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<div class="title">ExampleRuntimeDebugging.m</div>  </div>
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<p>Modifying class methods at runtime can be a very powerful debugging tool. Consider the following example:</p>
<p>You are finding that you have a memory leak with one of your view controllers, but you're not sure where the extra retain is being made. If you swap the retain implementation on UIViewController with your own method then you can set a breakpoint in the method and look at the stack each time retain is called. This method is similar to setting a breakpoint in GDB on the UIViewController's retain method but allows programmatic control over when you want to hit the breakpoint.</p>
<p>This example is part of <a class="el" href="Runtime-Class-Modifications.html">Runtime Class Modifications</a>.</p>
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<div class="line"><span class="keyword">@interface </span>UIViewController()</div>
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<div class="line">- (id)_retain;</div>
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<div class="line"><span class="keyword">@implementation </span>UIViewController()</div>
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<div class="line">- (id)_retain {</div>
<div class="line">  <span class="comment">// _retain has been swizzled so calling _retain here will actually call retain.</span></div>
<div class="line">  [<span class="keyword">self</span> _retain]; <span class="comment">// Set a breakpoint here.</span></div>
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<div class="line">    didFinishLaunchingWithOptions: (NSDictionary *)options {</div>
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<div class="line">  <span class="comment">// Swap the default retain implementation with our custom implementation with which we can</span></div>
<div class="line">  <span class="comment">// visually set a breakpoint.</span></div>
<div class="line">  <a name="a0"></a><a class="code" href="Runtime-Class-Modifications.html#gNISwapInstanceMethods">NISwapInstanceMethods</a>([UIViewController <span class="keyword">class</span>], <span class="keyword">@selector</span>(retain), <span class="keyword">@selector</span>(_retain));</div>
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