Runtime--Method Swizzling

关于OC的特性运行时，可以实现黑魔法 Method Swizzling。

实现代码举例：

+ (void)load {

    Classclz = [selfclass];

    SELoldSEL =@selector(viewDidAppear:);

    SELnewSEL =@selector(newviewffDidAppear:);

    MethodoriginalMethod =class_getInstanceMethod(clz, oldSEL);

    MethodswizzledMethod =class_getInstanceMethod(clz, newSEL);

    BOOLdidAddMethod =class_addMethod(clz,

                                       oldSEL,

                                       method_getImplementation(swizzledMethod),

                                       method_getTypeEncoding(swizzledMethod));

    if(didAddMethod) {

        class_replaceMethod(clz,

                            oldSEL,

                            method_getImplementation(originalMethod),

                            method_getTypeEncoding(originalMethod));

    }else{

        method_exchangeImplementations(originalMethod, swizzledMethod);

   }

}

- (void) newviewffDidAppear:(BOOL)animated {

    [selfnewviewDidAppear:animated];

    WRITE_PERFORMANCE

}