multithreading - 当主线程被阻塞时显示事件指示器(继续)

Question

继续previous question我希望能够显示一些事件指示器即使主线程被阻塞。(基于this article)。

基于所附代码的问题:

• 使用 Synchronize(PaintTargetWindow); 不会绘制窗口
• 我有时会遇到错误:Canvas 不允许绘图。 在行中:{FBitmap.}StretchDraw(Rect(Left, ImageRect.Top, Right, ImageRect.Bottom), FfgPattern)

这是我用来创建指标线程的代码:

unit AniThread;

interface

uses Windows, Classes, Graphics, Controls, Math;

const
  ANI_GRAD_FG_COLOR_BAGIN = $00CDFFCD;
  ANI_GRAD_FG_COLOR_END   = $0024B105;
  ANI_GRAD_BK_COLOR_BAGIN = $00F5F5F5;
  ANI_GRAD_BK_COLOR_END   = $00BDBDBD;

type
  TAnimationThread = class(TThread)
  private
    FWnd: HWND;
    FPaintRect: TRect;
    FInterval: Integer;
    FfgPattern, FbkPattern: TBitmap;
    FBitmap: TBitmap;
    FImageRect: TRect;
    procedure UpdatePattern(Pattern: TBitmap; ColorBegin, ColorEnd: TColor);
    function CreatePatternBitmap(AColorBegin, AColorEnd: TColor): TBitmap;
    procedure PaintTargetWindow;
  protected
    procedure Execute; override;
  public
    procedure Animate;
    constructor Create(PaintSurface: TWinControl; { Control to paint on }
      PaintRect: TRect;          { area for animation bar }
      Interval: Integer          { wait in msecs between paints}
      );
    destructor Destroy; override;
  end;

implementation

constructor TAnimationThread.Create(PaintSurface: TWinControl;
  PaintRect: TRect;
  Interval: Integer);
begin
  inherited Create(True); { suspended }
  FreeOnterminate := True;
  Priority := tpHigher;
  FInterval := Interval;
  FWnd := PaintSurface.Handle;
  FPaintRect := PaintRect;
  FfgPattern := CreatePatternBitmap(ANI_GRAD_FG_COLOR_BAGIN, ANI_GRAD_FG_COLOR_END);
  FbkPattern := CreatePatternBitmap(ANI_GRAD_BK_COLOR_BAGIN, ANI_GRAD_BK_COLOR_END);
end;

destructor TAnimationThread.Destroy;
begin
  inherited Destroy;
  FfgPattern.Free;
  FbkPattern.Free;
end;

procedure TAnimationThread.Animate;
begin
  Resume;
  Sleep(0);
end;

function TAnimationThread.CreatePatternBitmap(AColorBegin, AColorEnd: TColor): TBitmap;
begin
  Result := TBitmap.Create;
  Result.PixelFormat := pf24bit;
  UpdatePattern(Result, AColorBegin, AColorEnd);
end;

type
  PRGBTripleArray = ^TRGBTripleArray;
  TRGBTripleArray = array[0..32767] of TRGBTriple;
  TGradientColors = array[0..255] of TRGBTriple;

procedure PatternBuilder(const Colors: TGradientColors; Pattern: TBitmap);
var
  Y: Integer;
  Row: PRGBTripleArray;
begin
  Pattern.Width := 1;
  Pattern.Height := 256;
  for Y := 0 to 127 do
  begin
    Row := PRGBTripleArray(Pattern.ScanLine[Y]);
    Row[0] := Colors[Y];
    Row := PRGBTripleArray(Pattern.ScanLine[Y + 128]);
    Row[0] := Colors[255 - Y];
  end;
end;

procedure TAnimationThread.UpdatePattern(Pattern: TBitmap; ColorBegin, ColorEnd: TColor);
var
  Colors: TGradientColors;
  dRed, dGreen, dBlue: Integer;
  RGBColor1, RGBColor2: TColor;
  RGB1, RGB2: TRGBTriple;
  Index: Integer;
begin
  RGBColor1 := ColorToRGB(ColorBegin);
  RGBColor2 := ColorToRGB(ColorEnd);

