instancetype

1. instancetype is a contextual keyword that can be used as a result type to signal that a method returns a related result type.

@interface Person
+ (instancetype)personWithName:(NSString *)name;
@end

使用id类型，当指针类型不匹配时，编译器不报错：

@interface Foo : NSObject
+ (id)aConvenienceInit;
@end

/// 编译器不报错
NSArray* subviews = [Foo aConvenienceInit].subviews; 

使用instance类型，当指针类型不匹配时，编译器报错：

@interface Foo : NSObject
+ (instancetype)aConvenienceInit;
@end

/// 编译器报错
NSArray* subviews = [Foo aConvenienceInit].subviews; 

2. instancetype, unlike id, can only be used as the result type in a method declaration. instancetype只能用于方法声明时的返回值，id的用法更广泛。

参考资料：
https://stackoverflow.com/questions/36020540/why-is-instancetype-used
https://nshipster.com/instancetype/