Nested multiple request with dispatch_group - ios

Question

I would like to make multiple requests to server to fetch posts and comments one after another. So, I created this example with dispatch_group which fetches all the POSTS serially one after another and then after it finished with the POSTS, it fetches comments one after another.
Here is a rough schema about how this works.
Fetch post 1
Fetch post 2
Fetch post 3
....
Fetch post 50
Fetch comment 1
Fetch comment 2
...
Fetch comment 50
So, all these should work serially as shown, like it fetches post 1, finishes with that and then fetches post 2 finish and so on.
The following example works fine for the purpose. But, now I would want to have a call back to actually know when syncing of 50 posts were finished and when 50 comments were finished. I tried that by adding dispatch_group_notify after for loop in requestOne and requestTwo. But, the notify method seems to be called when all the tasks have been completed. How can that be achieved ? I am not native English speaker so, please write down if I need to improve the post, I can still try :)
#interface GroupTest ()
#property (nonatomic, readonly) dispatch_group_t group;
#property (nonatomic, readonly) dispatch_queue_t serialQueue;
#end
#implementation GroupTest
- (instancetype)init
{
if (self = [super init]) {
_group = dispatch_group_create();
_serialQueue = dispatch_queue_create("com.test.serial.queue",
DISPATCH_QUEUE_SERIAL);
}
return self;
}
- (void)start
{
dispatch_async(self.serialQueue, ^{
[self requestOneCompletion:^{
NSLog(#"Request 1 completed");
}];
[self requestTwoCompletion:^{
NSLog(#"Request 2 completed");
}];
});
}
- (void)requestTwoCompletion:(void(^)(void))completion
{
for (NSUInteger i = 1; i <= 50; i++) {
dispatch_group_enter(self.group);
[self requestComment:i
completion:^(id response){
NSLog(#"%#", response);
dispatch_group_leave(self.group);
}];
dispatch_group_wait(self.group, DISPATCH_TIME_FOREVER);
}
}
- (void)requestOneCompletion:(void(^)(void))completion
{
for (NSUInteger i = 1; i <= 50; i++) {
dispatch_group_enter(self.group);
[self requestPost:i
completion:^(id response){
NSLog(#"%#", response);
dispatch_group_leave(self.group);
}];
dispatch_group_wait(self.group, DISPATCH_TIME_FOREVER);
}
}
- (void)requestComment:(NSUInteger)comment
completion:(void(^)(id))completion
{
NSString *urlString = [NSString stringWithFormat:#"https://jsonplaceholder.typicode.com/comments/%lu", (unsigned long)comment];
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *dataTask = [session dataTaskWithURL:[NSURL URLWithString:urlString]
completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
id object = [NSJSONSerialization JSONObjectWithData:data
options:0
error:nil];
completion(object);
}];
[dataTask resume];
}
- (void)requestPost:(NSUInteger)post
completion:(void(^)(id))completion
{
NSString *urlString = [NSString stringWithFormat:#"https://jsonplaceholder.typicode.com/posts/%lu", (unsigned long)post];
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *dataTask = [session dataTaskWithURL:[NSURL URLWithString:urlString]
completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
id object = [NSJSONSerialization JSONObjectWithData:data
options:0
error:nil];
completion(object);
}];
[dataTask resume];
}
#end

Answer 1

I think what you want to do is the following. Note that this is made so each completion block for requestTwoCompletion and requestOneCompletion will be called after all 50 calls are done. The order of the 50 calls are not guaranteed.
The main changes I made were the dispatch_group_t is local to each method and I moved dispatch_group_wait outside of the for loop. In this case it takes away the benefit of completion since the wait will block unit it is done. If you are highly insistent on completion being used and it not blocking, you can wrap this all in a dispatch_async.
- (void)requestTwoCompletion:(void(^)(void))completion
{
dispatch_group_t group = dispatch_group_create();
for (NSUInteger i = 1; i <= 50; i++) {
dispatch_group_enter(group);
[self requestComment:i
completion:^(id response){
NSLog(#"%#", response);
dispatch_group_leave(group);
}];
}
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
completion();
}
- (void)requestOneCompletion:(void(^)(void))completion
{
dispatch_group_t group = dispatch_group_create();
for (NSUInteger i = 1; i <= 50; i++) {
dispatch_group_enter(group);
[self requestPost:i
completion:^(id response){
NSLog(#"%#", response);
dispatch_group_leave(group);
}];
}
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
}
As this is in a serial queue, the way this will work is requestOneCompletion will finish all 50, and then requestTwoCompletion will run all 50 next.