iOS 实例方法混淆

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IMS指的是 Instance Method Swizzling, 实例方法混淆.
下段代码是一个Instance Method Swizzling和一个Method Swizzling的例子:

// Man.m- (void)run{    NSLog(@"%s, %@", __func__, _name);}- (void)jump{    NSLog(@"%s, %@", __func__, _name);}- (void)handsUp{    NSLog(@"%s, %@", __func__, _name);}- (void)handsDown{    NSLog(@"%s, %@", __func__, _name);}//  ViewController.m- (void)viewDidLoad {    ...    Man *a = [Man manWithName:@"a"];    Man *b = [Man manWithName:@"b"];    [self swizzleInstanceMethodWithInstance:a originalSel:@selector(run) replacementSel:@selector(jump)];    [self swizzleInstanceMethodWithClass:[Man class] originalSel:@selector(handsUp) replacementSel:@selector(handsDown)];    [a run];    [b run];    [a handsUp];    [b handsUp];}// 输出的结果是2015-03-14 23:53:39.832 testRuntime[2196:629365] -[Man jump], a2015-03-14 23:53:39.833 testRuntime[2196:629365] -[Man run], b2015-03-14 23:53:39.833 testRuntime[2196:629365] -[Man handsDown], a2015-03-14 23:53:39.833 testRuntime[2196:629365] -[Man handsDown], b

为什么run方法是只有对象a被替换了,handsUp方法是都被替换了呢?
我们先来看下普通的Method Swizzling是如何实现的

- (void)swizzleInstanceMethodWithClass:(Class)clazz originalSel:(SEL)original replacementSel:(SEL)replacement{    Method a = class_getInstanceMethod(clazz, original);    Method b = class_getInstanceMethod(clazz, replacement);    // class_addMethod 为该类增加一个新方法    if (class_addMethod(clazz, original, method_getImplementation(b), method_getTypeEncoding(b)))    {        // 替换类方法的实现指针        class_replaceMethod(clazz, replacement, method_getImplementation(a), method_getTypeEncoding(a));    }    else    {        // 交换2个方法的实现指针        method_exchangeImplementations(a, b);    }}

Instance Method Swizzling是用了类似KVO的办法.
先动态添加一个类MySubclass继承自原来的类,然后修改对象a的isa为新类,再替换掉新类的方法.

- (void)swizzleInstanceMethodWithInstance:(id)object originalSel:(SEL)original replacementSel:(SEL)replacement{    Class newClass = objc_allocateClassPair([object class], "MySubclass", 0);    objc_registerClassPair(newClass);    Method a = class_getInstanceMethod(newClass, original);    Method b = class_getInstanceMethod([object class], replacement);    if (class_addMethod(newClass, original, method_getImplementation(b), method_getTypeEncoding(b)))    {        class_replaceMethod(newClass, replacement, method_getImplementation(a), method_getTypeEncoding(a));    }    else    {        method_exchangeImplementations(a, b);    }    object_setClass(object, newClass);}