how to use AFNetworking with parallel request and collect all result - ios

Question

My scene is like this, first I have a server json api which return some data for specify page, the api is like /data/page/1. For this case, suppose the response data is :
page 1 => ['a','b']
page 2 => ['c','d']
page 3 => ['e','f']
I use AFNetworking 2 to fetch data from api, for single page data request it works well.
The problem is now I want to implement parallel request for more than one page. I need one api for view controller which accept one pages number, and callback with all data for these pages collected. The api I need is:
typedef void (^DataBlock)(id data);
- (void) dataForPages:(NSInteger)pages withSuccessBlock:(DataBlock)block;
If view controller pass 3 for pages parameter, I want AFNetworking can request data parallel and then collected the 3 result then use in callback block.
I tried to use NSOperationQueue to process multi AFHTTPRequestOperation but failed, the code demo is like this:
- (void) dataForPages:(NSInteger)pages withSuccessBlock:(DataBlock)block
{
//want to use for each, here suppose pages is 3
NSMutableArray *result = [[NSMutableArray alloc] init];
AFHTTPRequestOperation *op1 = [[AFHTTPRequestOperation alloc] initWithRequest:#"/data/page/1"];
AFHTTPRequestOperation *op2 = [[AFHTTPRequestOperation alloc] initWithRequest:#"/data/page/2"];
AFHTTPRequestOperation *op3 = [[AFHTTPRequestOperation alloc] initWithRequest:#"/data/page/3"];
[op1 setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
[result addObjectsFromArray: responseObject]; //responseObject is ['a', 'b']
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
}];
[op2 setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
[result addObjectsFromArray: responseObject]; //responseObject is ['c', 'd']
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
}];
[op3 setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
[result addObjectsFromArray: responseObject]; //responseObject is ['e', 'f']
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
}];
NSOperationQueue *q = [[NSOperationQueue alloc] init];
[q addOperation:op1];
[q addOperation:op2];
[q addOperation:op3];
[q waitUntilAllOperationsAreFinished];
block(result);
}
In my test the result always empty, I'm not quite understand waitUntilAllOperationsAreFinished.
Anyone knows how to deal this problem with NSOperation or GCD?

Answer 1

After some code research, I found it's difficult to get what I want with NSOperation and NSOperationQueue, because AFNetworking has it's own completion block handler.
The final solution is use dispatch_group, all code is like this:
dispatch_group_t group = dispatch_group_create();
NSURLRequest *req1 = ...;
NSURLRequest *req2 = ...;
AFHTTPRequestOperation *op1 = [[AFHTTPRequestOperation alloc] initWithRequest:req1];
AFHTTPRequestOperation *op2 = [[AFHTTPRequestOperation alloc] initWithRequest:req2];
NSMutableArray *result = [[NSMutableArray alloc] init];
dispatch_group_enter(group); //enter group
[op1 setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
[result addObjectsFromArray: responseObject];
dispatch_group_leave(group); //leave group in completion handler
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
dispatch_group_leave(group);
}];
dispatch_group_enter(group);
[op2 setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
[result addObjectsFromArray: responseObject];
dispatch_group_leave(group);
} failure:^(AFHTTPRequestOperation *operation, NSError *error) {
dispatch_group_leave(group);
}];
[op1 start];
[op2 start];
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
block(result);
});