在启动器后面或在后台播放媒体时，TV 应用必须显示“正在播放”卡片。通过此卡片，用户可返回当前正在播放媒体的应用。
当存在活动的 MediaSession 时，Android 框架会在主屏幕上显示“正在播放”卡片。该卡片包含媒体元数据，如专辑封面、名称和应用图标。当用户选择该卡片时，系统会打开相应的应用。
本课程介绍如何利用 MediaSession 类实现“正在播放”卡片。
图 1. 在后台播放媒体时显示“正在播放”卡片。
启动媒体会话
当您的应用准备播放媒体时，创建一个 MediaSession。以下代码段举例说明了如何设置适当的回调和标记：
Kotlin
session = MediaSession(this, "MusicService").apply {
setCallback(MediaSessionCallback())
setFlags(
MediaSession.FLAG_HANDLES_MEDIA_BUTTONS or MediaSession.FLAG_HANDLES_TRANSPORT_CONTROLS
)
}
Java
session = new MediaSession(this, "MusicService");
session.setCallback(new MediaSessionCallback());
session.setFlags(MediaSession.FLAG_HANDLES_MEDIA_BUTTONS |
MediaSession.FLAG_HANDLES_TRANSPORT_CONTROLS);
注意：系统将仅为设置有 FLAG_HANDLES_TRANSPORT_CONTROLS 标记的媒体会话显示“正在播放”卡片。
显示“正在播放”卡片
系统仅为活动会话显示“正在播放”卡片。播放开始时，您必须调用 setActive(true)。此外，您的应用还必须请求音频焦点，如管理音频焦点中所述。
Kotlin
private fun handlePlayRequest() {
tryToGetAudioFocus()
if (!session.isActive) {
session.isActive = true
}
...
}
Java
private void handlePlayRequest() {
tryToGetAudioFocus();
if (!session.isActive()) {
session.setActive(true);
}
...
}
通过调用 setActive(false) 停用媒体会话或者另一个应用启动媒体播放时，系统会从启动器屏幕中移除该卡片。如果播放完全停止且没有活动的媒体，您的应用应立即停用媒体会话。如果播放暂停，您的应用应在延迟一段时间（通常为 5 到 30 分钟）后停用媒体会话。
更新播放状态
更新 MediaSession 中的播放状态，以便该卡片可以显示当前媒体的状态。
Kotlin
private fun updatePlaybackState() {
val position: Long =
mediaPlayer
?.takeIf { it.isPlaying }
?.currentPosition?.toLong()
?: PlaybackState.PLAYBACK_POSITION_UNKNOWN
val stateBuilder = PlaybackState.Builder()
.setActions(getAvailableActions()).apply {
setState(mState, position, 1.0f)
}
session.setPlaybackState(stateBuilder.build())
}
private fun getAvailableActions(): Long {
var actions = (PlaybackState.ACTION_PLAY_PAUSE
or PlaybackState.ACTION_PLAY_FROM_MEDIA_ID
or PlaybackState.ACTION_PLAY_FROM_SEARCH)
playingQueue?.takeIf { it.isNotEmpty() }?.apply {
actions = if (mState == PlaybackState.STATE_PLAYING) {
actions or PlaybackState.ACTION_PAUSE
} else {
actions or PlaybackState.ACTION_PLAY
}
if (currentIndexOnQueue > 0) {
actions = actions or PlaybackState.ACTION_SKIP_TO_PREVIOUS
}
if (currentIndexOnQueue < size - 1) {
actions = actions or PlaybackState.ACTION_SKIP_TO_NEXT
}
}
return actions
}
Java
private void updatePlaybackState() {
long position = PlaybackState.PLAYBACK_POSITION_UNKNOWN;
if (mediaPlayer != null && mediaPlayer.isPlaying()) {
position = mediaPlayer.getCurrentPosition();
}
PlaybackState.Builder stateBuilder = new PlaybackState.Builder()
.setActions(getAvailableActions());
stateBuilder.setState(mState, position, 1.0f);
session.setPlaybackState(stateBuilder.build());
}
private long getAvailableActions() {
long actions = PlaybackState.ACTION_PLAY_PAUSE |
PlaybackState.ACTION_PLAY_FROM_MEDIA_ID |
PlaybackState.ACTION_PLAY_FROM_SEARCH;
if (playingQueue == null || playingQueue.isEmpty()) {
return actions;
}
