指定初始化方法宏：
NS_DESIGNATED_INITIALIZER
不允许调用者乱用宏： NS_UNAVAILABLE
示例：
- (instancetype)initWithDelegate:(id<JXPagerViewDelegate>)delegate NS_DESIGNATED_INITIALIZER;
- (instancetype)init NS_UNAVAILABLE;
- (instancetype)initWithFrame:(CGRect)frame NS_UNAVAILABLE;
- (instancetype)initWithCoder:(NSCoder *)aDecoder NS_UNAVAILABLE;
这时如果调用init方法，就会编译错误 提示：
