown 和 OnMouseUp 中，调用表单上的 Invalidate ，或 Win32 InvalidateRect 函数，强制执行绘制周期。