Reference: 鏈接

總結(jié)一個(gè)通則：在任何地方，包括屬性，參數(shù)，block，block返回值/參數(shù)，通通用_Nullable形式的關(guān)鍵字，解決所有問題。

@property (nonatomic, strong) NSString * _Nullable identifier2; //屬性
- (void)doSomethingWithString:(NSString * _Nullable)str{}; //single pointer
- (void)compute:(NSError * _Nullable * _Nullable)error{}; //double pointer
- (void)success:(id _Nullable (^ _Nullable)(NSURLRequest * _Nullable request, NSHTTPURLResponse * _Nullable response, UIImage * image))success{};