  RGB1.rgbtRed := GetRValue(RGBColor1);
  RGB1.rgbtGreen := GetGValue(RGBColor1);
  RGB1.rgbtBlue := GetBValue(RGBColor1);

  RGB2.rgbtRed := GetRValue(RGBColor2);
  RGB2.rgbtGreen := GetGValue(RGBColor2);
  RGB2.rgbtBlue := GetBValue(RGBColor2);

  dRed := RGB2.rgbtRed - RGB1.rgbtRed;
  dGreen := RGB2.rgbtGreen - RGB1.rgbtGreen;
  dBlue := RGB2.rgbtBlue - RGB1.rgbtBlue;

  for Index := 0 to 255 do
    with Colors[Index] do
    begin
      rgbtRed := RGB1.rgbtRed + (Index * dRed) div 255;
      rgbtGreen := RGB1.rgbtGreen + (Index * dGreen) div 255;
      rgbtBlue := RGB1.rgbtBlue + (Index * dBlue) div 255;
    end;

  PatternBuilder(Colors, Pattern);
end;

procedure TAnimationThread.PaintTargetWindow;
var
  DC: HDC;
begin
  DC := GetDC(FWnd);
  if DC <> 0 then
    try
      BitBlt(DC,
        FPaintRect.Left,
        FPaintRect.Top,
        FImageRect.Right,
        FImageRect.Bottom,
        FBitmap.Canvas.handle,
        0, 0,
        SRCCOPY);
    finally
      ReleaseDC(FWnd, DC);
    end;
end;

procedure TAnimationThread.Execute;
var
  Left, Right: Integer;
  Increment: Integer;
  State: (incRight, incLeft, decLeft, decRight);
begin
  InvalidateRect(FWnd, nil, True);
  FBitmap := TBitmap.Create;
  try
    with FBitmap do
    begin
      Width := FPaintRect.Right - FPaintRect.Left;
      Height := FPaintRect.Bottom - FPaintRect.Top;
      FImageRect := Rect(0, 0, Width, Height);
    end;
    Left := 0;
    Right := 0;
    Increment := FImageRect.Right div 50;
    State := Low(State);
    while not Terminated do
    begin
      with FBitmap.Canvas do
      begin
        StretchDraw(FImageRect, FbkPattern);
        case State of
          incRight:
            begin
              Inc(Right, Increment);
              if Right > FImageRect.Right then begin
                Right := FImageRect.Right;
                Inc(State);
              end;
            end;
          incLeft:
            begin
              Inc(Left, Increment);
              if Left >= Right then begin
                Left := Right;
                Inc(State);
              end;
            end;
          decLeft:
            begin
              Dec(Left, Increment);
              if Left <= 0 then begin
                Left := 0;
                Inc(State);
              end;
            end;
          decRight:
            begin
              Dec(Right, Increment);
              if Right <= 0 then begin
                Right := 0;
                State := incRight;
              end;
            end;
        end;

        StretchDraw(Rect(Left, FImageRect.Top, Right, FImageRect.Bottom), FfgPattern);
      end; { with }

      // Synchronize(PaintTargetWindow); // not painting when the main thread is blocked
      PaintTargetWindow;

      SleepEx(FInterval, False);
    end; { While }
  finally
    FBitmap.Free;
  end;
end;

end.

