I add items to a list through a Model class and then add these items to an ArrayList<Object>. However, I always get a size of zero even if the <,java,android,asynchronous,arraylist,firebase-realtime-database"/>

时间戳

首页 > 解决方案 > 数组列表 copied from another ArrayList is empty even if there is data

I add items to a list through a Model class and then add these items to an ArrayList<Object>. However, I always get a size of zero even if the <

java - 数组列表 copied from another ArrayList is empty even if there is data

I add items to a list through a Model class and then add these items to an ArrayList<Object>. However, I always get a size of zero even if the <

问题描述

I add items to a list through a Model class and then add these items to an ArrayList<Object>. However, I always get a size of zero even if the commentsList.size() > 1.

I tried to convert the List to ArrayList and tried adding items. But always the size was 0 on ArrayList<Object>.

ArrayList<Comments> commentsList = new ArrayList<>(Arrays.asList(new Comments("username", "time", "date")));

Here are the functions that I am using.

private ArrayList<Object> getObject() {

    if (getComments() != null && getComments().size()>=1) {
        objects.add(getComments().get(0));
    }

    return objects;
}

public static ArrayList<Comments> getComments() {
    ArrayList<Comments> commentsList = new ArrayList<>();

    DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Comments");

    reference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            commentsList.clear();

            for (DataSnapshot shot : snapshot1.getChildren()) {
                 Comments comments = shot.getValue(Comments.class);
                 commentsList.add(comments);                          
           }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });

    return commentsList;
}

Second playback with exoplayer displays no video window

I have a simple file based playback implementation that works fine the first time. But, when I instantiate it again, I just hear the audio, but no video (just a black screen). Any help is appreciated

I've looked into lot of sample code, but nothing has helped so far

private void initiateFilePlayback(Uri uri)
{
    closePlayer();

    mDataSourceFactory = new DefaultDataSourceFactory(mContext, mAppName);
    mExtractorsFactory = new DefaultExtractorsFactory();

    mExoPlayer = ExoPlayerFactory.newSimpleInstance(this, new DefaultRenderersFactory(this), new DefaultTrackSelector(), new DefaultLoadControl());
    mExoPlayer.clearVideoSurface();

    mPlayerView = (PlayerView)findViewById(R.id.videoWindow);
    mPlayerView.setVisibility(View.VISIBLE);

    mPlayerView.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LOW_PROFILE
    | View.SYSTEM_UI_FLAG_IMMERSIVE
    | View.SYSTEM_UI_FLAG_FULLSCREEN
    | View.SYSTEM_UI_FLAG_LAYOUT_STABLE
    | View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY
    | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
    | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION);
    mPlayerView.setPlayer(mExoPlayer);

    mMediaSource = new ExtractorMediaSource.Factory(mDataSourceFactory).setExtractorsFactory(mExtractorsFactory).createMediaSource(uri);

    mExoPlayer.prepare(mMediaSource);
    mExoPlayer.setPlayWhenReady(true);
}

private void closePlayer()
{
    if (mExoPlayer != null)
    {
        mPlayerView.removeAllViews();

        mExoPlayer.stop();
        mExoPlayer.release();
        mExoPlayer.removeMetadataOutput(null);

        mPlayerView.setVisibility(View.INVISIBLE | View.GONE);
        mPlayerView = null;

        mMediaSource = null;
        mDataSourceFactory = null;
        mExtractorsFactory = null;
        mExoPlayer = null;
    }
}

And here's my layout: 

<com.google.android.exoplayer2.ui.PlayerView
    android:id="@+id/videoWindow"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

The 2nd playback just shows a black window with audio in the background

标签： javaandroidasynchronousarraylistfirebase-realtime-database

解决方案

在该getComments函数中，您正在执行一个异步操作，该操作正在从您的 firebase 数据库中检索数据。因此，您的commentsList. 当然，该函数只是ArrayList用零元素初始化一个新的，并创建一个ValueEventListener等待在您的应用程序中接收数据的函数。但是，此操作是异步的（在单独的线程中运行），因此在创建 之后ValueEventListener，该函数将立即返回空列表。因此，当您尝试在函数中构建时，您也会得到一个空ArrayList<Object>列表getObject。

onDataChange我建议编写另一个函数，在从 Firebase 数据库接收数据后调用该函数时将调用该函数。例如，在与以下相同的类中编写一个函数。

void nowCreateArrayListOfObjects() {
    // TODO: Call the getObject function here
}

现在从您的onDataChange函数中调用此函数，如下所示。

public static ArrayList<Comments> getComments() {
    ArrayList<Comments> commentsList = new ArrayList<>();

    DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Comments");

    reference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            commentsList.clear();

            for (DataSnapshot shot : snapshot1.getChildren()) {
                Comments comments = shot.getValue(Comments.class);
                commentsList.add(comments);
            }

            // Invoke the function here to get the values now
            nowCreateArrayListOfObjects();
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {
        }
    });

    return commentsList;
}

希望有帮助！

推荐阅读