cocoa touch - Can I pass a block as a @selector with Objective-C?

Question

Is it possible to pass an Objective-C block for the @selector argument in a UIButton? i.e., Is there any way to get the following to work?

    [closeOverlayButton addTarget:self 
                           action:^ {[anotherIvarLocalToThisMethod removeFromSuperview];} 
                 forControlEvents:UIControlEventTouchUpInside];

Thanks

Answer 1

Yes, but you'd have to use a category.

Something like:

@interface UIControl (DDBlockActions)

- (void) addEventHandler:(void(^)(void))handler 
        forControlEvents:(UIControlEvents)controlEvents;

@end

The implementation would be a bit trickier:

#import <objc/runtime.h>

@interface DDBlockActionWrapper : NSObject
@property (nonatomic, copy) void (^blockAction)(void);
- (void) invokeBlock:(id)sender;
@end

@implementation DDBlockActionWrapper
@synthesize blockAction;
- (void) dealloc {
  [self setBlockAction:nil];
  [super dealloc];
}

- (void) invokeBlock:(id)sender {
  [self blockAction]();
}
@end

@implementation UIControl (DDBlockActions)

static const char * UIControlDDBlockActions = "unique";

- (void) addEventHandler:(void(^)(void))handler 
        forControlEvents:(UIControlEvents)controlEvents {

  NSMutableArray * blockActions = 
                 objc_getAssociatedObject(self, &UIControlDDBlockActions);

  if (blockActions == nil) {
    blockActions = [NSMutableArray array];
    objc_setAssociatedObject(self, &UIControlDDBlockActions, 
                                        blockActions, OBJC_ASSOCIATION_RETAIN);
  }

  DDBlockActionWrapper * target = [[DDBlockActionWrapper alloc] init];
  [target setBlockAction:handler];
  [blockActions addObject:target];

  [self addTarget:target action:@selector(invokeBlock:) forControlEvents:controlEvents];
  [target release];

}

@end

Some explanation:

1. We're using a custom "internal only" class called DDBlockActionWrapper. This is a simple class that has a block property (the block we want to get invoked), and a method that simply invokes that block.
2. The UIControl category simply instantiates one of these wrappers, gives it the block to be invoked, and then tells itself to use that wrapper and its invokeBlock: method as the target and action (as normal).
3. The UIControl category uses an associated object to store an array of DDBlockActionWrappers, because UIControl does not retain its targets. This array is to ensure that the blocks exist when they're supposed to be invoked.
4. We have to ensure that the DDBlockActionWrappers get cleaned up when the object is destroyed, so we're doing a nasty hack of swizzling out -[UIControl dealloc] with a new one that removes the associated object, and then invokes the original dealloc code. Tricky, tricky. Actually, associated objects are cleaned up automatically during deallocation.

Finally, this code was typed in the browser and has not been compiled. There are probably some things wrong with it. Your mileage may vary.