Using Dispatch, Async Programming to make Code Block Wait

Question

I have 2 functions (func A() and func B()). I want to know how to make function B wait for function A to execute before proceeding. I've done a lot of reading, and know we can somehow do it with asynchronous programming and semaphores, or with functions such as dispatch_barrier_sync. However, I'm really new to swift, and am not understanding how to code this. I would really appreciate it if someone could help.

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
A()
B()

}

In function A and B both, I'm making queries to my Parse database — abcd123
– abcd123, Commented May 26, 2015 at 6:48
Parse has methods that include callbacks for when the operation has completed. You're better off calling your functions from there. — Mark
– Mark, Commented May 26, 2015 at 6:51
Could you direct me to those methods and how to use them for this scenario? I couldn't find them in the parse documentation. — abcd123
– abcd123, Commented May 26, 2015 at 6:55

Community · Accepted Answer · 2017-05-23 12:30:03Z

1

func a(completion: ()->Void){
  let query:PFQuery = PFQuery(className: "GameScore")
  query.getObjectInBackgroundWithId("xWMyZ4YEGZ", block: { (object, error) -> Void in
    if let object = object{
      completion()
    }
  })
}

func b(){
  //get your other object
}

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
  a(){ () -> Void in
    //Call B when A is completed
    b()
  }
}

Here we add a completion block to function A.
We call this block when the object has been returned.
When we call function A we use the { () -> Void in ……} to execute code only when function A has been completed.

(Code after abhishekkarwar)

edited May 23, 2017 at 12:30

CommunityBot

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answered May 26, 2015 at 8:59

milo526

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2 Comments

abcd123 Over a year ago

Thanks for the response! Where do I add in the rest of my code for the query in function A? I'm takling about: if error == nil { for object in objects {

abcd123 Over a year ago

also I'm using findObjectsInBackgroundWithBlock so would my code just look like : findObjectsInBackgroundWithBlock(block: { (object, error) -> Void in if let object = object{ completion() } })

abhishekkharwar · Accepted Answer · 2015-05-26 07:40:27Z

0

// Please convert bellow code in swift

- (UITableViewCell*)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{

 [self functionAWithCompletion:^(){
    //After completion of a cal b function
   [self functionB];
}];

}

- (void)functionAWithCompletion:(void (^)())completion{

PFQuery *query = [PFQuery queryWithClassName:@"GameScore"];
[query getObjectInBackgroundWithId:@"xWMyZ4YEGZ" block:^(PFObject *gameScore, NSError *error) {

     completion();
}];

}

- (void)functionb{


}

edited May 26, 2015 at 7:40

answered May 26, 2015 at 7:23

abhishekkharwar

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