ExampleRuntimeDebugging.m

Modifying class methods at runtime can be a very powerful debugging tool. Consider the following example:

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.

This example is part of Runtime Class Modifications.

//
// Copyright 2011-2014 NimbusKit
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//

@interface UIViewController()

- (id)_retain;

@end


@implementation UIViewController()

- (id)_retain {
  // _retain has been swizzled so calling _retain here will actually call retain.
  [self _retain]; // Set a breakpoint here.
}

@end


- (BOOL)              application: (UIApplication *)application
    didFinishLaunchingWithOptions: (NSDictionary *)options {

  // Swap the default retain implementation with our custom implementation with which we can
  // visually set a breakpoint.
  NISwapInstanceMethods([UIViewController class], @selector(retain), @selector(_retain));

}