CODEWITHSUNDEEP
objective-casynchronousobjective-c-blocksgrand-central-dispatch
allenlinli
allenlinli

Reputation: 2154

Block and Wait for Asynchronous Operations in Objective-C

When I want to get userID from Facebook, I need to wait the API "startWithCompletionHandler" to send me back the userID, then I continue my task. Just like the code listed below.

But I can not use dispatch_semaphore or dispatch_group to wait for the api to callback. Except for completionHandler, any other solution like "blockAndWait" in MagicalRecord that can help me solve question like this?

Also, the solution listed below isn't working. It seems blocked the main thread, so it'll keep logging "WAITING" and can not pop up the Facebook login view. 

-(void)getAccountIDAndWait{
__block BOOL isFinish = NO;
[[FBRequest requestForMe] startWithCompletionHandler:
 ^(FBRequestConnection *connection, NSDictionary<FBGraphUser> *aUser, NSError *error) {
     if (error) {
         NSLog(@"bug in getAccountIDAndWait");
         return;
     }

     self.accountID = [aUser objectForKey:@"id"];
     isFinish = YES;
 }];

while(isFinish==NO){
    sleep(0.1);
    NSLog(@"WAITING");
}

}

Upvotes: 0

Views: 1126

Answers (2)

Craig
Craig

Reputation: 8294

Expanding on what Chuck said I've slightly modified your code to allow the getAccountIdAndWait function to finish (and thus the main thread to idle) and when the completionHandler is called it will call the doTheNextThing function.

-(void)getAccountIDAndWait{
    [[FBRequest requestForMe] startWithCompletionHandler:
     ^(FBRequestConnection *connection, NSDictionary<FBGraphUser> *aUser, NSError *error) {
        if (error) {
            NSLog(@"bug in getAccountIDAndWait");
            return;
        }

        self.accountID = [aUser objectForKey:@"id"];
        [self doTheNextThing];
    }];

}

Upvotes: 1

Chuck
Chuck

Reputation: 237010

The approach you are using there is blocking. As you correctly observe, this is not really what you want. The design intent here is for you to put things that you want to happen after the request completes in the completion handler.

Upvotes: 0

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