在Mac平台下，WebRTC的demo默认使用VideoToolbox进行h264硬件编码。本文档的目标是调用OpenH264进行软编码。

配置与H264相关的宏

两条命令，第一条生成xcode下的工程文件，第二条查看可用的args选项。

gn gen --args='target_cpu="x64" proprietary_codecs=true rtc_use_h264=true use_openh264=true' --ide=xcode <out_dir>
gn args --list <out_dir>

The variable "rtc_use_264" was set as a build argument
but never appeared in a declare_args() block in any buildfile.
To view all possible args, run "gn args --list <out_dir>"
The build continued as if that argument was unspecified.

修改Demo

Demo调用硬编码的代码如下:

std::unique_ptr<VideoEncoder> ObjCVideoEncoderFactory::CreateVideoEncoder(
    const SdpVideoFormat &format) {
  RTCVideoCodecInfo *info = [[RTCVideoCodecInfo alloc] initWithNativeSdpVideoFormat:format];
  id<RTCVideoEncoder> encoder = [encoder_factory_ createEncoder:info];
  if ([encoder isKindOfClass:[RTCWrappedNativeVideoEncoder class]]) {
      // [read-note] 软编码实例化：vp8、vp9，还有这次加的OpenH264
    return [(RTCWrappedNativeVideoEncoder *)encoder releaseWrappedEncoder];
  } else {
      // [read-note] 硬编码实例化：videotoolbox-h264
      return std::unique_ptr<ObjCVideoEncoder>(new ObjCVideoEncoder(encoder));
  }
}

如上，vp8、vp9编码时调用RTCWrappedNativeVideoEncoder类，而H264实现的RTCVideoEncoderH264调用VT进行硬编码。所以思路是再封装一个RTCWrappedNativeVideoEncoder类进行OpenH264软编码。增加两个文件：RTCVideoEncoderOpenH264.h、RTCVideoEncoderOpenH264.mm，再创建Encoder时调用RTCVideoEncoderOpenH264，如下：

- (id<RTCVideoEncoder>)createEncoder:(RTCVideoCodecInfo *)info {
  if ([info.name isEqualToString:kRTCVideoCodecH264Name]) {
    //return [[RTCVideoEncoderH264 alloc] initWithCodecInfo:info];
      return [RTCVideoEncoderOpenH264 openH264Encoder];
  } else if ([info.name isEqualToString:kRTCVideoCodecVp8Name]) {
    return [RTCVideoEncoderVP8 vp8Encoder];
#if defined(RTC_ENABLE_VP9)
  } else if ([info.name isEqualToString:kRTCVideoCodecVp9Name]) {
    return [RTCVideoEncoderVP9 vp9Encoder];
#endif
  }

  return nil;
}

修改编译配置

由于Demo使用gn构建，ninja编译，所以需要修改gn脚本把刚才添加的两个文件包含到项目中。
修改sdk/build.gn

rtc_static_library("default_codec_factory_objc") {
       sources = [
         "objc/components/video_codec/RTCDefaultVideoDecoderFactory.h",
         "objc/components/video_codec/RTCDefaultVideoDecoderFactory.m",
         "objc/components/video_codec/RTCDefaultVideoEncoderFactory.h",
         "objc/components/video_codec/RTCDefaultVideoEncoderFactory.m",
       ]

       deps = [
         ":base_objc",
         ":native_video",
         ":videocodec_objc",
         ":videotoolbox_objc",
         ":vp8",
         ":OpenH264",
         ":vp9",
         ":vpx_codec_constants",
       ]
     }

... ...
 rtc_static_library("OpenH264") {
      visibility = [ "*" ]
      allow_poison = [ "software_video_codecs" ]
      sources = [
        "objc/api/video_codec/RTCVideoEncoderOpenH264.h",
        "objc/api/video_codec/RTCVideoEncoderOpenH264.mm",
      ]

     deps = [
        ":base_objc",
        ":wrapped_native_codec_objc",
        "../modules/video_coding:webrtc_h264",
      ]
    }

最终验证可以调用OpenH264编码，解码使用VT硬解码。