At the core of the ExoPlayer library is the ExoPlayer interface. An ExoPlayer exposes traditional high-level media player functionality such as the ability to buffer media, play, pause and seek. Implementations are designed to make few assumptions about (and hence impose few restrictions on) the type of media being played, how and where it is stored, and how it is rendered. Rather than implementing the loading and rendering of media directly, ExoPlayer implementations delegate this work to components that are injected when a player is created or when it’s prepared for playback. Components common to all ExoPlayer implementations are:

  • A MediaSource that defines the media to be played, loads the media, and from which the loaded media can be read. A MediaSource is injected via ExoPlayer.prepare at the start of playback.
  • Renderers that render individual components of the media. Renderers are injected when the player is created.
  • A TrackSelector that selects tracks provided by the MediaSource to be consumed by each of the available Renderers. A TrackSelector is injected when the player is created.
  • A LoadControl that controls when the MediaSource buffers more media, and how much media is buffered. A LoadControl is injected when the player is created.

The library provides default implementations of these components for common use cases. An ExoPlayer can use of these components, but may also be built using custom implementations if non-standard behaviors are required. Some use cases for custom implementations are:

  • Renderer – You may want to implement a custom Renderer to handle a media type not supported by the default implementations provided by the library.
  • TrackSelector – Implementing a custom TrackSelector allows an app developer to change the way in which tracks exposed by a MediaSource are selected for consumption by each of the available Renderers.
  • LoadControl – Implementing a custom LoadControl allows an app developer to change the player’s buffering policy.
  • Extractor – If you need to support a container format not currently supported by the library, consider implementing a custom Extractor class, which can then be used to together with ProgressiveMediaSource to play media of that type.
  • MediaSource – Implementing a custom MediaSource class may be appropriate if you wish to obtain media samples to feed to renderers in a custom way, or if you wish to implement custom MediaSource compositing behavior.
  • DataSource – ExoPlayer’s upstream package already contains a number of DataSource implementations for different use cases. You may want to implement you own DataSource class to load data in another way, such as over a custom protocol, using a custom HTTP stack, or from a custom persistent cache.

The concept of injecting components that implement pieces of player functionality is present throughout the library. The default implementations of the components delegate work to further injected components. This allows many sub-components to be individually replaced with custom implementations. For example the default MediaSource implementations require one or more DataSource factories to be injected via their own factories. By providing a custom DataSource factory it’s possible to load data from a non-standard source or through a different network stack.

When building custom components, we recommend the following:

  • If a custom component needs to report events back to the app, we recommend that you do so using the same model as existing ExoPlayer components, where an event listener is passed together with a Handler to the constructor of the component.
  • We recommended that custom components use the same model as existing ExoPlayer components to allow reconfiguration by the app during playback, as described in the section below. To do this, custom components should implement PlayerMessage.Target and receive configuration changes in the handleMessage method. Application code should pass configuration changes by calling ExoPlayer’s createMessage method, configuring the message, and sending it to the component using PlayerMessage.send.

Sending messages to components

It’s possible to send messages to ExoPlayer components. These can be created using ExoPlayer.createMessage and then sent using PlayerMessage.send. By default, messages are delivered on the playback thread as soon as possible, but this can be customized by setting another callback thread (using PlayerMessage.setHandler), or by specifying a delivery playback position (using PlayerMessage.setPosition). Sending messages to be delivered on the playback thread ensures that they are executed in order with any other operations being performed on the player.

Most of ExoPlayer’s out-of-the-box renderers support messages that allow changes to their configuration during playback. For example, the audio renderers accept messages to set the volume and the video renderers accept messages to set the surface. These messages should be delivered on the playback thread to ensure thread safety.