Objective-C Generic Response Types? - objective-c

Question

I have a wrapper for an API written in Objective-C that looks roughly like so:
#interface MyAPI : NSObject
- (void)getUsers :(UserRequest*)request :(void (^)(UserResponse *response))onResponse :(void (^)(APIError *error))onError;
- (void)getPosts :(PostRequest*)request :(void (^)(PostResponse *response))onResponse :(void (^)(APIError *error))onError;
- (void)getOtherStuff :(OtherStuffRequest*)request :(void (^)(OtherStuffResponse *response))onResponse :(void (^)(APIError *error))onError;
#end
#implementation MyAPI
- (void)getUsers :(UserRequest*)request :(void (^)(UserResponse *response))onResponse :(void (^)(APIError *error))onError
{
>>> NSString *url = "/api/users";
HTTPClient *client = [[HTTPClient alloc] init];
HTTPRequest *httpRequest = [[HTTPRequest alloc] initWithURL:url];
httpRequest.queryParams = request.toDictionary;
[httpClient send:httpRequest success:^(HTTPResponse *httpResponse) {
NSError *error;
>>> UserResponse *response = [[UserResponse alloc] initWithString:httpResposne.body error:&error];
if (error == nil) {
onResponse(response);
} else {
APIError *errorResponse = [[APIError alloc] initWithError:error];
onError(errorResponse);
}
}, error:^(NSError *error) {
APIError *errorResponse = [[APIError alloc] initWithError:error];
onError(errorResponse);
}];
}
- (void)getPosts :(PostRequest*)request :(void (^)(PostResponse *response))onResponse :(void (^)(APIError *error))onError
{
>>> NSString *url = "/api/posts";
HTTPClient *client = [[HTTPClient alloc] init];
HTTPRequest *httpRequest = [[HTTPRequest alloc] initWithURL:url];
httpRequest.queryParams = request.toDictionary;
[httpClient send:httpRequest success:^(HTTPResponse *httpResponse) {
NSError *error;
>>> PostResponse *response = [[PostResponse alloc] initWithString:httpResposne.body error:&error];
if (error == nil) {
onResponse(response);
} else {
APIError *errorResponse = [[APIError alloc] initWithError:error];
onError(errorResponse);
}
}, error:^(NSError *error) {
APIError *errorResponse = [[APIError alloc] initWithError:error];
onError(errorResponse);
}];
}
- (void)getOtherStuff :(OtherStuffRequest*)request :(void (^)(OtherStuffResponse *response))onResponse :(void (^)(APIError *error))onError
{
>>> NSString *url = "/api/otherStuff";
HTTPClient *client = [[HTTPClient alloc] init];
HTTPRequest *httpRequest = [[HTTPRequest alloc] initWithURL:url];
httpRequest.queryParams = request.toDictionary;
[httpClient send:httpRequest success:^(HTTPResponse *httpResponse) {
NSError *error;
>>> OtherStuffResponse *response = [[OtherStuffResponse alloc] initWithString:httpResposne.body error:&error];
if (error == nil) {
onResponse(response);
} else {
APIError *errorResponse = [[APIError alloc] initWithError:error];
onError(errorResponse);
}
}, error:^(NSError *error) {
APIError *errorResponse = [[APIError alloc] initWithError:error];
onError(errorResponse);
}];
}
#end
I've marked with >>> the only two lines that change from one method call to another - just the URL and the response type.
Ideally to avoid duplicate code I would like to do something like this:
#interface InternalImplementation<RequestType, ResponseType> : NSObject
+ (ResponseType)getAnything :(RequestType)request fromURL:(NSString*)url;
#end
#implementation InternalImplementation
+ (void)getAnything :(RequestType)request fromURL:(NSString*)url :(void (^)(ResponseType *response))onResponse :(void (^)(APIError *error))onError
{
HTTPClient *client = [[HTTPClient alloc] init];
HTTPRequest *httpRequest = [[HTTPRequest alloc] initWithURL:url];
httpRequest.queryParams = request.toDictionary;
[httpClient send:httpRequest success:^(HTTPResponse *httpResponse) {
NSError *error;
ResponseType *response = [[ResponseType alloc] initWithString:httpResposne.body error:&error];
if (error == nil) {
onResponse(response);
} else {
APIError *errorResponse = [[APIError alloc] initWithError:error];
onError(errorResponse);
}
}, error:^(NSError *error) {
APIError *errorResponse = [[APIError alloc] initWithError:error];
onError(errorResponse);
}];
}
#end
This way I could re-write my API with almost no code duplication:
#implementation MyAPI
- (void)getUsers :(UserRequest*)request :(void (^)(UserResponse *response))onResponse :(void (^)(APIError *error))onError
{
[InternalImplementation<UserRequest, UserResponse> getAnything:request fromURL:"/api/users" onResponse:onResponse onError:onError];
}
- (void)getPosts :(PostRequest*)request :(void (^)(PostResponse *response))onResponse :(void (^)(APIError *error))onError
{
[InternalImplementation<PostRequest, PostResponse> getAnything:request fromURL:"/api/users" onResponse:onResponse onError:onError];
}
- (void)getOtherStuff :(OtherStuffRequest*)request :(void (^)(OtherStuffResponse *response))onResponse :(void (^)(APIError *error))onError
{
[InternalImplementation<OtherStuffRequest, OtherStuffResponse> getAnything:request fromURL:"/api/users" onResponse:onResponse onError:onError];
}
#end
Unfortunately I can't quite figure out how to get this working. Can it even be done?

Answer 1

There's no need for generics here. Generics in Objective-C are "lightweight." They don't really do anything. They're mostly there to help bridge to Swift. (In theory they could provide better warnings about type mismatches in ObjC, but they rarely if ever do.)
Objective-C is duck typed. If something responds to a message, then it can accept that message. It doesn't matter what the "real" type of the object is. You can add a bunch of public methods like getUsers:... in order to get good warnings if the caller does something wrong, but getAnything can just accept Request. It doesn't need to be generic to do that (I'm assuming that UserRequest is a subclass of Request and UserResponse is a subclass of Response).
The only tricky thing is that you need to pass the class so you can init it. That's no problem:
+ (void)getAnythingWithRequest:(Request *)request
fromURL:(NSString *)url
ofType:(Class)responseType
success:(void (^)(Response *response))onResponse
failure:(void (^)(APIError *error))onError
With that, assuming that type has a initWithString:, you can just call it:
Response *response = [[responseType alloc] initWithString:#"..."];
Objective-C is a highly dynamic language. As long as you make sure everything responds to what you expect it to respond to, then it'll all work. There's no reason to add generics unless you intend to bridge this to Swift.
To make things a little nicer, getUsers:... etc can require that their success handler be of a more constrained type, and you can then just bridge across to that new type and add the URL string as you like:
+ (void)getUserWithRequest:(UserRequest *)request
ofType:(Class)responseType
success:(void (^)(UserResponse *response))onResponse
failure:(void (^)(APIError *error))onError {
[self getAnythingWithRequest:request
fromURL:#"/api/users"
ofType:[UserResponse self]
success:^(Response *response){onResponse((UserResponse*)response);}
failure:onError];
}