1.什么是ExoPlayer

ExoPlayer是谷歌官方主导基于低层媒体API（如MediaCodec，AudioTrack，MediaDrm）开发的Android媒体播放库，整体上的特点如下：

• 高度的可定制性，开发者可以根据实际需求对其进行修改
• 提供一系列MediaPlayer不提供的特性，例如，支持动态的自适应流HTTP(DASH) 和平滑流
• 独立于Android系统，可以构建系统版本自主升级

ExoPlayer各功能对系统版本的要求如下：

Use case	Android version number	Android API level
Audio playback	4.1	16
Video playback	4.1	16
DASH (no DRM)	4.1	16
DASH (Widevine CENC; “cenc” scheme)	4.4	19
SmoothStreaming (no DRM)	4.1	16
SmoothStreaming (PlayReady SL2000)	AndroidTV	AndroidTV
HLS (no DRM)	4.1	16
HLS (AES-128 encryption)	4.1	16

相关的网站如下：

• 项目官网：google.github.io/ExoPlayer/
• 开发指南：google.github.io/ExoPlayer/g…
• Github地址：github.com/google/ExoP…
• Medium博客：medium.com/google-exop…

框架简介

接口ExoPlayer是ExoPlayer的核心，提供了一些我们平时可能会用到的功能，如缓冲、暂停、播放或者快进快退。ExoPlayer通过控制组件实现了媒体的识别、加载和渲染。这些组件如下：

• MediaSource:是对播放对象的抽象，由于播放对象可能存在于内存、磁盘甚至网络中，MediaSource还需要提供对应的读取方式。
• Renderer:负责Media的渲染工作
• TrackSelecter: 选择MediaSource中的轨道（Track）交由TrackRenderer负责渲染,这里包括音频轨道和视频轨道。
• LoadControl: 负责媒体的加载，如何时进行加载以及加载的多少

以上四个组件，MediaSource需要在播放媒体前传入ExoPlayer，而后三者则需要在创建ExoPlayer时制定。

2.ExoPlayer 的Hello World

使用ExoPlayer在音频和视频的播放上没有太大区别，这里我们以视频播放为例，介绍一下如何使用ExoPlayer 播放视频，可以分为以下几步：

1. 将ExoPlayer添加到项目依赖中
2. 创建SimpleExoPlayer实例
3. 将player关联到到View上
4. 创建MediaSource并使player处于prepare状态
5. 释放资源

1.将ExoPlayer添加到项目依赖中

ExoPlayer发布在Jcenter上，所以需要在项目根目录的build.gralde文件中添加如下代码：

repositories {
    jcenter()
}

ExoPlayer对应的gradle地址如下：

compile 'com.google.android.exoplayer:exoplayer:r2.X.X'

2.创建一个SimpleExoPlayer实例

这里我们使用ExoPlayerFactory.newSimpleInstance(Context, TrackSelector, LoadControl)创建对应的ExoPlayer实例对象。
相关代码如下：

// 1. Create a default TrackSelector
BandwidthMeter bandwidthMeter = new DefaultBandwidthMeter();
TrackSelection.Factory videoTrackSelectionFactory = new AdaptiveVideoTrackSelection.Factory(bandwidthMeter);
TrackSelector trackSelector = new DefaultTrackSelector(videoTrackSelectionFactory);
// 2. Create a default LoadControl
LoadControl loadControl = new DefaultLoadControl();
// 3. Create the player
SimpleExoPlayer player =
    ExoPlayerFactory.newSimpleInstance(context, trackSelector, loadControl);

3.将player关联到到View上

ExoPlayer默认提供了一个实现SimpleExoPlayerView,它包括PlaybackControlView和用于视频渲染的Surface，通过调用SimpleExoPlayerView.setPlayer(SimpleExoPlayer)，就可以将SimpleExoPlayerView和Player进行绑定。
当然你可以对以上行为进行自定义，例如将PlaybackControlView作为单独的组件或者实现一个属于你自己的PlaybackControlView。

4.准备player

在ExoPlayer中，一个媒体片段用MediaSource表示，如果要播放一个片段，需要创建对应的MediaSource（ExoPlayer提供的实现有DashMediaSource、SsMediaSource、HlsMediaSource、ExtractorMediaSource）。如果我们需要播放一段mp4的视频，代码如下：

// Measures bandwidth during playback. Can be null if not required.
DefaultBandwidthMeter bandwidthMeter = new DefaultBandwidthMeter();
// Produces DataSource instances through which media data is loaded.
DataSource.Factory dataSourceFactory = new DefaultDataSourceFactory(this,
    Util.getUserAgent(this, "yourApplicationName"), bandwidthMeter);
// Produces Extractor instances for parsing the media data.
ExtractorsFactory extractorsFactory = new DefaultExtractorsFactory();
// This is the MediaSource representing the media to be played.
MediaSource videoSource = new ExtractorMediaSource(mp4VideoUri,
    dataSourceFactory, extractorsFactory, null, null);
// Prepare the player with the source.
player.prepare(videoSource);

当ExoPlayer准备就绪后，我们可以通过player控制视频的播放、前进或后退。

5.释放资源

当你不需要player时，可以通过调用ExoPlayer.release来释放资源。

3.MediaSource

之前已经提到过了，ExoPlayer中一个media片段用MediaSource表示。除了之前提到过的MediaSource，ExoPlayer中还提供了MergingMediaSource, LoopingMediaSource 和 ConcatenatingMediaSource等MediaSource，通过他们之间的相互组合，我们可以得到各种各样的播放效果

