How to add completion block to .childAdded firebase query?

manpreet Best Answer 2 years ago

I have the below fetch function. How can I add a completion block so when it finishes I can do something?

This query will run the code inside more than once.

func getFollowers() {
    print("get followers called")
    let ref = Database.database().reference()
    ref.child("users2").child((Auth.auth().currentUser?.uid)!).child("Following").observe(.childAdded) { (snap) in
        let personBeignFollow = snap.key
        self.peopleUserFollows.append(personBeignFollow)
        print("Appened: ", personBeignFollow)
        self.fetchAllUserFirstPostMedia(user: personBeignFollow)
    }
}

I have looked here but was not able to make it work.

Here is what I tried:

    func getFollowers(_: ()-> ()) {
    print("get followers called")
    let ref = Database.database().reference()
    ref.child("users2").child((Auth.auth().currentUser?.uid)!).child("Following").observe(.childAdded) { (snap) in
        let personBeignFollow = snap.key
        self.peopleUserFollows.append(personBeignFollow)
        print("Appened: ", personBeignFollow)
        self.fetchAllUserFirstPostMedia(user: personBeignFollow)
    }
}

Then where it is called:

getFollowers() {
   self.collectionView.reloadData()
}

manpreet 2 years ago

In your function declaration, you can add the following:

func getFollowers(_ completion: @escaping () -> Void) {
    print("get followers called")
    let ref = Database.database().reference()
    ref.child("users2").child((Auth.auth().currentUser?.uid)!).child("Following").observe(.childAdded) { (snap) in
        let personBeignFollow = snap.key
        self.peopleUserFollows.append(personBeignFollow)
        print("Appened: ", personBeignFollow)
        self.fetchAllUserFirstPostMedia(user: personBeignFollow)

       // tell the calling function to execute the completion handler again
       completion()
    }
}

then to use it you would do something like this:

getFollowers {
 // whatever you want to do after the query has run
}

This doesn't directly matter but just as a common design practice, it would be a good idea to pass your new data you retrieved from the query as a parameter in the completion handler rather than assigning a property on the class.

That would look something like this:

func getFollowers(_ completion: (String) -> Void) {
    ref.child("users2").child((Auth.auth().currentUser?.uid)!).child("Following").observe(.childAdded) { (snap) in
        let personBeignFollow = snap.key
        print("Appened: ", personBeignFollow)        
       // tell the calling function to execute the completion handler again
       completion(personBeignFollow)
    }
}

and then your calling site might look like this:

getFollowers { newUser in
    self.fetchAllUserFirstPostMedia(user: newUser)
}