如何查看流是否包含要转换的有效位图。

How can I tell if a stream contains a picture or not? I am working with Delphi xe8 FMX developing an iOS application. I have a listbox and am loading pictures into the items. I can do this:

如何判断流是否包含图片?我正在使用Delphi xe8 FMX开发iOS应用程序。我有一个列表框,正在将图片加载到项目中。我可以做这个:

          if not Assigned(S) then
            s:=TMemoryStream.Create;
          if not Assigned(clHTTP) then
            clHTTP := TIDHTTP.Create;
          with clHTTP do
          begin
            clHTTP.HandleRedirects := True;
            clHTTP.AllowCookies := True;
            clHTTP.RedirectMaximum := 110000;
            clHTTP.Get(someimageURL,s);
          end;
          s.Seek(0,soFromBeginning);
          try
            LItem.ItemData.Bitmap.LoadFromStream(s);
          except
            clHTTP.Get(DefaultImageURL,s);
            s.Seek(0,soFromBeginning);
            LItem.ItemData.Bitmap.LoadFromStream(s);
          end;
          s.Free;
          clHTTP.Free;

I would prefer not to use a try-except block because it appears this causes loading of the bitmaps to be inconsistent. For example, I have to scroll the listbox items out of view, then back into view to see the pictures.

我宁愿不使用try-except块,因为它看起来会导致位图的加载不一致。例如,我必须将列表框项目滚动到视图之外,然后返回视图以查看图片。

1 个解决方案

#1

A quick way would be to check for the file signature. For instance, here are some common image format signatures:

一种快速方法是检查文件签名。例如,以下是一些常见的图像格式签名:

PNG: 89 50 4E 47 0D 0A 1A 0A

PNG:89 50 4E 47 0D 0A 1A 0A

JPEG: FF D8 FF E0

JPEG:FF D8 FF E0

GIF87a: 47 49 46 38 37 61

GIF87a:47 49 46 38 37 61

GIF89a: 47 49 46 38 39 61

GIF89a:47 49 46 38 39 61

A comprehensive list can be found here: http://en.wikipedia.org/wiki/List_of_file_signatures

可在此处找到完整列表:http://en.wikipedia.org/wiki/List_of_file_signatures

This approach will not prove that the rest of the stream is valid, but it is a good start and will allow you at least to reject obvious ringers.

这种方法不能证明流的其余部分是有效的,但它是一个良好的开端,并且至少可以让你拒绝明显的铃声。

#1