拖动 TPaintBox 同时保持其内容静态

Question

我们有一个用于显示视频的 VCL TPaintBox。它的父级是 TScrollbox。

缩放时，PaintBox 变得比滚动框大，因此部分图像被隐藏。

我们已经实现了使用鼠标在滚动框中拖动绘画框的功能，效果很好。

然而，只要移动paintBox，即更改其left 或top 属性，其内容就会被擦除。

问题是，有没有办法防止油漆盒在移动时擦除其内容？

我创建了一个简单的 VCL Forms 示例，其中滚动框内有一个绘画框，代码如下。 单击键盘时，会在颜料盒上绘制一条线。

该示例表明，如果没有持续绘画，颜料盒的内容会在移动时被擦除。问题是，如何避免，或者应对？

#ifndef Unit1H
#define Unit1H
#include <System.Classes.hpp>
#include <Vcl.Controls.hpp>
#include <Vcl.StdCtrls.hpp>
#include <Vcl.Forms.hpp>
#include <Vcl.ExtCtrls.hpp>
#include "RzButton.hpp"

class TForm1 : public TForm
{
    __published:
        TPaintBox *PB;
        TScrollBox *ScrollBox1;
        void __fastcall ObjMouseDown(TObject *Sender, TMouseButton     Button, TShiftState Shift, int X, int Y);
        void __fastcall ObjMouseMove(TObject *Sender, TShiftState Shift, int X, int Y);
        void __fastcall ObjMouseUp(TObject *Sender, TMouseButton Button,  TShiftState Shift, int X, int Y);
        void __fastcall PBPaint(TObject *Sender);
        void __fastcall FormKeyDown(TObject *Sender, WORD &Key, TShiftState Shift);

    public:
        __fastcall TForm1(TComponent* Owner);

        bool mIsDragging;
        TPoint mMouseDownLocation;
        TPoint mDragStartingLocation;
};
extern PACKAGE TForm1 *Form1;
#endif

和来源

#include <vcl.h>
#pragma hdrstop
#include "Unit1.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma link "RzButton"
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
: TForm(Owner)
{}
//---------------------------------------------------------------------------
void __fastcall TForm1::ObjMouseDown(TObject *Sender, TMouseButton Button, TShiftState Shift,
      int X, int Y)
{
    mIsDragging = true;
    mMouseDownLocation = Mouse->CursorPos;
    mDragStartingLocation = TPoint(PB->Left, PB->Top);
}

//---------------------------------------------------------------------------
void __fastcall TForm1::ObjMouseMove(TObject *Sender, TShiftState Shift,
      int X, int Y)
{
     if(mIsDragging)
     {
        PB->Left = mDragStartingLocation.X + (Mouse->CursorPos.X - mMouseDownLocation.X);
        PB->Top = mDragStartingLocation.Y  + (Mouse->CursorPos.Y - mMouseDownLocation.Y);
     }
}

//---------------------------------------------------------------------------
void __fastcall TForm1::ObjMouseUp(TObject *Sender, TMouseButton Button,
      TShiftState Shift, int X, int Y)
{
     mIsDragging = false;
}

//---------------------------------------------------------------------------
void __fastcall TForm1::PBPaint(TObject *Sender)
{
//    PB->Canvas->Pen->Color = clRed;
//    PB->Canvas->Pen->Width = 3;
//    PB->Canvas->MoveTo(50,50);
//    PB->Canvas->LineTo(150, 150);
}

//---------------------------------------------------------------------------
void __fastcall TForm1::FormKeyDown(TObject *Sender, WORD &Key, TShiftState Shift)
{
  PB->Canvas->Pen->Color = clRed;
  PB->Canvas->Pen->Width = 3;
  PB->Canvas->MoveTo(50,50);
  PB->Canvas->LineTo(150, 150);
}