设置 Player 类

注意： 在继续阅读本指南之前，您必须完成 HbbTV 版 IMA SDK 使用入门中的步骤。

在 video_player.js 中，定义一个视频播放器封装类，以初始化和控制 dash.js 播放器。

设置宽带播放器

通过创建视频和封装容器标记，定义在应用中放置宽带播放器的位置：

<div id="broadband-wrapper">
    <video id="broadband-video"></video>
</div>
index.html

创建视频播放器

使用 HTML 元素、dash.js 播放器和回调的变量初始化视频播放器类，其他类方法可以使用这些变量。

/**
 * Video player wrapper class to control ad creative playback with dash.js in
 * broadband.
 */
var VideoPlayer = function() {
  this.videoElement = document.querySelector('video');
  this.broadbandWrapper = document.getElementById('broadband-wrapper');
  this.player = dashjs.MediaPlayer().create();
  this.onAdPodEndedCallback = null;

  // Function passed in VideoPlayer.prototype.setEmsgEventHandler.
  this.onCustomEventHandler = null;
  //  Scope (this) passed in VideoPlayer.prototype.setEmsgEventHandler.
  this.customEventHandlerScope = null;

  // Function to remove all of player event listeners.
  this.cleanUpPlayerListener = null;
  debugView.log('Player: Creating dash.js player');
};
video_player.js

定义播放控制函数

如需显示广告播放器并附加视频视图，请创建 VideoPlayer.play() 方法。之后，创建 VideoPlayer.stop() 方法，以在广告连播结束后处理清理工作。

/** Starts playback of ad stream. */
VideoPlayer.prototype.play = function() {
  debugView.log('Player: Start playback');
  this.show();
  this.player.attachView(this.videoElement);
};

/** Stops ad stream playback and deconstructs player. */
VideoPlayer.prototype.stop = function() {
  debugView.log('Player: Request to stop player');
  if (this.cleanUpPlayerListener) {
    this.cleanUpPlayerListener();
  }
  this.player.reset();
  this.player.attachView(null);
  this.player.attachSource(null);
  this.player = null;
  this.hide();
};
video_player.js

预加载广告流清单

如需验证在内容流期间和广告插播时间点开始之前广告是否已充分加载，请使用 VideoPlayer.preload() 和 VideoPlayer.isPreloaded()。

1. 预加载广告流

创建 VideoPlayer.preload() 方法，以预加载广告流清单并在广告插播时间点之前构建广告缓冲区。您必须将播放器流式传输 设置 'cacheInitSegments' 更新为 true。通过更新设置，您可以缓存 init 段，从而避免在切换到广告时出现延迟。

/**
 * Starts ad stream prefetching into Media Source Extensions (MSE) buffer.
 * @param {string} url manifest url for ad stream playback.
 */
VideoPlayer.prototype.preload = function(url) {
  if (!this.player) {
    this.player = dashjs.MediaPlayer().create();
  }
  debugView.log('Player: init with ' + url);
  this.player.initialize(/* HTMLVideoElement */ null, url, /* autoplay */ true);

  this.player.updateSettings({
    'debug': {
      'logLevel': dashjs.Debug.LOG_LEVEL_WARNING,
      'dispatchEvent': true,  // flip to false to hide all debug events.
    },
    'streaming': {
      'cacheInitSegments': true,
    }
  });
  this.player.preload();
  this.attachPlayerListener();
  debugView.log('Player: Pre-loading into MSE buffer');
};
video_player.js

2. 检查预加载的广告缓冲区

创建 VideoPlayer.isPreloaded() 方法，以检查与应用中设置的缓冲区阈值相比，预加载的广告缓冲区是否足够：

// Ads will only play with 10 or more seconds of ad loaded.
var MIN_BUFFER_THRESHOLD = 10;
video_player.js

/**
 * Checks if the ad is preloaded and ready to play.
 * @return {boolean} whether the ad buffer level is sufficient.
 */
VideoPlayer.prototype.isPreloaded = function() {
  var currentBufferLevel = this.player.getDashMetrics()
      .getCurrentBufferLevel('video', true);
  return currentBufferLevel >= MIN_BUFFER_THRESHOLD;
};
video_player.js

附加播放器监听器

如需为 dash.js 播放器事件添加事件监听器，请创建 VideoPlayer.attachPlayerListener() 方法：PLAYBACK_PLAYING、PLAYBACK_ENDED、LOG 和 ERROR。除了设置用于移除这些监听器的清理函数之外，此方法还会处理方案 ID URI 的事件。

var SCHEME_ID_URI = 'https://developer.apple.com/streaming/emsg-id3';
video_player.js

