RTPSenderVideo在整个框架中起到重要的作用，它把采集的数据进行编码，并且在流程中会进行将编码后的数据进行RTP打包，最后发送到网络层

RTPSenderVideo::SendVideo

//对编码数据打包
bool RTPSenderVideo::SendVideo(int payload_type,absl::optional<VideoCodecType> codec_type,uint32_t rtp_timestamp,int64_t capture_time_ms,rtc::ArrayView<const uint8_t> payload,const RTPFragmentationHeader* fragmentation,RTPVideoHeader video_header,absl::optional<int64_t> expected_retransmission_time_ms) {
#if RTC_TRACE_EVENTS_ENABLEDTRACE_EVENT_ASYNC_STEP1("webrtc", "Video", capture_time_ms, "Send", "type",FrameTypeToString(video_header.frame_type));
#endifRTC_CHECK_RUNS_SERIALIZED(&send_checker_);//如果是空帧直接返回if (video_header.frame_type == VideoFrameType::kEmptyFrame)return true;//如果有效荷载没有内容，直接返回falseif (payload.empty())return false;int32_t retra