Hi,

I am trying to store a PNG image in a core data store backed by an sqlite database. Since I intend to use this database on an iPhone I can't store NSImage objects directly. I wanted to use bindings and an NSValueTransformer subclass to handle the transcoding from the NSImage (obtained by an Image well on my GUI) to an NSData containing the PNG binary representation of the image. I wrote the following code for the ValueTransformer :

+ (Class)transformedValueClass
{
    return [NSImage class];
}

+ (BOOL)allowsReverseTransformation
{
    return YES;   
}

- (id)transformedValue:(id)value
{
    if (value == nil) return nil;

    return [[[NSImage alloc] initWithData:value] autorelease];
}

- (id)reverseTransformedValue:(id)value
{
    if (value == nil) return nil;

 if(![value isKindOfClass:[NSImage class]])
 {
  NSLog(@"Type mismatch. Expecting NSImage");
 }

 NSBitmapImageRep *bits = [[value representations] objectAtIndex: 0];

 NSData *data = [bits representationUsingType:NSPNGFileType properties:nil];

    return data;
}

The model has a transformable property configured with this NSValueTransformer. In Interface Builder a table column and an image well are both bound to this property and both have the proper value transformer name (an image dropped in the image well shows up in the table column). The transformer is registered and called every time an image is added or a row is reloaded (checked with NSLog() calls). The problem arises when I am trying to save the managed objects. The console output shows the error message :

[NSImage length]: unrecognized selector sent to instance 0x1004933a0

It seems like core data is using the value transformer to obtain the NSImage back from the NSData and then tries to save the NSImage instead of the NSData.

There are probably workarounds such as the one presented in this post but I would really like to understand why my approach is flawn.

Thanks in advance for your ideas and explanations.