iOS method swimming

+ (void)load {
    //1、单例，避免多次交互
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        
        Method oriMethod = class_getInstanceMethod(self, @selector(eat));
        Method swiMethod = class_getInstanceMethod(self, @selector(swi_eat));
        
        // 3、取得新方法swiMethod的实现和方法类型签名
        //把新方法实现放到旧方法名中，此刻调用-eat就是调用-swi_eat
        BOOL didAddMethod = class_addMethod(self, @selector(eat), method_getImplementation(swiMethod), method_getTypeEncoding(swiMethod));
        if (didAddMethod) {
            // 3.1、添加成功，当前类未实现，父类实现了-eat
            // 此时不必做方法交换，直接将swiMethod的IMP替换为父类oriMethod的IMP即可
            class_replaceMethod(self, @selector(swi_eat), method_getImplementation(oriMethod), method_getTypeEncoding(oriMethod));
        }else{
            // 3.2、正常情况，直接交换
            method_exchangeImplementations(oriMethod, swiMethod);
        }
        
    });
}