How to use a TControlCanvas in a component

iseekcode

于 2009-11-17 12:48:00 发布

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分类专栏： Graphic 文章标签： destructor constructor class

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本文介绍如何在自定义组件中使用TControlCanvas。通过创建TScrollingPaintBox类并覆盖构造函数与析构函数来实现设备上下文（DC）与窗口句柄（HWND）的绑定。

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How to use a TControlCanvas in a component

type
TScrollingPaintBox = class(TScrollingWinControl)
private
    FCanvas: TCanvas;
public
    constructor Create(aOwner: TComponent); override;
    destructor Destroy; override;
    property Canvas: TCanvas read FCanvas;
end;

constructor TScrollingPaintBox.Create(AOwner: TComponent);
begin
inherited Create(AOwner);
FCanvas := TControlCanvas.Create;
TControlCanvas(FCanvas).Control := Self;
end;

destructor TScrollingPaintBox.Destroy;
begin
FCanvas.Free;
inherited Destroy;
end;

A TControlCanvas is important because it creates a DC (device context) that belongs to the HWND of the control. Also, override is important on your constructor and destructor to ensure that they are actually called.