StackOverflow2013

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copied!<p>From the <a href="https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSObject_Class/Reference/Reference.html#//apple_ref/occ/instm/NSObject/performSelector:withObject:afterDelay:inModes:" rel="noreferrer">-[NSObject performSelector:withObject:afterDelay:inModes:] documentation</a>:</p> <p><strong>2013/11/01 Answer</strong></p> <p>The docs appear to be updated, as expected, now they don't say the target object is retained however they still mention an scheduled timer in the runloop.</p> <p>If an NSTimer is used then the object must be retained by somebody or there would be no guarantee the selector can be performed since no-one could assure the object would be still alive. If not using ARC's <code>weak</code> it will crash. In my opinion, Apple edited its documentation to not mention implementation details, although I think it is pretty obvious. </p> <p>Above document does NOT mention the word <em>retain</em> at all however, it does NOT mean the target is is not retained. </p> <p>This is what I tried in the debugger in the simulator iOS7.0:</p> <pre><code>(lldb) p (int)[self retainCount] (int) $1 = 8 (lldb) expr (void)[self performSelector:@selector(description) withObject:nil afterDelay:1.0] (lldb) p (int)[self retainCount] (int) $2 = 9 (lldb) expr (void)[NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(description) object:nil] (lldb) p (int)[self retainCount] (int) $3 = 8 </code></pre> <p>Conclusion: the object is retained, but it is not something we should care about. Mister solved :p</p> <hr /> <p><strong>Previous Answer:</strong> (still valid)</p> <blockquote> <p>This method retains the receiver and the anArgument parameter until after the selector is performed.</p> </blockquote> <p>Because if the object is not retained, the object might be released before it gets executed and your app will crash. </p> <p>Is the same logic when you set and try to fire a method using NSTimers. When the timer is actually fired if the target object does not exist anymore (is released) your app will crash. So <code>performSelector:withObject:afterDelay:...</code> and its friends are here to make our life a little bit easier because it makes sure the app won't crash when the timer is fired ;)</p> <p>Hope it helps</p>

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