looping

在类实例化后指定类型。初始化时尽量用instancetype代替id。

参考：

• https://nshipster.com/instancetype/
  [[NSArray array] mediaPlaybackAllowsAirPlay]; // 这句也会报错了

• https://stackoverflow.com/questions/8972221/would-it-be-beneficial-to-begin-using-instancetype-instead-of-id

• https://www.jonmsterling.com/posts/2012-02-05-typed-collections-with-self-types-in-objective-c.html

扩展：类簇（Class Clusters）、类属编程（Generic programming）｜https://www.generic-programming.org/

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[NSString string];

[NSArray array];

[UIView new];

[UIView alloc];

[[UIView alloc] init]; // 为什么只有这句报 ⚠ ‘Expression result unused’