Factor out 3 copies of identical code into wxInputStreamPeeker

Resolve the long standing "NOTE" comments about having the same code in
wxAnimationDecoder::CanRead(), wxImage::CanRead() and GetImageCount() by
extracting the common logic into a helper wxInputStreamPeeker class and
using it from all places.

This loses the possibility to log a debug message if rewinding the
stream fails, but this is probably not very valuable and the actual
error should be already logged by SeekI() itself when it fails on a
seekable stream.

No real changes.

include/wx/animdecod.h

@@ -96,24 +96,8 @@ public:
 
     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(wxT("Failed to rewind the stream in wxAnimationDecoder!"));
-
-            // reading would fail anyhow as we're not at the right position
-            return false;
-        }
-
-        return ok;
+        return wxInputStreamPeeker(stream).
+                    CallIfCanSeek(&wxAnimationDecoder::DoCanRead, this);
     }
 
     virtual wxAnimationDecoder *Clone() const = 0;

include/wx/stream.h

@@ -682,6 +682,53 @@ protected:
     wxDECLARE_NO_COPY_CLASS(wxWrapperInputStream);
 };
 
+// ----------------------------------------------------------------------------
+// wxPeekInputStream(): read some data from a stream and then rewind it back
+// ----------------------------------------------------------------------------
+
+// This semi-private class is used in wx code to read some data, using the
+// provided method of the class T, from a stream if it's seekable and then
+// rewind it back.
+class wxInputStreamPeeker
+{
+public:
+    explicit wxInputStreamPeeker(wxInputStream& stream)
+        : m_stream(stream),
+          m_ofsOrig(stream.IsSeekable() ? stream.TellI() : wxInvalidOffset)
+    {
+    }
+
+    // Call the given function if the stream is seekable, otherwise return 0.
+    template <typename R, typename T>
+    R CallIfCanSeek(R (T::*func)(wxInputStream&), T* obj)
+    {
+        return RewindAndReturn(CanSeek() ? (obj->*func)(m_stream) : R(0));
+    }
+
+    // Overload for const methods.
+    template <typename R, typename T>
+    R CallIfCanSeek(R (T::*func)(wxInputStream&) const, const T* obj)
+    {
+        return RewindAndReturn(CanSeek() ? (obj->*func)(m_stream) : R(0));
+    }
+
+private:
+    bool CanSeek() const { return m_ofsOrig != wxInvalidOffset; }
+
+    template <typename R>
+    R RewindAndReturn(R retval)
+    {
+        if ( CanSeek() && m_stream.SeekI(m_ofsOrig) == wxInvalidOffset )
+            return R(0);
+
+        return retval;
+    }
+
+    wxInputStream& m_stream;
+    const wxFileOffset m_ofsOrig;
+
+    wxDECLARE_NO_COPY_CLASS(wxInputStreamPeeker);
+};
 
 #endif // wxUSE_STREAMS
 

src/common/image.cpp

@@ -3445,25 +3445,8 @@ wxIMPLEMENT_ABSTRACT_CLASS(wxImageHandler, wxObject);
 #if wxUSE_STREAMS
 int wxImageHandler::GetImageCount( wxInputStream& stream )
 {
-    // NOTE: this code is the same of wxAnimationDecoder::CanRead and
-    //       wxImageHandler::CallDoCanRead
-
-    if ( !stream.IsSeekable() )
-        return false;        // can't test unseekable stream
-
-    wxFileOffset posOld = stream.TellI();
-    int n = DoGetImageCount(stream);
-
-    // restore the old position to be able to test other formats and so on
-    if ( stream.SeekI(posOld) == wxInvalidOffset )
-    {
-        wxLogDebug(wxT("Failed to rewind the stream in wxImageHandler!"));
-
-        // reading would fail anyhow as we're not at the right position
-        return false;
-    }
-
-    return n;
+    return wxInputStreamPeeker(stream).
+            CallIfCanSeek(&wxImageHandler::DoGetImageCount, this);
 }
 
 bool wxImageHandler::CanRead( const wxString& name )
@@ -3481,25 +3464,8 @@ bool wxImageHandler::CanRead( const wxString& name )
 
 bool wxImageHandler::CallDoCanRead(wxInputStream& stream)
 {
-    // NOTE: this code is the same of wxAnimationDecoder::CanRead and
-    //       wxImageHandler::GetImageCount
-
-    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(wxT("Failed to rewind the stream in wxImageHandler!"));
-
-        // reading would fail anyhow as we're not at the right position
-        return false;
-    }
-
-    return ok;
+    return wxInputStreamPeeker(stream)
+            .CallIfCanSeek(&wxImageHandler::DoCanRead, this);
 }
 
 #endif // wxUSE_STREAMS