Implementing drag and drop in NSTableView

Can anyone help me to implement drag and drop in an NSTableView? I used this code below, but these methods are not getting called during execution.

- (BOOL)tableView:(NSTableView *)tv writeRowsWithIndexes:(NSIndexSet *)rowIndexes toPasteboard:(NSPasteboard*)pboard
 {
     // Copy the row numbers to the pasteboard.
     NSData *data = [NSKeyedArchiver archivedDataWithRootObject:rowIndexes];
     [pboard declareTypes:[NSArray arrayWithObject:@".gif"] owner:self];
     [pboard setData:data forType:@".gif"];
     return YES;
 }
 
 - (NSDragOperation)tableView:(NSTableView*)tv validateDrop:(id <NSDraggingInfo>)info proposedRow:(NSInteger)row proposedDropOperation:(NSTableViewDropOperation)op
 {
     // Add code here to validate the drop
 
     if (row > [ m_imageArray count])
         return NSDragOperationNone;
 
     if (nil == [info draggingSource]) // From other application
     {
         return NSDragOperationNone;
     }
     else if (self == [info draggingSource]) // From self
     {
         return NSDragOperationNone;
     }
     else // From other documents 
     {
         [tv setDropRow: row dropOperation: NSTableViewDropAbove];
         return NSDragOperationCopy;
     }
 
     NSLog(@"validate Drop");
     return NSDragOperationCopy;
 }
 
 
 - (BOOL)tableView:(NSTableView *)aTableView acceptDrop:(id <NSDraggingInfo>)info
               row:(NSInteger)row dropOperation:(NSTableViewDropOperation)operation
 {
     NSPasteboard* pboard = [info draggingPasteboard];
     NSData* rowData = [pboard dataForType:@".gif"];
     NSIndexSet* rowIndexes = [NSKeyedUnarchiver unarchiveObjectWithData:rowData];
     NSInteger dragRow = [rowIndexes firstIndex];
 
     // Move the specified row to its new location...
 }
 

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