查找流

媒体流容器通常具有若干流，例如视频流和音频流。例如，你可以使用以下方法获取音频流：

// A Format Context - see Reading Data for more info
AVFormatContext *formatContext;

// Inspect packets of stream to determine properties
if (avformat_find_stream_info(formatContext, NULL) < 0){
    // Error finding info
}

// Find the stream and its codec
AVCodec* audioCodec;
int audioStreamIndex = av_find_best_stream(
    formatContext,        // The media stream
    AVMEDIA_TYPE_AUDIO,   // The type of stream we are looking for - audio for example
    -1,                   // Desired stream number, -1 for any
    -1,                   // Number of related stream, -1 for none
    &audioCodec,          // Gets the codec associated with the stream, can be NULL
    0                     // Flags - not used currently
);
if(audioStreamIndex = AVERROR_STREAM_NOT_FOUND || !audioCodec){
    // Error finding audio (ie. no audio stream?)
}

要获取其他类型的流，你只需要替换流的类型。以下是有效类型：

AVMEDIA_TYPE_VIDEO,
AVMEDIA_TYPE_AUDIO,
AVMEDIA_TYPE_SUBTITLE,
AVMEDIA_TYPE_DATA,        // Usually continuous
AVMEDIA_TYPE_ATTACHMENT,  // Usually sparse