Question

I have this strange crash relating to ARC auto-inserting objc_retains in my code.

I have the following two classes:

@interface MenuItem : NSObject
@property (weak, nonatomic) id target;
@property (unsafe_unretained, nonatomic) SEL action;
@property (strong, nonatomic) id object;
- (instancetype)initWIthTarget:(id)target action:(SEL)action withObject:(id)object;
- (void)performAction;
@end

@implementation MenuItem 
- (void)performAction
{
    if (self.target && self.action)
    {
      if (self.object)
      {
        [self.target performSelector:self.action withObject:self.object];
      }
      else
      {
        [self.target performSelector:self.action];
      }
    }
}
@end

@interface Widget : NSObject
- (void)someMethod:(id)sender;
@end

At some point I instantiate a MenuItem as such:

MenuItem *item = [MenuItem alloc] initWithTarget:widget action:@selector(someMethod:) object:nil];

Then elsewhere I invoke performAction on the menu item:

 [item performAction];

In the implementation of someMethod I get a crash:

@implementation Widget
- (void)someMethod:(id)sender
{
  // EXEC_BAD_ACCESS crash in objc_retain
}
@end

Why is this happening?

Answer 1

The reason for the crash was because I was using the wrong performSelector.

NSObject defines multiple versions of performSelector. The one I was invoking was:

- (id)performSelector:(SEL)aSelector;

However the method I was invoking took an id parameter. Eg:

- (void)someMethod:(id)sender;

Now ARC being the nice safe memory management system that it is tries to ensure that parameters are properly retained during the execution of a method. So even though my someMethod: was empty ARC was producing code that looked like this:

- (void)someMethod:(id)sender 
{
    objc_retain(sender);
    objc_release(sender);
}

The problem with this however was that I was invoking performSelector: and not supplying a value for the sender parameter. So sender was pointing at random junk on the stack. Therefore when objc_retain() was invoked the app crashed.

If I change:

MenuItem *item = [[MenuItem alloc] initWithTarget:widget 
                                          action:@selector(someMethod:) 
                                          object:nil];

to

MenuItem *item = [[MenuItem alloc] initWithTarget:widget 
                                          action:@selector(someMethod) 
                                          object:nil];

and

- (void)someMethod:(id)sender;

to

- (void)someMethod;

Then the crash goes away.

Similarly I can also change

[self.target performSelector:self.action];

to

[self.target performSelector:self.action withObject:nil];

if I want to follow the ‘standard’ form of target-action methods that take a single parameter. The benefit of the second form of performSelector is that if I’m invoking a method that doesn’t take a parameter it will still work fine.