Reference: 鏈接
總結一個通則：在任何地方，包括屬性湿酸，參數(shù)冶匹，block赏参，block返回值/參數(shù)求妹，通通用_Nullable形式的關鍵字揩懒，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{};
