从上文 Android音视频——NuPlayer框架的图来看
可以看到 NuPlayer 的解析模块主要是 NuPlayerSource和继承自它的HTTPLiveSource、RTSPSource、GenericSource等几个类。
在 NuPlaye r中调用 setDataSourceAsync ：
void NuPlayer::setDataSourceAsync( const sp<IMediaHTTPService> &httpService, const char *url, const KeyedVector<String8, String8> *headers) { sp<AMessage> msg = new AMessage(kWhatSetDataSource, this); size_t len = strlen(url); sp<AMessage> notify = new AMessage(kWhatSourceNotify, this); sp<Source> source; if (IsHTTPLiveURL(url)) { source = new HTTPLiveSource(notify, httpService, url, headers); } else if (!strncasecmp(url, "rtsp://", 7)) { source = new RTSPSource( notify, httpService, url, headers, mUIDValid, mUID); } else if ((!strncasecmp(url, "http://", 7) || !strncasecmp(url, "https://", 8)) && ((len >= 4 && !strcasecmp(".sdp", &url[len - 4])) || strstr(url, ".sdp?"))) { source = new RTSPSource( notify, httpService, url, headers, mUIDValid, mUID, true); } else { sp<GenericSource> genericSource = new GenericSource(notify, mUIDValid, mUID); // Don't set FLAG_SECURE on mSourceFlags here for widevine. // The correct flags will be updated in Source::kWhatFlagsChanged // handler when GenericSource is prepared. status_t err = genericSource->setDataSource(httpService, url, headers); if (err == OK) { source = genericSource; } else { ALOGE("Failed to set data source!"); } } msg->setObject("source", source); msg->post(); }
这里会根据不同协议选择不同的Source（视频源）对象，有了这个Source（视频源）对象
后，发送kWhatSetDataSource消息，代码如下：
case kWhatSetDataSource: { ALOGV("kWhatSetDataSource"); CHECK(mSource == NULL); status_t err = OK; sp<RefBase> obj; CHECK(msg->findObject("source", &obj)); if (obj != NULL) { mSource = static_cast<Source *>(obj.get()); } else { err = UNKNOWN_ERROR; } CHECK(mDriver != NULL); sp<NuPlayerDriver> driver = mDriver.promote(); if (driver != NULL) { driver->notifySetDataSourceCompleted(err); } break; }
也就是通过具体的 Source 解析完数据，再把 Source 强制转换成 mSource 给 Decoder 使用，这时里面就包含了数据相关信息，如几个 Track、是什么格式的等。其中的一个HTTPLiveSource，主要用于解析HLS协议。
从图中可以看出HTTPLiveSource主要封装了LiveSession。 LiveSession有** HTTPDownloader ** 模块、M3UParser 模块，以及取播放列表的 PlaylistFetcher 模块。
当调用 HTTPLiveSource 的 prepareAsync 函数时，代码如下：
void NuPlayer::HTTPLiveSource::prepareAsync() { if (mLiveLooper == NULL) { mLiveLooper = new ALooper; mLiveLooper->setName("http live"); mLiveLooper->start(); mLiveLooper->registerHandler(this); } sp<AMessage> notify = new AMessage(kWhatSessionNotify, this); mLiveSession = new LiveSession( notify, (mFlags & kFlagIncognito) ? LiveSession::kFlagIncognito : 0, mHTTPService); mLiveLooper->registerHandler(mLiveSession); mLiveSession->connectAsync( mURL.c_str(), mExtraHeaders.isEmpty() ? NULL : &mExtraHeaders); }
内部主要构建了一个LiveSession对象。通过LiveSession内部的 connectAsync函数，创建
一个会话：
void LiveSession::connectAsync( const char *url, const KeyedVector<String8, String8> *headers) { sp<AMessage> msg = new AMessage(kWhatConnect, this); msg->setString("url", url); if (headers != NULL) { msg->setPointer( "headers", new KeyedVector<String8, String8>(*headers)); } msg->post(); }
接下来是onConnect
void LiveSession::onConnect(const sp<AMessage> &msg) { CHECK(msg->findString("url", &mMasterURL)); // TODO currently we don't know if we are coming here from incognito mode ALOGI("onConnect %s", uriDebugString(mMasterURL).c_str()); KeyedVector<String8, String8> *headers = NULL; if (!msg->findPointer("headers", (void **)&headers)) { mExtraHeaders.clear(); } else { mExtraHeaders = *headers; delete headers; headers = NULL; } // create looper for fetchers if (mFetcherLooper == NULL) { mFetcherLooper = new ALooper(); mFetcherLooper->setName("Fetcher"); mFetcherLooper->start(false, false); } // create fetcher to fetch the master playlist addFetcher(mMasterURL.c_str())->fetchPlaylistAsync(); }
