AFNetworking background JSON parsing avoid block nesting - afnetworking

Question

I have some networking code with heavy JSON parsing going on. It needs to be done in the background to not block the main thread. The code looks like this :
-(void) getSomeDataWithParameters:(...)parameters completion:(void (^)(NSArray *data))completion
{
NSURLRequest *req = ...
AFJSONRequestOperation *op = [[AFJSONRequestOperation alloc] initWithRequest:req];
// sometimes I have more requests
// startOperations is a wrapper on AFHTTPClient enqueueBatchOfHTTPRequestOperations:progressBlock:completionBlock:
// that handles errors and loading views
[self startOperations:#[op] completionBlock:^(NSArray *operations) {
// getBgQueue = return dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(getBgQueue(), ^{
NSArray *data = [MyParserClass parseJSON:op.responseJSON inContext:self.localContext];
[self.localContext MR_saveToPersistentStoreWithCompletion:^(BOOL success, NSError *error) {
// this is executed on main thread
if(completion) completion(...);
}];
});
}];
}
(AFNetworking 1.x)
The above code works very fine, but it's a pain to setup and write. And often the whole method content is wrapped inside another block to fetch some required data first... basically the blocks just pile up and makes ugly code
I'm using enqueueBatchOfHTTPRequestOperations and not individual completion blocks on AFJSONRequestOperation because batch completion block would sometimes fire before all individual operations completion blocks... (I also read somewhere that Mattt discouraged doing this)
Any pointers on how to do better than this?

Answer 1

I'm not sure what you want here, but just like "longcat is long", it's somewhat inherent in the pattern: 'continuation-passing style is continuation-passing style'. If you want to flatten things out a bit, you could make local block variables, but to a certain degree, you're stuck because you need the completion for -MR_saveToPersistentStoreWithCompletion to close over data in order to pass it to the -getSomeDataWithParameters... completion, but data won't exist until the -startOperations completion is executed.
You could probably achieve a less-nested appearance by using a bunch of __block variables, and splitting the code into several local blocks, but to me that feels kind of like cutting off your nose to spite your face. This code is readily understandable the way it is.
By the way... I notice that you're closing over op in the -startOperations completion block. This is fine because you're enqueuing op by doing -startOperations: #[op] ... but it would arguably be cleaner to get op from the operations parameter to the completion. I tightened this up as much as seemed reasonable:
- (void)getSomeDataWithParameters:(...)parameters completion:(void (^)(NSArray *data))completion
{
NSURLRequest *req = ...;
AFJSONRequestOperation *op = [[AFJSONRequestOperation alloc] initWithRequest:req];
[self startOperations:#[op] completionBlock:^(NSArray *operations) {
for (AFJSONRequestOperation *op in operations) {
dispatch_async(getBgQueue(), ^{
NSArray *data = [MyParserClass parseJSON:op.responseJSON inContext:self.localContext];
void (^mrSaveCompletion)(BOOL, NSError*) = completion ? ^(BOOL success, NSError *error) { completion(data); } : nil;
[self.localContext MR_saveToPersistentStoreWithCompletion: mrSaveCompletion];
});
}
}];
}
This will fan out each response potentially to a different thread. If you want all responses to execute on a single background thread, just swap the nesting of the for loop and the dispatch_async.
From there, the only really "superfluous" code is the dispatch_async. You could eliminate that by making -startOperations:... take a queue parameter where you would pass in the queue you wanted the completion to be called. Maybe like this:
- (void)startOperations: (NSArray*)ops completionQueue: (dispatch_queue_t)queue completionBlock: (void (^)(NSArray*))completion
{
void (^completionWrapper)(NSArray*) = !completion ? nil : ^(NSArray* ops) {
if (queue)
dispatch_async(queue, ^{ completion(ops); });
else
completion(ops);
};
[self startOperations: ops completionBlock: completionWrapper];
}
- (void)getSomeDataWithParameters:(...)parameters completion:(void (^)(NSArray *data))completion
{
NSURLRequest *req = ...;
AFJSONRequestOperation *op = [[AFJSONRequestOperation alloc] initWithRequest:req];
[self startOperations:#[op] completionQueue: getBgQueue() completionBlock:^(NSArray *operations) {
for (AFJSONRequestOperation *op in operations) {
NSArray *data = [MyParserClass parseJSON:op.responseJSON inContext:self.localContext];
void (^mrSaveCompletion)(BOOL, NSError*) = !completion ? nil : ^(BOOL success, NSError *error) { completion(data); };
[self.localContext MR_saveToPersistentStoreWithCompletion: mrSaveCompletion];
});
}];
}