使用 C++ Builder 在 Windows 上截取屏幕截图的线程安全性

Question

在 Windows 上截屏不是线程安全的吗？

我的以下代码有时会拍摄一些照片，但在大多数情况下，imgScreenshot（这只是一个 TImage）一直是纯白色...

我是不是漏掉了什么？

void __fastcall TCaptureThread::Execute()
{
    int CurWidth = 1600;
    int CurHeight = 900;

    std::unique_ptr<TCanvas> Canvas(new TCanvas);
    Canvas->Handle = GetDC(0);    

    FBMP = new TBitmap;                     // private class field
    FBMP->Width     = CurWidth;
    FBMP->Height    = CurHeight;    

    FR = Rect(0, 0, CurWidth, CurHeight);   // private class field

    while(!Terminated)
    {
        FBMP->Canvas->CopyRect(FR, Canvas, FR);       
        Synchronize(&UpdatePicture);

        Sleep(100);                 
    }

    delete FBMP;
    FBMP = NULL;
}

void __fastcall TCaptureThread::UpdatePicture()
{
    FMainForm->imgScreenshot->Canvas->CopyRect(FR, FBMP->Canvas, FR);
}

环境是 C++ Builder 10.1.2 Berlin

Answer 1

Isn't taking a screenshot on Windows thread-safe?

在使用默认非线程安全的 VCL 包装器类时不是。如果您直接使用纯 Win32 API 函数，那么是的，可以编写线程安全代码。

您的代码失败的主要原因是因为 VCL 旨在在多个对象之间共享 GDI 资源，而主 UI 线程经常释放 unused/dormant GDI 资源。所以你的工作线程的 TBitmap 图像数据很可能在你调用 Synchronize() 将它复制到你的 TImage 之前被破坏。

话虽这么说，但如果您对 Canvas 中的对象调用 Lock()/Unlock()，您的尝试可以完成工作线程，例如：

struct CanvasLocker
{
    TCanvas *mCanvas;
    CanvasLocker(TCanvas *C) : mCanvas(C) { mCanvas->Lock(); }
    ~CanvasLocker() { mCanvas->Unlock(); }
};

void __fastcall TCaptureThread::Execute()
{
    int CurWidth = 1600;
    int CurHeight = 900;

    std::unique_ptr<TCanvas> Canvas(new TCanvas);
    std::unique_ptr<TBitmap> BMP(new TBitmap);
    FBMP = BMP.get();

    {
    CanvasLocker lock(Canvas); // <-- add this!
    Canvas->Handle = GetDC(0);    
    }

    {
    CanvasLocker lock(BMP->Canvas); // <-- add this!
    BMP->Width = CurWidth;
    BMP->Height = CurHeight;    
    }

    FR = Rect(0, 0, CurWidth, CurHeight);

    while (!Terminated)
    {
        {
        CanvasLocker lock1(Canvas); // <-- add this!
        CanvasLocker lock2(BMP->Canvas); // <-- add this!
        BMP->Canvas->CopyRect(FR, Canvas.get(), FR);
        }

        Synchronize(&UpdatePicture);

        Sleep(100);                 
    }
}

void __fastcall TCaptureThread::UpdatePicture()
{
    CanvasLocker lock1(FBMP->Canvas); // <-- add this!
    CanvasLocker lock2(FMainForm->imgScreenshot->Canvas); // <-- add this!

    FMainForm->imgScreenshot->Canvas->CopyRect(FR, FBMP->Canvas, FR);
    // or: FMainForm->imgScreenshot->Picture->Bitmap->Assign(FBMP);
}

话虽这么说，因为 TCanvas 是可锁定的，您可能能够完全移除 Synchronize()：

void __fastcall TCaptureThread::Execute()
{
    int CurWidth = 1600;
    int CurHeight = 900;

    std::unique_ptr<TCanvas> Canvas(new TCanvas);
    std::unique_ptr<TBitmap> BMP(new TBitmap);

    {
    CanvasLocker lock(Canvas);
    Canvas->Handle = GetDC(0);    
    }

    {
    CanvasLocker lock(BMP->Canvas);
    BMP->Width = CurWidth;
    BMP->Height = CurHeight;    
    }

    TRect r = Rect(0, 0, CurWidth, CurHeight);

    while (!Terminated)
    {
        {
        CanvasLocker lock1(BMP->Canvas);

        {
        CanvasLocker lock2(Canvas);
        BMP->Canvas->CopyRect(r, Canvas.get(), r);
        }

        CanvasLocker lock3(FMainForm->imgScreenshot->Canvas);
        FMainForm->imgScreenshot->Canvas->CopyRect(r, BMP->Canvas, r);
        // or: FMainForm->imgScreenshot->Picture->Bitmap->Assign(BMP);
        }

        Sleep(100);                 
    }
}

使用 C++ Builder 在 Windows 上截取屏幕截图的线程安全性

Thread safety for taking screenshots on Windows with C++ Builder

c++

vcl

c++builder

thread-safety