网络请求，开始取播放列表。在取到播放列表后，会进行回调：
void LiveSession::onMasterPlaylistFetched(const sp<AMessage> &msg) { AString uri; CHECK(msg->findString("uri", &uri)); ssize_t index = mFetcherInfos.indexOfKey(uri); if (index < 0) { ALOGW("fetcher for master playlist is gone."); return; } // no longer useful, remove mFetcherLooper->unregisterHandler(mFetcherInfos[index].mFetcher->id()); mFetcherInfos.removeItemsAt(index); CHECK(msg->findObject("playlist", (sp<RefBase> *)&mPlaylist)); if (mPlaylist == NULL) { ALOGE("unable to fetch master playlist %s.", uriDebugString(mMasterURL).c_str()); postPrepared(ERROR_IO); return; } // We trust the content provider to make a reasonable choice of preferred // initial bandwidth by listing it first in the variant playlist. // At startup we really don't have a good estimate on the available // network bandwidth since we haven't tranferred any data yet. Once // we have we can make a better informed choice. size_t initialBandwidth = 0; size_t initialBandwidthIndex = 0; int32_t maxWidth = 0; int32_t maxHeight = 0; if (mPlaylist->isVariantPlaylist()) { Vector<BandwidthItem> itemsWithVideo; for (size_t i = 0; i < mPlaylist->size(); ++i) { BandwidthItem item; item.mPlaylistIndex = i; item.mLastFailureUs = -1ll; sp<AMessage> meta; AString uri; mPlaylist->itemAt(i, &uri, &meta); CHECK(meta->findInt32("bandwidth", (int32_t *)&item.mBandwidth)); int32_t width, height; if (meta->findInt32("width", &width)) { maxWidth = max(maxWidth, width); } if (meta->findInt32("height", &height)) { maxHeight = max(maxHeight, height); } mBandwidthItems.push(item); if (mPlaylist->hasType(i, "video")) { itemsWithVideo.push(item); } } // remove the audio-only variants if we have at least one with video if (!itemsWithVideo.empty() && itemsWithVideo.size() < mBandwidthItems.size()) { mBandwidthItems.clear(); for (size_t i = 0; i < itemsWithVideo.size(); ++i) { mBandwidthItems.push(itemsWithVideo[i]); } } CHECK_GT(mBandwidthItems.size(), 0u); initialBandwidth = mBandwidthItems[0].mBandwidth; mBandwidthItems.sort(SortByBandwidth); for (size_t i = 0; i < mBandwidthItems.size(); ++i) { if (mBandwidthItems.itemAt(i).mBandwidth == initialBandwidth) { initialBandwidthIndex = i; break; } } } else { // dummy item. BandwidthItem item; item.mPlaylistIndex = 0; item.mBandwidth = 0; mBandwidthItems.push(item); } mMaxWidth = maxWidth > 0 ? maxWidth : mMaxWidth; mMaxHeight = maxHeight > 0 ? maxHeight : mMaxHeight; mPlaylist->pickRandomMediaItems(); changeConfiguration( 0ll /* timeUs */, initialBandwidthIndex, false /* pickTrack */); }
上面的代码主要是根据 URL返回的 M3U文件，获取对应的 Bandwidthltem，如果熟悉M3U文件，可以知道M3U文件有一级索引和二级索引，M3U一级索引内容举例如下
#EXTM3U #EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=200000 gear1/prog_index.m3u8 #EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=311111 gear2/prog_index.m3u8 #EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=484444 gear3/prog_index.m3u8 #EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=737777 gear4/prog_index.m3u8
其主要作用是动态码率适应，BANDWIDTH 越大，分辨率越高。BANDWIDTH 实际上就是带宽。
例如，想要获得某个 Track的信息，得到视频、音频、字幕相关信息，可以通过LiveSession 的 getTrackInfo 函数：
sp<AMessage> LiveSession::getTrackInfo(size_t trackIndex) const { if (mPlaylist == NULL) { return NULL; } else { if (trackIndex == mPlaylist->getTrackCount() && mHasMetadata) { sp<AMessage> format = new AMessage(); format->setInt32("type", MEDIA_TRACK_TYPE_METADATA); format->setString("language", "und"); format->setString("mime", MEDIA_MIMETYPE_DATA_TIMED_ID3); return format; } return mPlaylist->getTrackInfo(trackIndex); } }
