Retaining repeating NSTimer for later access?

Question

I am creating an NSTimer in the createTimer method that I want to refer back to in the later cancelTimer method. To facilitate this I am taking ownersh

Answer 1

You release before you call invalidate. That means by the time you call invalidate, you've already relinquished ownership of the timer. In practice, you end up calling invalidate on a deallocated timer instance.

What you should do is call invalidate before you call release. Since you are using a retained property, you can just set the property to nil:

// Schedule the timer.
self.walkTimer = [NSTimer scheduledTimerWith...];

// Cancel the timer.
[self.walkTimer invalidate];
self.walkTimer = nil;

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Update to clear up any confusion regarding memory management

It's important to keep in mind the Memory Management Rules of Objective-C — you own an object if you call alloc, copy or retain on it, and if you own an object, you have to eventually call release. In this case, setWalkTimer: retains the timer because the property is declared as retain — that means you own the timer and must call release on it down the road. The invalidate method does not count as relinquishing ownership of the timer.

When you schedule a timer, the run loop retains it, and when the timer fires or is invalidated, the run loop releases it. But really, you don't need to know that — it's an implementation detail. The call to release by invalidate is only to balance the retain when the timer was scheduled on the run loop.