为什么此方法在另一个方法完成之前运行？

Question

I am kinda new to Android programming. I am building a simple application and I have a "follow-unfollow" concept on it. What I simply want is, if the current user follows the user he/she is exploring, I want him/her to see "unfollow" button. If not following, there should be a "follow" button. On my UserProfileActivity class I have a method called onPrepareOptionsMenu() and inside this method I can set the buttons.

@Override
public boolean onPrepareOptionsMenu(Menu menu) {
    MenuItem follow = menu.findItem(R.id.action_follow);
    MenuItem unfollow = menu.findItem(R.id.action_unfollow);
    Bundle bundle = this.getIntent().getExtras();
    if(isFollowing(bundle.getString("userid")) == true){
        follow.setVisible(false);
        unfollow.setVisible(true);
    }
    else{
        follow.setVisible(true);
        unfollow.setVisible(false);
    }
    return super.onPrepareOptionsMenu(menu);
}

Also, I have another method called isFollowing() and it returns a boolean "true" if current user follows the other user, it returns "false" if not. It is the simplest way that I have thought to solve this issue.

    public boolean isFollowing(String userID){
    isFollowingResult = false;

    firebaseDatabase = FirebaseDatabase.getInstance();
    databaseReference = firebaseDatabase.getReference();
    final DatabaseReference followingData = databaseReference.child("followingData");

    followingData.child(currentUser.getUid()).child(userID).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if(dataSnapshot.exists())
                isFollowingResult = true;
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    return isFollowingResult;
}

When I run this, and when I click on a user's profile onPrepareOptionsMenu() method is called and inside it isFollowing() method is called. The problem is, it does not wait for the isFollowing() method to run and finish running and it immediately sees it as "false" and always shows "follow" button. Any suggestion would be appreciated.

Thanks.

Answer 1

There are multiple issues with the code.

Firstly, your isFollowing() function is setting up the listener, on the data field, but it'll only get called when the data changes. In this case, you may only want to read the data once: https://firebase.google.com/docs/database/android/read-and-write#read_data_once

Secondly, the use of a listener implies asynchronicity. Meaning, you'll need to wait until you get the callback later in order to get the value you want.

The ideal solution in order to maintain responsiveness of your app is to maintain a local "copy" of the value in your database with a listener that constantly updates that value. That way, you can query the state of your variable quickly (since it's stored/replicated locally) and still be up to date with your database (with the listener).

This will also prevent each "read" from going all the way to the service and back and also remove the need for your UI to wait to render correctly (accurately).

Answer 2

Make follow and unfollow the fields of the class. Then change their visibility in onDataChange(DataSnapshot) method.