Generic UITableViewDelegate

Question

I am writing a subclass of UITableView and want my subclass to process some of the UITableViewDelegate methods before passing them along with the “real” delegate, as well as redirect all UITableViewDelegate methods not implemented by my subclass.

In a subclass, I have a private property:

@property (nonatomic, assign) id <UITableViewDelegate> trueDelegate; 

which contains the "real delegate" to which all unrealized methods should pass. In both my init methods, I set

 self.delegate = self; 

and I override - (void) setDelegate: (id) like this

 -(void)setDelegate:(id<UITableViewDelegate>)delegate { if (delegate != self) { _trueDelegate = delegate; } else { [super setDelegate:self]; } } 

I then redefine them to handle message forwarding

 -(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector { NSMethodSignature *sig; sig = [[self.delegate class] instanceMethodSignatureForSelector:aSelector]; if (sig == nil) { sig = [NSMethodSignature signatureWithObjCTypes:"@^v^c"]; } return sig; } - (void)forwardInvocation:(NSInvocation *)anInvocation { SEL selector = anInvocation.selector; if ([self respondsToSelector:selector]) { [anInvocation invokeWithTarget:self]; } else { [anInvocation invokeWithTarget:_trueDelegate]; } } 

The problem is that unrealized delegate methods are never called in the table view, so they are not given the opportunity to be redirected to the _trueDelegate object.

I tried checking them out here:

 - (BOOL)respondsToSelector:(SEL)aSelector { } 

but this method is never called for UITableViewDelegate methods, although it is well versed in other methods.

Answer 1

For performance, the UITableView checks and remembers which delegation methods are available as soon as the delegate is installed. First you set the delegate to self , then trueDelegate . Therefore, at the moment the delegate is set to a UITableView , trueDelegate is nil , and therefore -respondsToSelector: this always returns NO .

To fix this, set the delegate after trueDelegate . In addition, you can simplify the forwarding code. Delete all the code you specified above, except for the property, and replace it with:

 - (void)setDelegate:(id <UITableViewDelegate>)delegate { if (delegate == self) return; self.trueDelegate = delegate; [super setDelegate:self]; } - (BOOL)respondsToSelector:(SEL)aSelector { if ([super respondsToSelector:aSelector]) return YES; return [self.trueDelegate respondsToSelector:aSelector]; } - (id)forwardingTargetForSelector:(SEL)aSelector { return self.trueDelegate; }