iOS开发那些事儿（四）the dark arts of the Objective-C runtime

一."Black Magic"：Method Swizzling 利用 Runtime 特性把一个方法的实现与另一个方法的实现进行替换，也可以用runtime的四维理解——修改Dispatch Table让一个方法的IMP对应到我们指定的IMP上去

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二.实例说明：比如我们想要在APP中记录每一个ViewController的出现次数

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三.实例分析：

1. 第一种思路就是在ViewController出现的一瞬间（viewDidAppear）我就用记录工具记录一条日志。

    @implementation MyViewController ()
   
    - (void)viewDidAppear:(BOOL)animated
    {
        [super viewDidAppear:animated];
   
        // Custom code 
   
        // Logging
        [Logging logWithEventName:@“my view did appear”];
    }
   
    - (void)myButtonClicked:(id)sender
    {
        // Custom code 
   
        // Logging
        [Logging logWithEventName:@“my button clicked”];
    }

   暴力方法

   *缺点就在于，我要记录所有的VC那么我就要在所有VC中去手写这些代码？那聪明的你一定会想到Catagory。但是仔细想一想你需要继承UIViewController、UITableViewController、UICollectionViewController所有这些 VC添加类别，而且违背了苹果设计Catagory的目的（为了想类添加一些代码而不是替换）

2. 第二种思路就是我们今天的主角method swizzling

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三.Demo：

 #import <objc/runtime.h>

 @implementation UIViewController (Tracking)

 + (void)load {
     //保证只替换一次
     static dispatch_once_t onceToken;
     dispatch_once(&onceToken, ^{
         Class class = [self class];
         SEL originalSelector = @selector(viewWillAppear:);
         SEL swizzledSelector = @selector(xxx_viewWillAppear:);

         Method originalMethod = class_getInstanceMethod(class, originalSelector);
         Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

         BOOL didAddMethod =
             class_addMethod(class,
                 originalSelector,
                 method_getImplementation(swizzledMethod),
                 method_getTypeEncoding(swizzledMethod));

         if (didAddMethod) {
             class_replaceMethod(class,
                 swizzledSelector,
                 method_getImplementation(originalMethod),
                 method_getTypeEncoding(originalMethod));
         } else {
             method_exchangeImplementations(originalMethod, swizzledMethod);
         }
     });
 }

 #pragma mark - Method Swizzling

 - (void)xxx_viewWillAppear:(BOOL)animated {
     //这里需要解释一下，因为我们在ViewwillAppear中需要去call父类的实现。这个这行代码调用的时候xxx_viewWillAppear与viewWillAppear方法已经替换。所以不是表面看上去的递归形式
     [self xxx_viewWillAppear:animated];

      [Logging logWithEventName:NSStringFromClass([self class])];
 }

 @end        

为UIViewController添加分类

　　解释1.这里唯一可能需要解释的是 class_addMethod 。要先尝试添加原 selector 是为了做一层保护，因为如果这个类没有实现 originalSelector ，但其父类实现了，那 class_getInstanceMethod 会返回父类的方法。这样 method_exchangeImplementations 替换的是父类的那个方法，这当然不是你想要的。所以我们先尝试添加 orginalSelector ，如果已经存在，再用 method_exchangeImplementations 把原方法的实现跟新的方法实现给交换掉

　　解释2：Swizzling总是在+load中执行，Because method swizzling affects global state, it is important to minimize the possibility of race conditions. +load is guaranteed to be loaded during class initialization, which provides a modicum of consistency for changing system-wide behavior. By contrast, +initialize provides no such guarantee of when it will be executed—in fact, it may never be called, if that class is never messaged directly by the app.da大概意思是因为方+load是保证是在类初始化加载，从而改变系统行为提供了些许的一致性。相比之下，没有提供这样的保证+init()，如果APP没有直接调用，可能永远不会执行。

　　

 void (gOriginalViewDidAppear)(id, SEL, BOOL);

 void newViewDidAppear(UIViewController *self, SEL _cmd, BOOL animated)
 {
     // call original implementation
     gOriginalViewDidAppear(self, _cmd, animated);

     // Logging
     [Logging logWithEventName:NSStringFromClass([self class])];
 }

 + (void)load
 {
     Method originalMethod = class_getInstanceMethod(self, @selector(viewDidAppear:));
     gOriginalViewDidAppear = (void *)method_getImplementation(originalMethod);

     if(!class_addMethod(self, @selector(viewDidAppear:), (IMP) newViewDidAppear, method_getTypeEncoding(originalMethod))) {
         method_setImplementation(originalMethod, (IMP) newViewDidAppear);
     }
 }

直接取代老方法而不是替换

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四.总结：What are the Dangers of Method Swizzling in Objective C?

• Method swizzling is not atomic
• Changes behavior of un-owned code
• Possible naming conflicts
• Swizzling changes the method's arguments
• The order of swizzles matters
• Difficult to understand (looks recursive)
• Difficult to debug