用法:在主窗体上放置一个TButton和一个TPanel。

uses AniThread;

procedure TForm1.Button1Click(Sender: TObject);
var
  at: TAnimationThread;
begin
  at := TAnimationThread.Create(Panel1, Panel1.ClientRect, 10);
  Button1.Enabled := False;
  try
    at.Animate;
    Sleep(3000); // sleep 3 sec. block main thread
  finally
    at.Terminate;
    Button1.Enabled := True;
  end;
end;

我知道你们中的许多人会不赞成这种方法。但现在对我来说主要是一个挑战，让它运作良好。任何有关此问题的帮助将不胜感激。

编辑:

这是original article (作者:B 组 Peter Below)。我只实现了渐变绘画。

Answer 1

Canvas 不允许绘图。 行中出现异常:

FBitmap.StretchDraw(Rect(Left, ImageRect.Top, Right, ImageRect.Bottom), FfgPattern)

这是由于 TBitmap Canvas 不是线程安全的，除非您锁定它(即使在主 UI 线程中)。根据我的经验，即使您将 Canvas 锁定在工作线程中，它的 DC 可能 会被 Graphics.pas 垃圾收集/GDI 缓存释放，而消息在主 UI TWinControl.MainWndProc 中处理。正在访问的每个位图 Canvas 都需要锁定，包括我的代码中的 FBitmap + FbkPattern + FfgPattern。

参见Graphis.pas中的FreeMemoryContexts:

{ FreeMemoryContexts is called by the VCL main winproc to release
  memory DCs after every message is processed (garbage collection).
  Only memory DCs not locked by other threads will be freed.
}

可能的解决方案是不直接使用TBitmap.Canvas，而是使用CreateCompatibleDC，如下所述:How to load images from disk in background (multiple threads) [AKA: TBitmap is not thread-safe]或锁定您使用的每个 TCanvas。

更多引用:
How threadsafe is TBitmap
GDI handle leak using TGIFImage in a second thread
QC: TJPEGImage.Draw() is not thread safe

<小时/>

为我工作的代码确保每个 TBitmap.Canvas 都被锁定在工作线程上下文中:
Working TAnimationThread
无论主 UI 线程是否被阻塞，这都可以正常工作。

procedure TForm1.Button1Click(Sender: TObject);
var
  at1, at2, at3, at4, at5: TAnimationThread;
begin
  at1 := TAnimationThread.Create(Panel1, Panel1.ClientRect, 10);
  at2 := TAnimationThread.Create(Panel2, Panel2.ClientRect, 10);
  at3 := TAnimationThread.Create(Panel3, Panel3.ClientRect, 10);
  at4 := TAnimationThread.Create(Panel4, Panel4.ClientRect, 10);
  at5 := TAnimationThread.Create(Panel5, Panel5.ClientRect, 10);
  // Sleep(5000); // do some work for 5 seconds, block main thread
  // at1.Terminate; at2.Terminate; at3.Terminate; at4.Terminate; at5.Terminate;
end;

现在，如果我省略例如锁定FfgPattern.Canvas.Lock;，则当我移动 UI 表单时，TBitmap 的 DC 将被终止(以防万一)我不会阻塞主线程，即不会休眠 5 秒并且不会终止线程)。

我的结论:

1. “除了主线程之外，您无法从任何其他地方使用 VCL 控件”(摘自评论)。不对!任何主 VCL 窗口控制 DC 都可以从工作线程访问，不会出现任何问题(事实上，许多应用程序直接绘制到桌面窗口 DC)。

2. TBitmap Canvas 是线程安全的，如果您知道在何处/何时锁定它。

3. 由于我不确定在何处/何时锁定它，最好不要在工作线程中使用 TBitmap Canvas 。使用API​​位图操作，使用CreateCompatibleDC/CreateBitmap； TWICImage 位于 Windows 成像组件之上。 TBitmap 垃圾收集是邪恶的!

4. 我不推荐这种方法。更好的方法是在工作线程的上下文中创建一个纯 API 窗口并在那里显示事件指示器，例如Displaying splash screen in Delphi when main thread is busy

5. 最好的方法(正如评论中已经提到的)是在工作线程中完成艰苦的工作，并在工作线程工作时在主 UI 线程中显示事件指示器。