總體流程

下載一個媒體檔案的流程

1、初始化
2、根據URL下載m3u8檔案（即PlayList檔案）
3、判斷m3u8檔案的型別，是Master PlayList還是Media PlayList
4、如果是Master PlayList，那麼分析這個PlayList的內容，並選擇一個合適的Media PlayList，注意這一步只是分析Master PlayList的內容，並沒有下載Media PlayList
5、處理Media PlayList，因為步驟3下載的可能就是一個Media PlayList，因此，在處理之前先把舊的內容釋放掉，然後重新下載Media PlayList，再分析Media PlayList裡面的內容
6、Media PlayList分析完成之後，就得到TS媒體檔案的資訊了
7、根據TS媒體檔案資訊，下載TS檔案
8、清理

程式碼實現

// 下載一個視訊
HRESULT HLSDownloader::download_hls_video(std::string surl,std::string filename,bool livestreaming){

	curl_global_init(CURL_GLOBAL_ALL);

    av_register_all(); // ffmpge的函式，註冊所有的編解碼器

    const char * url = surl.c_str();
    char *hlsfile_source;
    struct hls_media_playlist media_playlist;

	// 從伺服器上下載m3u8檔案（Master PlayList或者Media PlayList）
    if (get_data_from_url(url, &hlsfile_source, NULL, STRING,m_strUserAgent) == 0) {
        Log(LogID_MMHLSDownloader, LogLevel_Error, __FUNCTION__,
                    "");
        MSG_ERROR("No result from server.\n");
        return E_FAIL;
    }

	// 獲取m3u8檔案的型別（是Master PlayList還是Media PlayList）
    int playlist_type = get_playlist_type(hlsfile_source);

	// 如果是Master PlayList
    if (playlist_type == MASTER_PLAYLIST) {
        struct hls_master_playlist master_playlist;
        master_playlist.source = hlsfile_source;
        master_playlist.url = strdup(url);
		// 讀取Master PlayList中的內容並分析
        if (handle_hls_master_playlist(&master_playlist)) {
            return E_FAIL;
        }

        int max = 0;
        for (int i = 0; i < master_playlist.count; i++) {
            if (master_playlist.media_playlist[i].bitrate > master_playlist.media_playlist[max].bitrate) {
                max = i;
            }
        }
        
        int quality_choice = max;

		// 選取一個Media PlayList
        media_playlist = master_playlist.media_playlist[quality_choice];
        master_playlist_cleanup(&master_playlist);
    } else if (playlist_type == MEDIA_PLAYLIST) {
		// 如果是Media PlayList
        media_playlist.bitrate = 0;
        media_playlist.url = strdup(url);
    } else {
        return E_FAIL;
    }

	// 處理Media PlayList
    if (handle_hls_media_playlist(&media_playlist,m_strUserAgent)) {
        return E_FAIL;
    }

    if (media_playlist.encryption) {
        Log(LogID_MMHLSDownloader, LogLevel_Info, __FUNCTION__,
                    "HLS Stream is %s encrypted.",
                  media_playlist.encryptiontype == ENC_AES128 ? "AES-128" : "SAMPLE-AES");
    }

    Log(LogID_MMHLSDownloader, LogLevel_Info, __FUNCTION__,
                        "Media Playlist parsed successfully");

	// 下載HLS視訊
    if (download_hls(&media_playlist,livestreaming,m_iMaxSize,filename.c_str(),m_strUserAgent,true)) {
        Log(LogID_MMHLSDownloader, LogLevel_Error, __FUNCTION__,
                        "Download HLS Failed");
        return E_FAIL;
    }
    
    media_playlist_cleanup(&media_playlist);
    curl_global_cleanup();

    return S_OK;
}