如何让 TGPImage 直接从流中加载图片? 回复 "datouxuke " 的问题

问题来源: http://www.cnblogs.com/del/archive/2009/01/23/1232856.html#post

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方法一:

Delphi 的 TBitmap 类可以方便地操作流, 如果能让 TGPImage 和 TBitmap 沟通起来就好了;
TGPImage 有一个子类 TGPBitmap 可以和 TBitmap 沟通... ok 了!

代码如下:

---

uses GDIPOBJ, GDIPAPI;

procedure TForm1.FormPaint(Sender: TObject);
const
  ImgPath = 'c:\temp\test.bmp';
var
  img: TGPImage;
  bit: TBitmap;
  g: TGPGraphics;
begin
  bit := TBitmap.Create;
  bit.LoadFromFile(ImgPath); {因为 TBitmap 操作流特别方便, 这里就没有演示}

  img := TGPBitmap.Create(bit.Handle, bit.Palette);

  g := TGPGraphics.Create(Canvas.Handle);
  g.DrawImage(img, 0, 0);

  g.Free;
  bit.Free;
  img.Free;
end;

方法二:

TGPImage 需要的是 IStream 而非 TStream, 直接给它一个 IStream 参数不就完了?
那怎么实现 IStream 接口呢? Classes 单元下有一个 TStreamAdapter 类就是专门实现 IStream 接口的.

---

uses GDIPOBJ, GDIPAPI;

procedure TForm1.FormPaint(Sender: TObject);
const
  ImgPath = 'c:\temp\test.bmp';
var
  img: TGPImage;
  MemStream: TMemoryStream;
  MyIStream: TStreamAdapter;
  g: TGPGraphics;
begin
  MemStream := TMemoryStream.Create;
  MemStream.LoadFromFile(ImgPath);
  MyIStream := TStreamAdapter.Create(MemStream);
  img := TGPImage.Create(MyIStream);

  g := TGPGraphics.Create(Canvas.Handle);
  g.DrawImage(img, 0, 0);

  g.Free;
  img.Free;
  MemStream.Free;
  MyIStream := nil; {注意这里}
end;

注意 TStreamAdapter 的本质还是个接口, 只能 MyIStream := nil; 而不能 MyIStream.free;
如果这样写可能会容易理解一些, 不过要多 uses 一个 ActiveX 单元:

---

uses GDIPOBJ, GDIPAPI, ActiveX;

procedure TForm1.FormPaint(Sender: TObject);
const
  ImgPath = 'c:\temp\test.bmp';
var
  img: TGPImage;
  MemStream: TMemoryStream;
  MyIStream: IStream; {这里和上面不同}
  g: TGPGraphics;
begin
  MemStream := TMemoryStream.Create;
  MemStream.LoadFromFile(ImgPath);
  MyIStream := TStreamAdapter.Create(MemStream);
  img := TGPImage.Create(MyIStream);

  g := TGPGraphics.Create(Canvas.Handle);
  g.DrawImage(img, 0, 0);

  g.Free;
  img.Free;
  MemStream.Free;
  MyIStream := nil;
end;

TStreamAdapter.Create 方法还有一个 TStreamOwnership 类型的默认参数, 默认值是 soReference;
如果改为 soOwned, 那连 MyIStream := nil; 也不需要了; 因为和 MyIStream 关联的对象会一同释放它.

---

uses GDIPOBJ, GDIPAPI;

procedure TForm1.FormPaint(Sender: TObject);
const
  ImgPath = 'c:\temp\test.bmp';
var
  img: TGPImage;
  MemStream: TMemoryStream;
  MyIStream: TStreamAdapter;
  g: TGPGraphics;
begin
  MemStream := TMemoryStream.Create;
  MemStream.LoadFromFile(ImgPath);
  MyIStream := TStreamAdapter.Create(MemStream, soOwned);
  img := TGPImage.Create(MyIStream);

  g := TGPGraphics.Create(Canvas.Handle);
  g.DrawImage(img, 0, 0);

  g.Free;
  img.Free;
  MemStream.Free;
//  MyIStream := nil; {此时如果有这句会出错}
end;