#reference counting

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iOS 5 introduced ARC, automatic reference counting, and greatly simplified programming for Apple's mobile devices.

Prior to ARC, coders had to explicitly release and retain objects for memory to be recycled. In the event that an object is retained and never released, your app is said to leak memory. ARC is compile time logic that automatically retains and release your objects for you. The magic works via reference counting (kind of like Java). When an object no longer as any pointers pointing to it, it can be assumed that the object won't be used again and it is deallocated.

For the most part this all works as intended and hackers can focus on what they're writing instead of retaining and releasing. However, there are instances when the compiler doesn't do the right thing. These common points of trouble are:

Circular references

Passed blocks

In the case of circular references, an object cannot be released because it points to another object which points back at itself (the reference count on each object is then 1, not 0). To avoid circular references you can declare properties like so:

@property (weak, nonatomic) id delegate;

Here the reference to the delegate is a weak reference. A weak reference is ignored and not counted when the compiler considers if an object should be deallocated or not.

The reference must be weak because in this case the instantiating object (not shown) adds this view to its view hierarchy and sets this property to point to itself creating a circular reference.

That was a pretty typical example and you'll want to use weak references for most of your IB outlet connections and delegate properties.

A not so obvious use for weak references are blocks that capture self references, or any references for that matter:

- (void)loopThisBlock { [UIView animateWithDuration:0.2 animations:^{ someView.alpha = (someView.alpha + 1.0) % 2; } completion:^(BOOL finished) { [self loopThisBlock]; }]; }

This very contrived method call will animate the alpha of someView between 0.0 and 1.0. However you'll notice that a memory leak is created. Since the block loops infinitely, it captures a reference to self and the containing object is never released.

This can be solved by using an explicit weak reference:

- (void)loopThisBlock { __weak someContainerClass *weakSelf = self; [UIView animateWithDuration:0.2 animations:^{ someView.alpha = (someView.alpha + 1.0) % 2; } completion:^(BOOL finished) { [weakSelf loopThisBlock]; }]; }

Now the container object will dealloc as expected.

The previous example is a case where ARC fails to release an object. Here's an example where ARC releases a reference too early.

- (void)passMeTheSameBlock:(void (^)())completeCallback { completeCallback(); } - (void)passMeABlock:(void (^)())completeCallback { [self passMeTheSameBlock:completeCallback]; }

What do you think will happen if we call:

[self passMeABlock:^{ NSLog(@"YEEHAAWWW!!!"); }];

The block will be deallocated before it can be executed. How can we fix this? Make sure to copy the block.

- (void)passMeTheSameBlock:(void (^)())completeCallback { completeCallback(); } - (void)passMeABlock:(void (^)())completeCallback { void (^completeCallbackCopy)() = [completeCallback copy]; [self passMeTheSameBlock:completeCallback]; }