document that CanRead() and GetImageCount() functions of wxImageHandlers do NOT modify the current stream position and that they require seekable streams; rename current GetImageCount() functions to DoGetImageCount() and put save-and-restore stream position logic in GetImageCount(); add comments in the various DoCanRead() and in DoGetImageCount() where the stream position is modified; remove unneeded SeekI(0) calls from DoCanRead() and DoGetImageCount() functions: they didn't allow to load images from non-seekable streams; implement forward-seeking in wxInputStream::SeekI() also for non-seekable streams

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@60852 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/animdecod.h

@@ -93,7 +93,28 @@ public:
     }
 
     virtual bool Load( wxInputStream& stream ) = 0;
-    virtual bool CanRead( wxInputStream& stream ) const = 0;
+    
+    bool CanRead( wxInputStream& stream ) const
+    {
+        // NOTE: this code is the same of wxImageHandler::CallDoCanRead
+        
+        if ( !stream.IsSeekable() )
+            return false;        // can't test unseekable stream
+
+        wxFileOffset posOld = stream.TellI();
+        bool ok = DoCanRead(stream);
+
+        // restore the old position to be able to test other formats and so on
+        if ( stream.SeekI(posOld) == wxInvalidOffset )
+        {
+            wxLogDebug(_T("Failed to rewind the stream in wxAnimationDecoder!"));
+
+            // reading would fail anyhow as we're not at the right position
+            return false;
+        }
+
+        return ok;
+    }
 
     virtual wxAnimationDecoder *Clone() const = 0;
     virtual wxAnimationType GetType() const = 0;
@@ -129,6 +150,12 @@ public:
     unsigned int GetFrameCount() const { return m_nFrames; }
 
 protected:
+    // checks the signature of the data in the given stream and returns true if it
+    // appears to be a valid animation format recognized by the animation decoder;
+    // this function should modify the stream current position without taking care
+    // of restoring it since CanRead() will do it.
+    virtual bool DoCanRead(wxInputStream& stream) const = 0;
+    
     wxSize m_szAnimation;
     unsigned int m_nFrames;