1.Using instancetype instead of id in appropriate places improves type safety in your Objective-C code

@interface MyObject : NSObject+ (instancetype)factoryMethodA;+ (id)factoryMethodB;@end @implementation MyObject+ (instancetype)factoryMethodA { return [[[self class] alloc] init]; }+ (id)factoryMethodB { return [[[self class] alloc] init]; }@end void doSomething() {    NSUInteger x, y;     x = [[MyObject factoryMethodA] count]; // Return type of +factoryMethodA is taken to be "MyObject *"    y = [[MyObject factoryMethodB] count]; // Return type of +factoryMethodB is "id"}复制代码

Because of the instancetype return type of +factoryMethodA, the type of that message expression is MyObject *. Since MyObject doesn’t have a -count method, the compiler gives a warning about the x line:

main.m: ’MyObject’ may not respond to ‘count’ However, because of the id return type in +factoryMethodB, the compiler can give no warning about the y line. Since an object of type id can be any class, and since a metho