Is there a way to retrieve every responder that has handled a UITouch?

Question

I'm trying to debug some touchesBegan/Moved/Ended related slowdown in my game; I think that some of my touch responders are not unloading properly, and so as the game runs on more and more of them stack up and the touches get less responsive because they have to pass through a larger and larger responder chain.

Is there some way to view/retrieve the path taken by a UITouch as it moves through the chain? Or simply some way to retrieve a list of all active responders?

Thanks, -S

Answer 1

You can hijack the desired methods on UIResponder to add logging and then call the original method. Here's an example:

#import <objc/runtime.h>

@interface UIResponder (MYHijack)
+ (void)hijack;
@end

@implementation UIResponder (MYHijack)
+ (void)hijackSelector:(SEL)originalSelector withSelector:(SEL)newSelector
{
    Class class = [UIResponder class];
    Method originalMethod = class_getInstanceMethod(class, originalSelector);
    Method categoryMethod = class_getInstanceMethod(class, newSelector);
    method_exchangeImplementations(originalMethod, categoryMethod);
}

+ (void)hijack
{
    [self hijackSelector:@selector(touchesBegan:withEvent:) withSelector:@selector(MYHijack_touchesBegan:withEvent:)];
}

- (void)MYHijack_touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    NSLog(@"touches!");
    [self MYHijack_touchesBegan:touches withEvent:event]; // Calls the original version of this method
}
@end

Then somewhere in your app (I sometimes put it in main() itself), just call [UIResponder hijack]. As long as the UIResponder subclass calls super at some point, your code will be injected.

method_exchangeImplementations() is a beautiful thing. Be careful with it of course; it's great for debugging, but very confusing if used indiscriminately.