Cascades of delegates and hijacking delegate callbacks in Objective-C - ios

Question

Say I write a UITextField subclass and want to have control over the text written into it by the user. I would set the input field's delegate to be myself and implement -textField:shouldChangeCharactersInRange:replacementString:.
However, I would still want to allow whatever part of code uses me as a text field to implement the usual delegate methods. An approach for that would be to store a second delegate reference and map them like so:
- (id)init {
self = [super init];
super.delegate = self;
return self;
}
- (void)setDelegate:(id)delegate {
self.nextDelegate = delegate;
}
- (id)delegate {
return self.nextDelegate;
}
I would then proceed to implement all UITextFieldDelegate methods and forward them to the next delegate as I wish. Obviously, I may want to modify some parameters before passing them on to the next delegate, like in -textField:shouldChangeCharactersInRange:replacementString:.
Another problem I'm thinking of is when the user's sets nextDelegate to the text field itself (for whatever reason), resulting in an infinite loop.
Is there a more elegant way to hijack delegate callbacks like in the example code I posted?

Answer 1

The problem with your approach is the overridden delegate accessor: There's no guarantee that Apple's code always uses the delegate ivar directly and does not use the getter to access the delegate. In that case it would just call through to the nextDelegate, bypassing your sneaked in self delegate.
You might have checked that your approach works in the current implementation but this could also change in future UIKit versions.
Is there a more elegant way to hijack delegate callbacks like in the example code I posted?
No, I'm not aware of any elegant solutions. You could not override the delegate accessor and instead set up secondary delegate (to which you have to manually pass all delegate messages).
To solve the actual problem of filtering text input it might be worthwhile looking into
- (void)replaceRange:(UITextRange *)range withText:(NSString *)text;
This method is implemented by UITextField (as it adopts UITextInput) and could be overridden to filter the text argument.

Answer 2

I think you're thinking about this correctly, and the approach you outlined will work fine (I've done it).
There's no circularity issue because you shouldn't expose nextDelegate in the subclass's public interface, so no caller will have the chance to setup a cycle. (You could also test in the setter that delegate != self.
It would be better, though, if you could avoid this altogether. For example, if you just want to tweak the text field text as it changes, you can get the control event:
[self addTarget:self action:#selector(didChange:) forControlEvents:UIControlEventEditingChanged];
Then,
- (void)textFieldDidChange:(id)sender {
self.text = [self alteredText];
}
- (NSString *)alteredText {
// do whatever transform to user input you wish, like change user input 'a' to 'x'
return [self.text stringByReplacingOccurrencesOfString:#"a" withString:#"x"];
}
This will work as well, but with the odd side effect that the delegate won't see the alteredText in shouldChangeCharactersInRange:. That's fixable by making alteredText public and having the class customers call it instead of the standard getter.

Answer 3

All of the problems with subclassing can be avoided by using a different approach of intercepting delegate messages: A "delegate proxy".
The idea is to use an intermediate object (derived from NSProxy) that either responds to a delegate message or passes it along to the next delegate. It's basically what you did by subclassing the UITextField but instead of using the text field object we'll use a custom object that handles only the interception of some delegate messages.
These customized delegate proxys form a set of reusable building blocks which are simply plugged into each other to customize the behavior of any object that uses delegation.
Here's an example (code on github) of a chain of delegates:
UITextField -> TextFilterDelegate -> SomeViewController
The UITextField passes delegate messages to TextFilterDelegate which responds to textField:shouldChangeCharactersInRange:replacementString: and passes other delegate messages on to its own delegate (the view controller).