runtime --- 方法实现交换

+ (void)load {
    //获取类方法
    Method olld = class_getClassMethod([self class], @selector(URLWithString:));
    
    Method new = class_getClassMethod([self class], @selector(AZ_URLWithString:));
    //交换两个方法的实现
    method_exchangeImplementations(olld, new);
    
}

+ (instancetype)AZ_URLWithString:(NSString *)str{
    
    NSURL *url  =[NSURL AZ_URLWithString:str];
    if (!url) {
        NSLog(@"交换方法");
    }
    return url;
}