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实现在Windows Phone Silverlight应用中播放FLV视频（下）

August 3rd, 2014 |

Author:

暗影吉他手

实现GetSampleAsync()

接下来需要实现MediaStreamSource.GetSampleAsync()。当系统解码器需要新的视频或音频sample的时候会调用这个函数，这个函数需要根据MediaStreamType返回相应的下一个sample。至于是预加载所有的sample还是用on demand的方式获取下一个sample都可以，前者可能会在播放视频的头几秒有所卡顿，后者可能会导致seek的过程略长。

以下代码演示on demand的方式。代码风格很糟糕，仅供演示用：

private MediaStreamSample GetAudioSample()
{
MediaStreamSample mediaStreamSample = new MediaStreamSample(this.audioStreamDescription, null, 0, 0, 0, emptyDict);
try
{
var sample = audioSamples[audioStreamIndex];
mediaStreamSample = new MediaStreamSample(this.audioStreamDescription, this.mediaStream, sample.Offset, sample.Count, sample.Timestamp, emptyDict);
}
catch { }
}

private MediaStreamSample GetVideoSample()
{
MediaStreamSample mediaStreamSample = new MediaStreamSample(this.videoStreamDescription, null, 0, 0, 0, emptyDict);

try
{
var sample = videoSamples[videoStreamIndex];
MemoryStream stream = new MemoryStream();

if (videoStreamIndex == 1)
{
// 处理第二个视频帧（即第一个包含实际视频数据的视频帧，第一个视频帧用来OpenMediaAsync了）
mediaStream.Seek(videoSamples[0].Offset, SeekOrigin.Begin);
BinaryReader binaryReader = new BinaryReader(mediaStream);

var configurationVersion = binaryReader.ReadByte();
var avcProfileIndication = binaryReader.ReadByte();
var avcCompatibleProfiles = binaryReader.ReadByte();
var avcLevelIndication = binaryReader.ReadByte();
byte lengthSizeMinusOne = binaryReader.ReadByte();
var naluLengthSize = (byte)(1 + (lengthSizeMinusOne & 3));

byte nSPS = (byte)(binaryReader.ReadByte() & 0x1f);
var sPs = new List<byte[]>(nSPS);
for (uint i = 0; i < nSPS; i++)
{
byte[] buffer = new byte[Mp4Util.BytesToUInt16BE(binaryReader.ReadBytes(2))];
mediaStream.Read(buffer, 0, buffer.Length);
sPs.Add(buffer);
}

byte nPPS = binaryReader.ReadByte();
var pPs = new List<byte[]>(nPPS);
for (uint i = 0; i < nPPS; i++)
{
byte[] buffer = new byte[Mp4Util.BytesToUInt16BE(binaryReader.ReadBytes(2))];
mediaStream.Read(buffer, 0, buffer.Length);
pPs.Add(buffer);
}

var sps = sPs[0];
var pps = pPs[0];

stream.Write(startCode, 0, startCode.Length);
stream.Write(sps, 0, sps.Length);
stream.Write(startCode, 0, startCode.Length);
stream.Write(pps, 0, pps.Length);
}

mediaStream.Seek(sample.Offset, SeekOrigin.Begin);

BinaryReader reader = new BinaryReader(this.mediaStream);
var sS = sample.Count;
ulong count;

while (sS > 4L)
{
var ui32 = reader.ReadUInt32();
count = OldSkool.swaplong(ui32);
stream.Write(startCode, 0, startCode.Length);
stream.Write(reader.ReadBytes((int)count), 0, (int)count);
sS -= 4 + (uint)count;
}

mediaStreamSample = new MediaStreamSample(this.videoStreamDescription, stream, 0, stream.Length, sample.Timestamp, emptyDict);
}
catch { }
return mediaStreamSample;
}

protected override void GetSampleAsync(MediaStreamType mediaStreamType)
{
MediaStreamSample mediaStreamSample = null;
FlvTag s = null;

