Collect all data from FirebaseDatabse and call notifyDataSetChanged() once

问题

in brief: I have a list of users ID and I want to iterate over database and find profiles of those users and put them on the list. But I have a problem as follows:

final List<Friend> friendsProfiles = new ArrayList<>();
    for (final FriendId friendId : friendIds) {
        mUserRef.child(friendId).addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                // Get the friend profile
                Friend friend = dataSnapshot.getValue(Friend.class);
                // Add to the list
                friendsProfiles.add(friend);
                // The problem is here, because its called as many times as the size of
                // the friendIds list. loadnewData() contains notifyDataSetChanged()
                mFriendsFragment.loadNewData(friendsProfiles);
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }
    // It gives 0, because it's called before onDatachange()
    // So I can't call loadNewData() here
    Log.d(TAG, updatedFriendsRequestList.size());

How to do it in the nice, proper way?

回答1:

You can simply count how many you've already loaded, and then call notifyDataSetChanged() only once you've loaded the last one:

final List<Friend> friendsProfiles = new ArrayList<>();
for (final FriendId friendId : friendIds) {
    mUserRef.child(friendId).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            // Get the friend profile
            Friend friend = dataSnapshot.getValue(Friend.class);
            // Add to the list
            friendsProfiles.add(friend);

            if (friendsProfiles.size() == friendIds.length) {
                mFriendsFragment.loadNewData(friendsProfiles);
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            throw databaseError.toException(); // don't ignore errors, as they break the logic of your app
        }
    });
}

来源：https://stackoverflow.com/questions/53284983/collect-all-data-from-firebasedatabse-and-call-notifydatasetchanged-once