AFHTTPSessionManager and NSOperation queues with dependencies (AFNetworking 2) - ios

Question

On previous versions of AFNetworking I could make use of AFHTTPRequestOperation to create multiple requests, create dependencies between them and enqueue them pretty easily. Example (inside of an AFHTTPClient subclass):
NSURLRequest *categoriesRequest = [self requestWithMethod:#"GET" path:#"categories" parameters:nil];
AFHTTPRequestOperation *categoriesOperation = [self HTTPRequestOperationWithRequest:categoriesRequest success:^(AFHTTPRequestOperation *operation, id responseObject) {
NSArray *jsonCategories = responseObject;
for (NSDictionary *jsonCategory in jsonCategories) {
SPOCategory *category = [[SPOCategory alloc] initWithDictionary:jsonCategory];
[self.categories addObject:category];
}
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
// …
}];
NSURLRequest *incidencesRequest = [self requestWithMethod:#"GET" path:#"incidences" parameters:nil];
AFHTTPRequestOperation *incidencesOperation = [self HTTPRequestOperationWithRequest:incidencesRequest success:^(AFHTTPRequestOperation *operation, id responseObject) {
NSArray *jsonIncidences = responseObject;
for (NSDictionary *jsonIncidence in jsonIncidences) {
SPOIncidence *incidence = [[SPOIncidence alloc] initWithDictionary:jsonIncidence];
[self.incidences addObject:incidence];
}
completionBlock(self.incidences, self.categories, nil);
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
// …
}];
[incidencesOperation addDependency:categoriesOperation];
[self enqueueBatchOfHTTPRequestOperations:#[categoriesOperation, incidencesOperation] progressBlock:^(NSUInteger numberOfFinishedOperations, NSUInteger totalNumberOfOperations) {
// Processing…
} completionBlock:^(NSArray *operations) {
// Completed
}];
I know I can continue to make use of AFHTTPRequestOperation but, I'd like to know if there is a similar way to achieve the same thing inside a subclass of AFHTTPSessionManager, using NSURLSession as the backing library instead of NSURLConnection.
Thank you!

Answer 1

AFHTTPSessionManager's connection factory methods create connections which will be represented by a NSURLSessionDataTask object.
Unlike AFHTTPRequestOperation these are not NSOperation subclasses, and thus declaring dependencies is not possible.
One could imagine to wrap a factory method like
- (NSURLSessionDataTask *)GET:(NSString *)URLString
parameters:(NSDictionary *)parameters
success:(void (^)(NSURLSessionDataTask *task, id responseObject))success
failure:(void (^)(NSURLSessionDataTask *task, NSError *error))failure;
into a helper method/function which returns a NSOperation object. That might (will) become cumbersome and looks quite weird, though.
If you are courageous enough to consider another third party library, you can solve your problem as explained below:
The idea is to represent the eventual result of the asynchronous operation by a "Promise". Think of a Promise as a placeholder of the result, which will eventually be set by the operation. So, basically you wrap a factory method into one which then effectively yields a method having this signature:
-(Promise*) fetchCategories;
or
-(Promise*) fetchCategoriesWithParameters:(NSDictionary*)parameters;
Notice that above methods are asynchronous - yet they have no completion handler. The Promise will instead provide this facility.
Initially, when fetchCategories returns, the promise object does not "contain" the result.
You obtain (at some tme later) the eventual result respectively and error by "registering" a completion handler block respectively an error handler block with a then property like so (pseudo code):
[self.fetchCategoriesWithParameters].then(
<success handler block>,
<failure handler block> );
A more complete code snippet:
Promise* categoriesPromise = [self fetchCategories];
categoriesPromise.then(^id(id result){
self.categories = result;
... // (e.g, dispatch on main thread and reload table view)
return nil;
}, ^id(NSError* error){
NSLog(#"Error: %#", error);
return nil;
});
Note: The parameter result of the success handler block is the eventual result of the operation, aka the responseObject.
Now, in order to "chain" multiple asynchronous operations (including the handlers), you can do this:
self.categoriesPromise = [self fetchCategories];
Promise* finalResult = self.categoriesPromise.then(^id(id result){
NSArray *jsonCategories = result;
for (NSDictionary *jsonCategory in jsonCategories) {
SPOCategory *category = [[SPOCategory alloc] initWithDictionary:jsonCategory];
[self.categories addObject:category];
}
return [self fetchIncidencesWithParams:result);
}, nil)
.then(^id(id result){
NSArray *jsonIncidences = result;
for (NSDictionary *jsonIncidence in jsonIncidences) {
SPOIncidence *incidence =
[[SPOIncidence alloc] initWithDictionary:jsonIncidence];
[self.incidences addObject:incidence];
}
return #[self.incidences, self.categories];
}, nil)
.then(^id(id result){
NSArray* incidences = result[0];
NSArray* categories = result[1];
...
return nil;
}, nil /* error handler block */);
You create and "resolve" (that is, setting the result) a Promise as shown below:
- (Promise*) fetchCategories {
Promise* promise = [[Promise alloc] init];
NSURLRequest *categoriesRequest = [self requestWithMethod:#"GET" path:#"categories" parameters:nil];
AFHTTPRequestOperation *categoriesOperation = [self HTTPRequestOperationWithRequest:categoriesRequest success:^(AFHTTPRequestOperation *operation, id responseObject) {
[promise fulfillWithResult:responseObject];
}
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
[promise rejectWithReason:error];
}];
return promise;
}
Disclaimer:
There are a few third party Objective-C libraries which implement a Promise in this or a similar way. I'm the author of RXPromise which implements a promise according the Promises/A+ specification.