Adding context to a UI control or NSObject

Question

It's great to be able to addTarget on a UIButton. I only wish there was some sneaky way I could attach state to the UIButton so that when the target method is invoked, I could magically pull that state (any id) from the sender.

Something like:

[button shoveMyObjectInThere:foo];
[button addTarget:self action:@selector(touchyTouchy:) forControlEvents:UIControlEventTouchUpInside];

Followed by:

-(void) touchyTouchy:(id) sender {
    UIButton button = (UIButton*)sender;
    id foo = [button getByObjectBack];
    // do something interesting with foo
}

Would be great if UIButton had an 'id context' property where developers could shove stuff, but that doesn't seem to be the case. Objective-C is a very dynamic language though, so I wonder if there is some sneaky way I can add method or fields to an object at runtime?

Wow I LOLed when I saw that selector name!!! :) – Jacob Relkin Jan 06 '11 at 00:17 — Jacob Relkin, Jan 06 '11 at 00:17

score 5 · Accepted Answer · edited May 18 '12 at 16:57

5

You could try making an associative reference

#import <objc/runtime.h>

objc_setAssociatedObject(id object, void *key, id value, objc_AssociationPolicy policy); 
objc_getAssociatedObject(id object, void *key);

edited May 18 '12 at 16:57

Berik

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answered Jan 10 '11 at 17:05

Rich

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score 0 · Answer 2 · answered Jan 06 '11 at 00:22

0

What about something like setValue:forKey:, a part of the Key-Value coding feature of Objective-C?

answered Jan 06 '11 at 00:22

fbrereto

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Gave that a whirl and docs and my experimentation seem to indicate that is only for setting existing fields/accessors. – David Blevins Jan 06 '11 at 01:28
Doesn't seem possible to add new member data using KVC. See [Why is my object not key value coding-compliant?](http://stackoverflow.com/q/11491957/1804403) – Skotch May 30 '13 at 21:06

score 0 · Answer 3 · answered Jan 06 '11 at 00:34

So I just did a quick test and found some interesting results.

UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[button setTitle:@"Hello" forState:UIControlStateNormal];
[button addTarget:self action:@selector(buttonClicked:) forControlEvents:UIControlEventTouchUpInside];
button.frame = CGRectMake(100, 100, 100, 100);
[self.window addSubview:button];

// ...

- (void)buttonClicked:(id)sender {
    NSLog(@"button clicked %@", [sender class]);
}

This prints: button clicked UIRoundedRectButton

So it seems like this should be possible... Truth be told, I ran into some problems subclassing UIButton to get the full example working but this seems promising. :)

This answer does not appear to be related to the question. You have not added any context to the UIButton that can be pulled out in `buttonClicked:` — Skotch, May 30 '13 at 21:08

score 0 · Answer 4 · answered Dec 06 '17 at 06:14

The official solution is to use the "tag" property:

[self.someMutableArray addObject:foo];
button.tag = self.someMutableArray.count - 1;

[button addTarget:self action:@selector(touchyTouchy:) forControlEvents:UIControlEventTouchUpInside];

Then:

-(void) touchyTouchy:(id) sender {
    UIButton button = (UIButton*)sender;
    id foo = self.someMutableArray[button.tag];
    // do something interesting with foo
}

In most situations you'd use an enum or constant for the tag, but an array is obviously more flexible.

Adding context to a UI control or NSObject

4 Answers4