if (mState == PlaybackState.STATE_PLAYING) {
actions |= PlaybackState.ACTION_PAUSE;
} else {
actions |= PlaybackState.ACTION_PLAY;
}
if (currentIndexOnQueue > 0) {
actions |= PlaybackState.ACTION_SKIP_TO_PREVIOUS;
}
if (currentIndexOnQueue < playingQueue.size() - 1) {
actions |= PlaybackState.ACTION_SKIP_TO_NEXT;
}
return actions;
}
显示媒体元数据
使用 setMetadata() 方法设置 MediaMetadata。使用媒体会话对象的这一方法，您可以向“正在播放”卡片提供有关曲目的信息，如名称、字幕和各种图标。以下示例假定您的曲目数据存储在自定义数据类 MediaData 中。
Kotlin
private fun updateMetadata(myData: MediaData) {
val metadataBuilder = MediaMetadata.Builder().apply {
// To provide most control over how an item is displayed set the
// display fields in the metadata
putString(MediaMetadata.METADATA_KEY_DISPLAY_TITLE, myData.displayTitle)
putString(MediaMetadata.METADATA_KEY_DISPLAY_SUBTITLE, myData.displaySubtitle)
putString(MediaMetadata.METADATA_KEY_DISPLAY_ICON_URI, myData.artUri)
// And at minimum the title and artist for legacy support
putString(MediaMetadata.METADATA_KEY_TITLE, myData.title)
putString(MediaMetadata.METADATA_KEY_ARTIST, myData.artist)
// A small bitmap for the artwork is also recommended
putBitmap(MediaMetadata.METADATA_KEY_ART, myData.artBitmap)
// Add any other fields you have for your data as well
}
session.setMetadata(metadataBuilder.build())
}
Java
private void updateMetadata(MediaData myData) {
MediaMetadata.Builder metadataBuilder = new MediaMetadata.Builder();
// To provide most control over how an item is displayed set the
// display fields in the metadata
metadataBuilder.putString(MediaMetadata.METADATA_KEY_DISPLAY_TITLE,
myData.displayTitle);
metadataBuilder.putString(MediaMetadata.METADATA_KEY_DISPLAY_SUBTITLE,
myData.displaySubtitle);
metadataBuilder.putString(MediaMetadata.METADATA_KEY_DISPLAY_ICON_URI,
myData.artUri);
// And at minimum the title and artist for legacy support
metadataBuilder.putString(MediaMetadata.METADATA_KEY_TITLE,
myData.title);
metadataBuilder.putString(MediaMetadata.METADATA_KEY_ARTIST,
myData.artist);
// A small bitmap for the artwork is also recommended
metadataBuilder.putBitmap(MediaMetadata.METADATA_KEY_ART,
myData.artBitmap);
// Add any other fields you have for your data as well
session.setMetadata(metadataBuilder.build());
}
响应用户操作
当用户选择“正在播放”卡片时，系统会打开拥有该会话的应用。如果您的应用向 setSessionActivity() 提供了 PendingIntent，系统会启动您指定的 Activity，如下所示。否则，系统将会打开默认系统 Intent。您指定的 Activity 必须提供允许用户暂停或停止播放的播放控件。
Kotlin
val pi: PendingIntent = Intent(context, MyActivity::class.java).let { intent ->
PendingIntent.getActivity(
context, 99 /*request code*/,
intent,
PendingIntent.FLAG_UPDATE_CURRENT
)
}
session.setSessionActivity(pi)
Java
Intent intent = new Intent(context, MyActivity.class);
PendingIntent pi = PendingIntent.getActivity(context, 99 /*request code*/,
intent, PendingIntent.FLAG_UPDATE_CURRENT);
session.setSessionActivity(pi);