lock (syncRootGetSample)
{
if (mediaStreamType == MediaStreamType.Audio)
{
if (audioStreamIndex <= audioSamples.Count - 1)
{
mediaStreamSample = this.GetAudioSample();
audioStreamIndex++;
}
else
{
try
{
while (audioStreamIndex > audioSamples.Count - 1)
{
s = new FlvTag(this.mediaStream, ref fileOffset);
if (s.TagType == TagType.Audio)
audioSamples.Add(s);
else if (s.TagType == TagType.Video)
videoSamples.Add(s);
}
mediaStreamSample = this.GetAudioSample();
audioStreamIndex++;
}
catch
{
// 音频流末尾
mediaStreamSample = new MediaStreamSample(this.audioStreamDescription, null, 0, 0, 0, emptyDict);
Debug.WriteLine("audio end: " + audioStreamIndex + "," + audioSamples.Count);
}
}
}
else if (mediaStreamType == MediaStreamType.Video)
{
if (videoStreamIndex <= videoSamples.Count - 1)
{
mediaStreamSample = this.GetVideoSample();
videoStreamIndex++;
}
else
{
try
{
while (videoStreamIndex > videoSamples.Count - 1)
{
s = new FlvTag(this.mediaStream, ref fileOffset);
if (s.TagType == TagType.Audio)
audioSamples.Add(s);
else if (s.TagType == TagType.Video)
videoSamples.Add(s);
}
mediaStreamSample = this.GetVideoSample();
videoStreamIndex++;
}
catch
{
// 视频流末尾
mediaStreamSample = new MediaStreamSample(this.videoStreamDescription, null, 0, 0, 0, emptyDict);
Debug.WriteLine("video end: " + videoStreamIndex + "," + videoSamples.Count);
}
}
}
}

this.ReportGetSampleCompleted(mediaStreamSample);
}

实现SeekAsync()

seek的原理很简单，根据传来的timestamp找到相对最近的视频和音频sample然后传给系统解码器即可：

protected override void SeekAsync(long seekToTime)
{
if (seekToTime > 0)
{
lock (syncRootGetSample)
{
// 如果seekToTime超过正在播放的时间点，需要继续填充sample list
try
{
while (videoSamples[videoSamples.Count - 1].Timestamp < seekToTime)
{
var s = new FlvTag(this.mediaStream, ref fileOffset);
if (s.TagType == TagType.Audio)
audioSamples.Add(s);
else if (s.TagType == TagType.Video)
videoSamples.Add(s);
}
}
catch { }

try
{
while (audioSamples[audioSamples.Count - 1].Timestamp < seekToTime)
{
var s = new FlvTag(this.mediaStream, ref fileOffset);
if (s.TagType == TagType.Audio)
audioSamples.Add(s);
else if (s.TagType == TagType.Video)
videoSamples.Add(s);
}
}
catch { }

// 开始seek
try
{
for (int i = videoSamples.Count - 1; i >= 0; i--)
{
if (videoSamples[i].Timestamp <= seekToTime)
{
videoStreamIndex = i;
break;
}
}
}
catch { }

try
{
for (int i = audioSamples.Count - 1; i >= 0; i--)
{
if (audioSamples[i].Timestamp <= seekToTime)
{
audioStreamIndex = i;
break;
}
}
}
catch { }
}
}
this.ReportSeekCompleted(seekToTime);
}

实现CloseMedia()

在这里释放资源，清除sample list，关闭mediaStream。

结语

需要说明的是，该方法只在Windows Phone Silverlight应用中有效，在针对Windows Phone 8.1的RT应用中需要改变实现方式，不过变化不是特别大，参考MSDN上的相关例程再开动一下脑筋很容易就能移植过去。

关于本系列文章中引用的大量谜之class（如FlvTag等）请自行阅读参考资料[1]。

参考资料：
[1] 在Silverlight应用程序中实现对FLV视频格式的支持

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One Response to “实现在Windows Phone Silverlight应用中播放FLV视频（下）”

praise says:

2014-09-18 at 08:04

求相关完整demo~~~

Reply