• load 方法是在初始化时调用
• 使用单利是为了只执行一遍、

+ (void)load {
    
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        Class cls = [self class];
        
        /// 想要替换的实例方法
        SEL originalSelector = @selector(init);
        /// 替换的新方法名
        SEL swizzledSelector = @selector(x_init);
        
        Method originalMethod = class_getInstanceMethod(cls, originalSelector);
        Method swizzlingMethod = class_getInstanceMethod(cls, swizzledSelector);
        
        /// 在交换方法之前要先判断将要替换的方法是否存在
        BOOL isExist = class_addMethod(cls, originalSelector, method_getImplementation(swizzlingMethod), method_getTypeEncoding(swizzlingMethod));

        /// 实现方法交换
        if (isExist) {
            class_replaceMethod(cls, swizzlingMethod, method_getImplementation(swizzlingMethod), method_getTypeEncode(swizzlingMethod));
        } else {
            method_exchangeImplementations(originMethod, swizzlingMethod);
        }
        
        
        /// 如果替换的是类方法
        Class classB = Object_getClass(self);
        
//        /// 想要替换的类方法
//        SEL originalSelector = @selector(array);
//        /// 替换的新方法名
//        SEL swizzledSelector = @selector(x_array) ;
//
//        Method originalMethod = class_getClassMethod(classB, originalSelector);
//        Method swizzlingMethod = class_getClassMethod(classB, swizzledSelector);
        
    });
    
}