**__nullable, _Nullable和nullable

Reference: 链接

总结一个通则：在任何地方，包括属性，参数，block，block返回值/参数，通通用_Nullable形式的关键字，解决所有问题。

@property (nonatomic, strong) NSString * _Nullable identifier2; //属性
- (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{};