废弃掉本身的初始化方法

NS_UNAVAILABLE

使用自定义的初始化方法

NS_DESIGNATED_INITIALIZER

例子

```

- (instancetype)initCustom:(BOOL)custom NS_DESIGNATED_INITIALIZER;

- (instancetype)initWithFrame:(CGRect)frame NS_UNAVAILABLE;

- (instancetype)initWithCoder:(NSCoder *)coder NS_UNAVAILABLE;

- (instancetype)init NS_UNAVAILABLE;

```