合并视频和字幕

MediaSource videoSource = new ExtractorMediaSource(videoUri, dataSourceFactory, extractorsFactory, null, null);
Format format = Format.createTextSampleFormat(null, MimeTypes.APPLICATION_TTML, null, Format.NO_VALUE, Format.NO_VALUE, "en", null);
MediaSource subtitleSource = new SingleSampleMediaSource(subtitleUri, dataSourceFactory, format, C.TIME_UNSET);
MergingMediaSource mergedSource = new MergingMediaSource(videoSource, subtitleSource);

循环播放视频

MediaSource source = new ExtractorMediaSource(videoUri, ...);
// Loops the video indefinitely.
LoopingMediaSource loopingSource = new LoopingMediaSource(source);
// Plays the video twice.
LoopingMediaSource loopingSource = new LoopingMediaSource(source,2);

连续播放视频

MediaSource firstSource = new ExtractorMediaSource(firstVideoUri, ...);
MediaSource secondSource = new ExtractorMediaSource(secondVideoUri, ...);
// Plays the first video, then the second video.
ConcatenatingMediaSource concatenatedSource = new ConcatenatingMediaSource(firstSource, secondSource);

4.事件监听

在platback的过程中，应用可以可以根据ExoPlayer的状态做出相应的调整，对播放过程进行控制。为此，ExoPlayer提供了两个不同方式

事件回调（High Level Event）

ExoPlayer提供了一个ExoPlay.EventListener这一接口，通过addListener和removeListener可以添加和删除相关接口。这个接口可以监听player状态的更改，提供的方法如下。除此以外，SimpleExoPlayer提供了setVideoListener负责接收与视频渲染有关的事件，setVideoDebugListener和setAudioDebugListener用于调试信息。

public interface ExoPlayer {
  interface EventListener {
    /**
     * Called when the timeline and/or manifest has been refreshed.
     * 

     * Note that if the timeline has changed then a position discontinuity may also have occurred.
     * For example the current period index may have changed as a result of periods being added or
     * removed from the timeline. The will not be reported via a separate call to
     * {@link #onPositionDiscontinuity()}.
     *
     * @param timeline The latest timeline. Never null, but may be empty.
     * @param manifest The latest manifest. May be null.
     */
    void onTimelineChanged(Timeline timeline, Object manifest);
    /**
     * Called when the available or selected tracks change.
     *
     * @param trackGroups The available tracks. Never null, but may be of length zero.
     * @param trackSelections The track selections for each {@link Renderer}. Never null and always
     *     of length {@link #getRendererCount()}, but may contain null elements.
     */
    void onTracksChanged(TrackGroupArray trackGroups, TrackSelectionArray trackSelections);
    /**
     * Called when the player starts or stops loading the source.
     *
     * @param isLoading Whether the source is currently being loaded.
     */
    void onLoadingChanged(boolean isLoading);
    /**
     * Called when the value returned from either {@link #getPlayWhenReady()} or
     * {@link #getPlaybackState()} changes.
     *
     * @param playWhenReady Whether playback will proceed when ready.
     * @param playbackState One of the {@code STATE} constants defined in the {@link ExoPlayer}
     *     interface.
     */
    void onPlayerStateChanged(boolean playWhenReady, int playbackState);
    /**
     * Called when an error occurs. The playback state will transition to {@link #STATE_IDLE}
     * immediately after this method is called. The player instance can still be used, and
     * {@link #release()} must still be called on the player should it no longer be required.
     *
     * @param error The error.
     */
    void onPlayerError(ExoPlaybackException error);
    /**
     * Called when a position discontinuity occurs without a change to the timeline. A position
     * discontinuity occurs when the current window or period index changes (as a result of playback
     * transitioning from one period in the timeline to the next), or when the playback position
     * jumps within the period currently being played (as a result of a seek being performed, or
     * when the source introduces a discontinuity internally).
     * 

     * When a position discontinuity occurs as a result of a change to the timeline this method is
     * not called. {@link #onTimelineChanged(Timeline, Object)} is called in this case.
     */
    void onPositionDiscontinuity();
  }

Handler(Low Level Event)

通过调用ExoPlayer.void sendMessages(ExoPlayerMessage... )和ExoPlayer.void blockingSendMessages(ExoPlayerMessage... messages)实现和ExoPlayer交互，再由内部的Handler进行消息的分发和处理。其实，在具体实现上，回调是通过Handler.handleMessage(Message)实现的，两者在实现上是一样的。

5.使用Stetho对ExoPlayer进行网络调试

Stetho是FaceBook出品的Android调试利器，在它的配合下，可以使用Chrome进行调试。当然，ExoPlayer也支持Stetho调试，开启方式如下：

1.实现添加相关库依赖

由于ExoPlayer的数据读取是由接口com.google.android.exoplayer2.upstream.DataSource实现的，这里的我们采用的类是com.google.android.exoplayer2.ext.okhttp.OkHttpDataSourceFactory，所以需要添加如下依赖

compile 'com.google.android.exoplayer:extension-okhttp:r2.1.1'
compile 'com.squareup.okhttp3:okhttp:3.5.0'
compile 'com.facebook.stetho:stetho:1.4.2'
compile 'com.facebook.stetho:stetho-okhttp3:1.4.2'

2.网络和Stetho的初始化

通过Stetho.initializeWithDefaults(Context)完成Stetho的初始化。
创建OkHttpClient，并添加StethoInterceptor到NetworkInterceptor

Stetho.initializeWithDefaults(this);
OkHttpClient okHttpClient = new OkHttpClient.Builder()
        .addNetworkInterceptor(new StethoInterceptor())
        .build();

3.创建OkHttpDataSource

DataSource.Factory dataSourceFactory =
        new OkHttpDataSourceFactory(okHttpClient, userAgent, bandwidthMeter);

4.查看调试结果

打开chrome://inspect，我们就可以在Developer Tools的Network部分查看到相关的调试结果: