Crash in objc_retain in method performed with performSelector - xcode

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.