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completionhandler - How to return outer function from inside async inner function in Objective-C

I want to return from outer function from inside async inner function. In swift, I would have used completion handler for this purpose which would escape from function. But in Objective-C, completion handler won't actually return from function:

My function looks like this:

-(void)chosenFrom:(NSDictionary<NSString *,id> *)info{

    [self asyncCode:info withCompletion:^(NSData *imageData) {
        if(imageData) {
             // I want to return from  chosenFrom function ***inside here.***
        }
    }];

// This is to illustrate  completion handler don't escape
[self checkCompletionEscaping:^(NSString * lad) {
    NSLog(@"Check me %@", lad);// It would print all 3 lines below.
}];
}

-(void) checkCompletionEscaping:(void(^)(NSString * lad)) completion {
completion(@"Hello"); // completion handler should've escaped from func.
completion(@"Shivam");
completion(@"How are you");
}

If I were to use Swift, I could have easily returned from outer function from inside inner function using completion handler:

private func getHistoryKeys(searchterm: String, completion: @escaping () -> ()) {
  let url = PubmedAPI.createEsearchURL(searchString: searchterm)
  let task = session.dataTask(with: url) { (data, response, error) in
   if let error = error {
      completion() // This would work as return
   } else {
      completion() // Same as return
   }
  }
 task.resume()
}

PS: escaping means returning from function just like return statement.

See Question&Answers more detail:os

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1 Reply

0 votes
by (71.8m points)

Easier is just to call another function that tells it's completed.

-(void)chosenFrom:(NSDictionary<NSString *,id> *)info{

    [self asyncCode:info withCompletion:^(NSData *imageData) {
        if(imageData) {
             [self completedAsync:imageData];
        }
    }];

}

-(void)completedAsync:(NSData*) imageData {
  // do your thang.
}

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