A project that demonstrates the use of Objective-C's message passing capabilities to prevent retain cycles caused by interaction with NSTimer, NSThread or CADisplayLink instances.

Introduction

As you may already know, NSTimer, NSThread, and CADisplayLink instances retain their targets. If the target retains an instance of one of these classes as well, we have a retain cycle: neither the target nor the instance will ever be deallocated.

Why would you want to retain an instance of one of these classes? Imagine you have a certain view animation that requires the use of a CADisplayLink instance. As soon as the animation has finished, the CADisplayLink instance is not needed anymore and should be paused to prevent your app from waisting resources. In order to pause it, you will have to keep a reference to it.

At this point you might wonder: why not keep a weak reference to the instance instead of a strong one? Doesn't this solve all our problems? The answer is no, because it doesn't change the fact that the target is retained. For example, a UIViewController instance that has a weak reference to a repeating NSTimer object will never be deallocated because it is retained by the timer.

What does it do

It enables you to create NSTimer, NSThread and CADisplayLink objects without having to worry about retain cycles.

How does it work

Internally a RDRIntermediateTarget object keeps a weak reference to the actual target (your UIViewController for example). It forwards all invocations originated by the NSTimer, NSThread or CADisplayLink to the actual target.

How to use

Note that the RDRIntermediateTarget is not retained! Check out the sample project to learn more.

RDRIntermediateTarget *target = 
[RDRIntermediateTarget intermediateTargetWithTarget:self];
self.timer = [NSTimer timerWithTimeInterval:1.0f
                                     target:target
                                   selector:@selector(_timerFired:)
                                   userInfo:nil
                                    repeats:YES];

[[NSRunLoop currentRunLoop] addTimer:timer
                             forMode:NSDefaultRunLoopMode];

About the sample project

The sample project features a single UIViewController subclass called ViewController with a timer, a switch, a label and a button. The timer is repeating and acts as a counter - on every tick, an integer is increased and subsequently displayed on the label. The switch allows you to toggle between a default implementation and an implementation where RDRIntermediateTarget is used. The latter is the case when the switch is on. Clicking on the button causes the application to reset the application window's rootViewController, which is an instance of ViewController. If rootViewController is successfully deallocated, you will notice a "DEALLOC" message in the console. If there is a retain cycle, it will not deallocate and thus nothing is logged.

Inside ViewController you can change the strong keyword for the timer property to weak to see for yourself that this change does not make a difference.

Requirements

• ARC

License

The code is licensed under the MIT license. See LICENSE for more details.