协议子类化和一致性

Question

Given

@protocol Response <NSObject>
@end

@protocol DataResponse <Response>
@end

@interface ListUsersResponse : NSObject <DataResponse>
@end

@interface RequestExecutor : NSObject
+(id<Response>) execute: (id<Request>) request receptor: (Class) model;
@end

ListUsersRequest * request = [self buildListUsersRequest];
ListUsersResponse * result = [RequestExecutor execute:request receptor:[ListUsersResponse class]];

I get an Initializing 'ListUsersResponse *__strong' with an expression of incompatible type 'id<Response>' error. Why is that? Cant the compiler detect protocol conformance? How to solve it?

Answer 1

These warning is because your RequestExecutor 's execute: method returns an object of a general id<Response> type. The variables, however, are declared as ListUsersResponse * so the compiler expects a more specific type (and it isn't sure if that type cast is correct or not, it has no way of knowing).

---

You can get rid of the warning by either:

a) Declaring your variables with id<Response> type instead of ListUsersRequest * type like:

id<Response> result = [RequestExecutor execute:request receptor:[ListUsersResponse class]];

Or

b) Casting them on the fly (if you are sure that at that point they will be of the appropriate class):

ListUsersResponse *result = (ListUsersRequest *)[RequestExecutor execute:request receptor:[ListUsersResponse class]];