/** Attaches event listener for various dash.js events.*/
VideoPlayer.prototype.attachPlayerListener = function() {
  var playingHandler = function() {
    this.onAdPodPlaying();
  }.bind(this);
  this.player.on(dashjs.MediaPlayer.events['PLAYBACK_PLAYING'], playingHandler);
  var endedHandler = function() {
    this.onAdPodEnded();
  }.bind(this);
  this.player.on(dashjs.MediaPlayer.events['PLAYBACK_ENDED'], endedHandler);
  var logHandler = function(e) {
    this.onLog(e);
  }.bind(this);
  this.player.on(dashjs.MediaPlayer.events['LOG'], logHandler);
  var errorHandler = function(e) {
    this.onAdPodError(e);
  }.bind(this);
  this.player.on(dashjs.MediaPlayer.events['ERROR'], errorHandler);

  var customEventHandler = null;
  if (this.onCustomEventHandler) {
    customEventHandler =
        this.onCustomEventHandler.bind(this.customEventHandlerScope);
    this.player.on(SCHEME_ID_URI, customEventHandler);
  }

  this.cleanUpPlayerListener = function() {
    this.player.off(
        dashjs.MediaPlayer.events['PLAYBACK_PLAYING'], playingHandler);
    this.player.off(dashjs.MediaPlayer.events['PLAYBACK_ENDED'], endedHandler);
    this.player.off(dashjs.MediaPlayer.events['LOG'], logHandler);
    this.player.off(dashjs.MediaPlayer.events['ERROR'], errorHandler);
    if (customEventHandler) {
      this.player.off(SCHEME_ID_URI, customEventHandler);
    }
  };
};
video_player.js

设置播放器事件回调

如需根据播放器事件管理广告连播播放，请创建 VideoPlayer.onAdPodPlaying()、VideoPlayer.onAdPodEnded() 和 VideoPlayer.onAdPodError() 方法：

/**
 * Called when ad stream playback buffered and is playing.
 */
VideoPlayer.prototype.onAdPodPlaying = function() {
  debugView.log('Player: Ad Playback started');
};

/**
 * Called when ad stream playback has been completed.
 * Will call the restart of broadcast stream.
 */
VideoPlayer.prototype.onAdPodEnded = function() {
  debugView.log('Player: Ad Playback ended');
  this.stop();
  if (this.onAdPodEndedCallback) {
    this.onAdPodEndedCallback();
  }
};

/**
 * @param {!Event} event The error event to handle.
 */
VideoPlayer.prototype.onAdPodError = function(event) {
  debugView.log('Player: Ad Playback error from dash.js player.');
  this.stop();
  if (this.onAdPodEndedCallback) {
    this.onAdPodEndedCallback();
  }
};
video_player.js

为 onAdPodEnded 事件创建 setter

通过创建 VideoPlayer.setOnAdPodEnded() 方法，设置在广告连播结束时运行的回调函数。应用类使用此方法在广告插播后恢复内容广播。

/**
 * Sets a callback function for when an ad pod has ended.
 * @param {!Function} callback Callback function.
 */
VideoPlayer.prototype.setOnAdPodEnded = function(callback) {
  this.onAdPodEndedCallback = callback;
};
video_player.js

处理流元数据事件

通过创建 VideoPlayer.setEmsgEventHandler() 方法，设置根据 emsg 事件运行的回调函数。对于本指南，请添加 scope 参数，因为您在 video_player.js 之外调用 setEmsgEventHandler()。

/**
 * Sets emsg event handler.
 * @param {!Function} customEventHandler Event handler.
 * @param {!Object} scope JS scope in which event handler function is called.
 */
VideoPlayer.prototype.setEmsgEventHandler = function(
    customEventHandler, scope) {
  this.onCustomEventHandler = customEventHandler;
  this.customEventHandlerScope = scope;
};
video_player.js

显示和隐藏广告插播的视频播放器

如需在广告插播期间显示视频播放器，并在广告插播结束后隐藏播放器，请创建 VideoPlayer.show() 和 VideoPlayer.hide() 方法：

/** Shows the video player. */
VideoPlayer.prototype.show = function() {
  debugView.log('Player: show');
  this.broadbandWrapper.style.display = 'block';
};

/** Hides the video player. */
VideoPlayer.prototype.hide = function() {
  debugView.log('Player: hide');
  this.broadbandWrapper.style.display = 'none';
};
video_player.js

接下来，创建一个广告管理器类，以使用 IMA SDK 发出流请求、获取广告连播清单、监听 IMA 流事件，并将 emsg 事件传递给 IMA SDK。