runtime 使用情况（一）

动态运行时添加方法，可能大家都知道，但是何时使用，可能不是很清楚。其中一种运用情况就是支持新旧两种API。

- (void)doSomething {
    // legacy code goes here...
}

- (void)doSomethingWithNewAPI {
    // do the same thing, but use new API.
｝

+ (void)initialise {

    //check self
    if (self != [MyObject class]) 
        return;

    //check if support newAPI
    if ([[SomeSystemClass instancesResponsedToSelector:@selector(theNewAPI)] == NO)]
    return;

    Method legacy = class_getInstanceMethod(self, @selector(doSomething));
    Method newAPI = class_getInstanceMethod(self, @selector(doSomethingWithNewAPI));

    //change two methods
    method_exchangeImplementations(legacy, newAPI);
}