MediaPlayer(四)--MediaPlayer()流程

小蚂蚁与大象

发布于 2020-04-17 17:42:05

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发布于 2020-04-17 17:42:05

文章被收录于专栏：构建FFmpeg Android播放器

基于Android8.1代码

java MediaPlayer()

先从源头开始看

frameworks/base/media/java/android/media/MediaPlayer.java

    public MediaPlayer() {
        super(new AudioAttributes.Builder().build(),
                AudioPlaybackConfiguration.PLAYER_TYPE_JAM_MEDIAPLAYER);

        //1
        Looper looper;
        if ((looper = Looper.myLooper()) != null) {
            mEventHandler = new EventHandler(this, looper);
        } else if ((looper = Looper.getMainLooper()) != null) {
            mEventHandler = new EventHandler(this, looper);
        } else {
            mEventHandler = null;
        }

        //2
        mTimeProvider = new TimeProvider(this);
        //3
        mOpenSubtitleSources = new Vector<InputStream>();

       //4
        /* Native setup requires a weak reference to our object.
         * It's easier to create it here than in C++.
         */
        native_setup(new WeakReference<MediaPlayer>(this));
        //5
        baseRegisterPlayer();
    }

1 创建EventHandler（重点）

后面jni回调java时会调用到postEventFromNative, postEventFromNative会将消息发给handler处理

2 mTimeProvider

对这个不太了解，这是MediaPlayer的一个内部类，继承自MediaTimeProvider，好像是一个用了提供Meida 播放，seek，buffer时间数据的类。

3 mOpenSubtitleSources

字幕源

4 native_setup（重点）

是对 native层MediaPlayer和回调listener的创建和初始化

5 baseRegisterPlayer()

这个函数在MediaPlayer的父类PlayerBase.主要是获取AppOpsService，即Application Operations Service，是关于系统应用权限管理的，这些API不对第三方应用开放。

这里主干创建流程主要关注1 和 4. 即创建handle和native层的MediaPlayer，后续native层即可回调java层接口，java层将信息发送给handle处理

native_setup(new WeakReference<MediaPlayer>(this));

frameworks/base/media/jni/android_media_MediaPlayer.cpp

static void
android_media_MediaPlayer_native_setup(JNIEnv *env, jobject thiz, jobject weak_this)
{
    ALOGV("native_setup");
    sp<MediaPlayer> mp = new MediaPlayer();
    if (mp == NULL) {
        jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
        return;
    }

    // create new listener and give it to MediaPlayer
    sp<JNIMediaPlayerListener> listener = new JNIMediaPlayerListener(env, thiz, weak_this);
    mp->setListener(listener);

    // Stow our new C++ MediaPlayer in an opaque field in the Java object.
    setMediaPlayer(env, thiz, mp);
}

native_setup做了几件事情，

1 创建native层播放器实例MediaPlayer()

2 创建了JNIMediaPlayerListener，将java的MediaPlayer实例传递进去

3 将JNIMediaPlayerListener 设置给MediaPlayer()

4 将FFMediaPlayer 设置给Java层的mNativeContext

后面native层的MediaPlayer 就通过JNIMediaPlayerListener 回调java层接口。JNIMediaPlayerListener 能够访问全局变量 fields.post_event，同时拥有 java的MediaPlayer实例，所以JNIMediaPlayerListener能够回调java层的postEventFromNative，并将MediaPlayer实例传递回去

关于这一部分listener的介绍可以参考

搭建ffmpeg player(一)--搭建上层框架

c++ MediaPlayer()

frameworks/av/media/libmedia/mediaplayer.cpp

MediaPlayer::MediaPlayer()
{
    ALOGV("constructor");
    mListener = NULL;     // MediaPlayerListener, JNIMediaPlayerListener的父类，回调java层
    mCookie = NULL;
    mStreamType = AUDIO_STREAM_MUSIC;
    mAudioAttributesParcel = NULL;
    mCurrentPosition = -1;
    mCurrentSeekMode = MediaPlayerSeekMode::SEEK_PREVIOUS_SYNC;
    mSeekPosition = -1;
    mSeekMode = MediaPlayerSeekMode::SEEK_PREVIOUS_SYNC;
    mCurrentState = MEDIA_PLAYER_IDLE;    //播放器当前状态 IDLE
    mPrepareSync = false;
    mPrepareStatus = NO_ERROR;
    mLoop = false;
    mLeftVolume = mRightVolume = 1.0;
    mVideoWidth = mVideoHeight = 0;
    mLockThreadId = 0;
    mAudioSessionId = (audio_session_t) 
    AudioSystem::newAudioUniqueId(AUDIO_UNIQUE_ID_USE_SESSION);
    AudioSystem::acquireAudioSessionId(mAudioSessionId, -1);
    mSendLevel = 0;
    mRetransmitEndpointValid = false;
}

在构造函数里将mCurrentState 设置成MEDIA_PLAYER_IDLE

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