c++ - 为什么鼠标按下会阻止组件的重新绘制？

Question

我正在尝试创建一个自定义组件来捕获鼠标事件，尤其是 MouseMove。

我从 TWinControl 派生，但我也尝试过 TGraphicControl、TCustomControl、TTrackBar 等。

我的问题是当我在组件上按住鼠标时，它没有被重新绘制。

Paint() 方法在我释放鼠标按钮之前不会被调用，即使我调用了 Invalidate()。

TrackBar 是我想要制作的最接近的组件。您选择勾号，然后用鼠标移动它。但是您不必释放鼠标就可以看到刻度线同时移动(再次绘制组件)。

如果我直接调用 Paint()，它可以工作，但背景不会被删除。

我错过了什么？

编辑:我再次尝试并确认我是否按住鼠标，Invalidate();仅当我松开鼠标时才会考虑调用。用我下面的代码试试你自己，paint 只在发布时调用:

__fastcall TMyCustomComponent::TMyCustomComponent(TComponent* Owner)
    : TCustomTransparentControl(Owner)
{
    mValue = 0;
}

void __fastcall TMyCustomComponent::MouseDown(System::Uitypes::TMouseButton Button, System::Classes::TShiftState Shift, int X, int Y)
{
    if (Button == mbLeft)
        {
        mValueStart = 0;        
        }
}

void __fastcall TMyCustomComponent::MouseMove(System::Classes::TShiftState Shift, int X, int Y)
{       
    Invalidate();       
}

void __fastcall TMyCustomComponent::Paint(void)
{
    TGraphicControl::Paint();   
    Canvas->Font->Name = "Arial";
    Canvas->Font->Size = 8;
    Canvas->Font->Style = TFontStyles() << fsBold;
    Canvas->Font->Color = clInfoText;
    Canvas->Brush->Color = clInfoBk;
    Canvas->FillRect(TRect(0, 0, 104, 21));
    mValue++;
    Canvas->TextOut(0, 2, AnsiString(mValue));
    Canvas->Brush->Color = clBtnShadow; 
}

Answer 1

以下对我来说效果很好:

__fastcall TMyCustomComponent::TMyCustomComponent(TComponent* Owner)
    : TCustomTransparentControl(Owner)
{
    mValue = 0;
    InterceptMouse = true; // <-- needed for TCustomTransparentControl to trigger Mouse...() methods!
}

void __fastcall TMyCustomComponent::MouseDown(System::Uitypes::TMouseButton Button, System::Classes::TShiftState Shift, int X, int Y)
{
    if (Button == mbLeft)
    {
        mValue = 0;
        Invalidate();
    }
    TCustomTransparentControl::MouseDown(Button, Shift, X, Y);
}

void __fastcall TMyCustomComponent::MouseMove(System::Classes::TShiftState Shift, int X, int Y)
{
    ++mValue;
    Invalidate();
    TCustomTransparentControl::MouseMove(Shift, X, Y);
}

void __fastcall TMyCustomComponent::Paint()
{
    TCustomTransparentControl::Paint();
    Canvas->Font->Name = "Arial";
    Canvas->Font->Size = 8;
    Canvas->Font->Style = TFontStyles() << fsBold;
    Canvas->Font->Color = clInfoText;
    Canvas->Brush->Color = clInfoBk;
    Canvas->FillRect(TRect(0, 0, ClientWidth, ClientHeight));
    Canvas->TextOut(0, 2, String(mValue));
    Canvas->Brush->Color = clBtnShadow;
}

按下鼠标左键会将 mValue 重置为 0 并绘制它。在控件周围移动鼠标会增加 mValue 并绘制它，无论是否按住鼠标按钮。