Objective-C 奇巧淫技--IMS

奇技淫巧 指过于奇巧而无益的技艺与制品.
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], a
2015-03-14 23:53:39.833 testruntime[2196:629365] -[man run], b
2015-03-14 23:53:39.833 testruntime[2196:629365] -[man handsdown], a
2015-03-14 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);
}