記得剛學(xué)Android開(kāi)發(fā)的時(shí)候在2011年敦锌，那個(gè)時(shí)候除了hello world馒疹，第一個(gè)接觸的組件就是這個(gè)，這也是目前Android手機(jī)運(yùn)用到最多的組件乙墙，有人說(shuō)沒(méi)怎么使用過(guò)颖变，目前的各類(lèi)播放器，短視頻听想，都屬于多媒體播放腥刹，作為Android的多媒體開(kāi)始鼻祖當(dāng)屬M(fèi)ediaPlayer了。

回顧下官方的API接口

以官方貼圖為切入點(diǎn)汉买，API地址

• 被重載的方法必須改變參數(shù)列表赵讯；
• 被重載的方法可以改變返回類(lèi)型盈咳；
• 被重載的方法可以改變?cè)L問(wèn)修飾符耿眉；
• 被重載的方法可以聲明新的或更廣的檢查異常边翼；
• 方法能夠在同一個(gè)類(lèi)中或者在一個(gè)子類(lèi)中被重載
  那么如果返回類(lèi)型不一樣算是重載嗎？答案是：不是的

setDataSorece

下面常用方法中鸣剪，列出不同重載后的方法组底。這時(shí)播放器接收到媒體資源后準(zhǔn)備進(jìn)入初始狀態(tài)。這里需要注意的是筐骇，這個(gè)順序不能改變债鸡，setDataSourece一定是播放器處于空閑態(tài)的時(shí)候調(diào)用的，否則就會(huì)出錯(cuò)铛纬。具體我們用代碼來(lái)分析：

 public void setDataSource(FileDescriptor fd, long offset, long length)
            throws IOException, IllegalArgumentException, IllegalStateException {
        try (ParcelFileDescriptor modernFd = FileUtils.convertToModernFd(fd)) {
            if (modernFd == null) {
                _setDataSource(fd, offset, length);
            } else {
                _setDataSource(modernFd.getFileDescriptor(), offset, length);
            }
        } catch (IOException e) {
            Log.w(TAG, "Ignoring IO error while setting data source", e);
        }
    }

非Idle調(diào)用會(huì)拋出IllegalStateException異常厌均，剩下兩個(gè)是在重載的時(shí)候拋出的。至于上面的異常告唆，官方給出注釋了

Sets the data source (MediaDataSource) to use.
Params:
dataSource – the MediaDataSource for the media you want to play
Throws:
IllegalStateException – if it is called in an invalid state
IllegalArgumentException – if dataSource is not a valid MediaDataSource

看下底層的代碼是如何實(shí)現(xiàn)的棺弊，路徑：framework/av/media/mediaplayer.cpp
看起重載后的其中一個(gè)方法：

status_t MediaPlayer::setDataSource(const sp<IDataSource> &source)
{
    ALOGV("setDataSource(IDataSource)");
    status_t err = UNKNOWN_ERROR;
    const sp<IMediaPlayerService> service(getMediaPlayerService());
    if (service != 0) {
        sp<IMediaPlayer> player(service->create(this, mAudioSessionId, mOpPackageName));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(source))) {
            player.clear();
        }
        err = attachNewPlayer(player);
    }
    return err;
}

這里關(guān)注幾個(gè)點(diǎn)就可以
需要獲取mediaplayer的服務(wù)也就是getMediaPlayerService晶密，這里就要來(lái)到經(jīng)常提起的binder機(jī)制了，其實(shí)就是一個(gè)典型的CS模型

• Server端
  BnMediaPlayerService 為MediaPlayerService在server端的具體實(shí)現(xiàn)模她；
• Client端
  BpMediaPlayerService為MediaPlayerService在client端的具體代理稻艰；
  通過(guò)BpMediaPlayerService，可以享用MediaPlayerService所提供的服務(wù)
  通過(guò)BpMediaPlayerService可以獲取到IMediaRecorder侈净、IMediaMetadataRetriever尊勿、IMediaPlayer、IOMX在client端的代理類(lèi)BpXXX（IMediaRecorder畜侦、IMediaMetadataRetriever元扔、IMediaPlayer、IOMX旋膳，這些皆是Bp和Bn的共同接口摇展，和MediaPlayerService一樣，都有Bp和Bn構(gòu)成）
  引用鏈接

status_t MediaPlayer::setDataSource(
        const sp<IMediaHTTPService> &httpService,
        const char *url, const KeyedVector<String8, String8> *headers)
{
    ALOGV("setDataSource(%s)", url);
    status_t err = BAD_VALUE;
    if (url != NULL) {
        const sp<IMediaPlayerService> service(getMediaPlayerService());
        if (service != 0) {
            sp<IMediaPlayer> player(service->create(this, mAudioSessionId, mOpPackageName));
            if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
                (NO_ERROR != player->setDataSource(httpService, url, headers))) {
                player.clear();
            }
            err = attachNewPlayer(player);
        }
    }
    return err;
}

status_t MediaPlayer::setDataSource(int fd, int64_t offset, int64_t length)
{
    ALOGV("setDataSource(%d, %" PRId64 ", %" PRId64 ")", fd, offset, length);
    status_t err = UNKNOWN_ERROR;
    const sp<IMediaPlayerService> service(getMediaPlayerService());
    if (service != 0) {
        sp<IMediaPlayer> player(service->create(this, mAudioSessionId, mOpPackageName));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(fd, offset, length))) {
            player.clear();
        }
        err = attachNewPlayer(player);
    }
    return err;
}

status_t MediaPlayer::setDataSource(const sp<IDataSource> &source)
{
    ALOGV("setDataSource(IDataSource)");
    status_t err = UNKNOWN_ERROR;
    const sp<IMediaPlayerService> service(getMediaPlayerService());
    if (service != 0) {
        sp<IMediaPlayer> player(service->create(this, mAudioSessionId, mOpPackageName));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(source))) {
            player.clear();
        }
        err = attachNewPlayer(player);
    }
    return err;
}

prepareAsync

status_t MediaPlayer::prepareAsync()
{
    ALOGV("prepareAsync");
    //防止重復(fù)調(diào)用溺忧，它的作用域到函數(shù)結(jié)束
    Mutex::Autolock _l(mLock);
    return prepareAsync_l();
}
// must call with lock held
status_t MediaPlayer::prepareAsync_l()
{
    if ( (mPlayer != 0) && ( mCurrentState & (MEDIA_PLAYER_INITIALIZED | MEDIA_PLAYER_STOPPED) ) ) {
        if (mAudioAttributesParcel != NULL) {
            mPlayer->setParameter(KEY_PARAMETER_AUDIO_ATTRIBUTES, *mAudioAttributesParcel);
        } else {
            mPlayer->setAudioStreamType(mStreamType);
        }
        mCurrentState = MEDIA_PLAYER_PREPARING;
        return mPlayer->prepareAsync();
    }
    ALOGE("prepareAsync called in state %d, mPlayer(%p)", mCurrentState, mPlayer.get());
    return INVALID_OPERATION;
}

• prepare
• start
• pause
• stop
• reset
• seekTo
• release
• setDisplay
• setSurface
• setVolume
• getTrackInfo
• selectTrack