Queueing

The Cast framework provides queueing classes that support the creation of lists of MediaQueueItem instances, which can be built from MediaInfo instances such as video or audio streams, to play sequentially on the Cast receiver. This queue of content items can be edited, reordered, updated, and so forth.

The Cast Receiver SDK maintains the queue and responds to operations on the queue as long as the queue has at least one item currently active (playing or paused). Senders can join the session and add items to the queue. The receiver maintains a session for queue items until the last item completes playback or the sender stops the playback and terminates the session, or until a sender loads a new queue on the receiver. The receiver does not maintain any information about terminated queues by default. Once the last item in the queue finishes, the media session ends and the queue vanishes.

Create and load media queue items

A media queue item is represented in the Cast framework as a MediaQueueItem instance. When you create a media queue item, if you are using the Media Player Library with adaptive content, you can set the preload time so that the player can begin buffering the media queue item before the item ahead of it in the queue finishes playing. Setting the item's autoplay attribute to true allows the receiver to play it automatically. For example, you can use a builder pattern to create your media queue item as follows:

MediaQueueItem queueItem = new MediaQueueItem.Builder(mediaInfo)
  .setAutoplay(true)
  .setPreloadTime(20)
  .build();

Load an array of media queue items in the queue by using the appropriate queueLoad method of RemoteMediaClient.

Receive media queue status updates

When the receiver loads a media queue item, it assigns a unique ID to the item which persists for the duration of the session (and the life of the queue). Your app can learn the status of the queue in terms of which item is currently loaded (it might not be playing), loading, or preloaded. The MediaStatus class provides this status information:

  • getPreloadedItemId() method - If the next item has been preloaded, returns the preloaded item ID.
  • getLoadingItemId() method - Returns the item ID of the item that is currently loading (but isn't active in the queue) on the receiver.
  • getCurrentItemId() method - Returns the item ID of the item that that was active in the queue (it might not be playing) at the time the media status change happened.
  • getQueueItems() (Deprecated, use MediaQueue instead) method - Returns the list of MediaQueueItem instances as an unmodifiable list.

Your app can also get the list of items using the MediaQueue class. The class is a sparse data model of the media queue. It keeps the list of item IDs in the queue, which is automatically synchronized with the receiver. MediaQueue doesn't keep all the MediaQueueItem because it will take too much memory when the queue is very long. Instead, it fetches the items on demand and keeps an LRU cache of recently accessed items. You can use these methods to access the media queue:

  • getItemIds() method - Returns the list of all item IDs in order.
  • getItemAtIndex() method - Returns the cached item at a given index. If the item is not cached, MediaQueue will return null and schedule to fetch the item. When the item is fetched, MediaQueue.Callback#itemsUpdatedAtIndexes() will be called, and calling getItemAtIndex() with the same ID again will return the item.
  • fetchMoteItemsRelativeToIndex() is used when the user scrolls the queue UI to the top or bottom, and your app wants to fetch more items from the cloud.

Use these methods together with the other media status methods to inform your app about the status of the queue and the items in the queue. In addition to media status updates from the receiver, your app can listen for changes to the queue by implementing RemoteMediaClient.Callback and MediaQueue.Callback.

Also CAF provides two utility classes to conveniently create UI for queueing.

For example, to create a RecyclerView using MediaQueueRecyclerViewAdapter:

public class MyRecyclerViewAdapter extends MediaQueueRecyclerViewAdapter<MyViewHolder> {
    public MyRecyclerViewAdapter(MediaQueue mediaQueue) {
        super(mediaQueue);
    }

    @Override
    public void onBindViewHolder(MyViewHolder holder, position) {
      MediaQueueItem item = getItem(position);

      // Update the view using `item`.
      ...
    }
}

public class MyViewHolder implements RecyclerView.ViewHolder {
  // Implement your own ViewHolder.
  ...
}

public void someMethod() {
    RecyclerView.Adapter adapter = new MyRecyclerViewAdapter(
        getRemoteMediaClient().getMediaQueue());
    RecyclerView recyclerView = (RecyclerView) getActivity().findViewById(R.id.my_recycler_view_id);
    recyclerView.setAdapter(adapter);
}

Edit the queue

To operate on the items in the queue, use the queue methods of the RemoteMediaClient class. These let you load an array of items into a new queue, insert items into an existing queue, update the properties of items in the queue, make an item jump forward or backward in the queue, set the properties of the queue itself (for example, change the repeatMode algorithm that selects the next item), remove items from the queue, and reorder the items in the queue.