gistfile1.j

var theClass = [self class],
    dragEnterEventImplementation = class_getMethodImplementation(theClass, @selector(fileDraggingEntered:)),
    dragEnterEventCallback = function (anEvent) {
    if (![self validateDraggedFiles:anEvent.dataTransfer.files]) 
    {
        return NO;
    }
    else
    {
       anEvent.dataTransfer.dropEffect = "copy"; 
       anEvent.stopPropagation(); 
       dragEnterEventImplementation(self, nil, anEvent);
    }
},