Alternatives to lots of nested completion blocks? - ios

Question

I'm using OAuthSwift to access the goodreads api. First I have an api call to get the user id. Then, using that id, I get the books the user has in a shelf. Then I check if there are any books with a certain title, and if there aren't, I call the api again to create that book. And then, being sure the book is there, I post a status update on its progress.
This is the code to get the user id:
oAuth.client.get(
"https://www.goodreads.com/api/auth_user",
success: {
data, response in
// Here I use the returned ID to get the books.
},
failure: {
error in
//…
}
)
Once I got the ID, I need to use it for the next api call, which gets the books. The thing is, I can't just add the next call after that first one, because they're asynchronous, so the second call would start before I got the ID from the first one.
What I'm doing, then, is to add the second api call inside that success closure (I pointed it out in the code with a comment). So I have two very similar pieces of code one inside the other; when the first api call succeeds it calls the next. But then I have to use the result of that second call to decide if I'll add a new book or not. So I have a third api call (to add the book) inside the success block of the second call, which is inside the success block of the first call… and you can see things are starting to get pretty confusing. And it doesn't even stop there: I have a fourth api call to post a status update, and it's inside the completion handler of the third call, which is inside that of the second, which is inside that of the first.
So… what should I do instead of that? I'm sorry if it's a silly question, but I'm new at swift and at programming overall.

Answer 1

I think Apple has provided best way of handling this scenario using GCD, GCD provides serial queue to execute blocks/API in serial manner. That is in FIFO Order
let serialQueue : dispatch_queue_t = dispatch_queue_create("com.yourSerialQueue.queue", DISPATCH_QUEUE_SERIAL);
dispatch_async(serialQueue) { () -> Void in
//Call First API
print("First API call succeed")
};
dispatch_async(serialQueue) { () -> Void in
//Call Second API
print("Second API call succeed")
};
dispatch_async(serialQueue) { () -> Void in
//Call Third API
print("Third API call succeed")
};
dispatch_async(serialQueue) { () -> Void in
//Call Fourth API
print("Fourth API call succeed")
};
OutPut:
First API call succeed
Second API call succeed
Third API call succeed
Fourth API call succeed

Answer 2

I do not have deep knowledge of GCD yet, for your scenario what i can suggest you in real quick is call each API with completion block as below, (this is what i can help you as of now)
dispatch_queue_t backgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(backgroundQueue, ^{
[self callGetUserIdAPIAndCompletion:^(NSDictionary *dictResponse) {
[self callGetBooksAPIForUserId:dictResponse[#"userId"] AndCompletion:^(NSDictionary *dictResponse) {
[self callAddBookAPIForUserId:dictResponse[#"userId"] bookId:dictResponse[#"bookId"] AndCompletion:^(NSDictionary *dictResponse) {
[self callPostStatusUpdateAPIForUserId:dictResponse[#"userId"] status:#"status" AndCompletion:^(NSDictionary *dictResponse) {
}];
}];
}];
}];
});
And write your API calls functions as below
-(void)callGetUserIdAPIAndCompletion:(void(^)(NSDictionary *dictResponse))completion {
NSDictionary *dictResponse;
//Call First API
NSLog(#"API 1 completes");
completion(dictResponse);
}
-(void)callGetBooksAPIForUserId:(NSString*)userId AndCompletion:(void(^)(NSDictionary *dictResponse))completion {
NSDictionary *dictResponse;
//Call Second API with userId
NSLog(#"API 2 completes");
completion(dictResponse);
}
-(void)callAddBookAPIForUserId:(NSString*)userId bookId:(NSString*)bookId AndCompletion:(void(^)(NSDictionary *dictResponse))completion {
NSDictionary *dictResponse;
//Call Third API with userId and bookId
NSLog(#"API 3 completes");
completion(dictResponse);
}
-(void)callPostStatusUpdateAPIForUserId:(NSString*) userId status:(NSString*) status AndCompletion:(void(^)(NSDictionary *dictResponse))completion {
NSDictionary *dictResponse;
//Call Fourth API with userId and status
NSLog(#"API 4 completes");
completion(dictResponse);
}