当使用ios版webrtc objective-c的api时，我们使用RTCEAGLVideoView来显示远端图像

@interface RTCEAGLVideoView : UIView <RTCVideoRenderer>

@property(nonatomic, weak) id<RTCEAGLVideoViewDelegate> delegate;

@end

我们可以实现并设置RTCEAGLVideoViewDelegate

@protocol RTCEAGLVideoViewDelegate

- (void)videoView:(RTCEAGLVideoView *)videoView didChangeVideoSize:(CGSize)size;

@end

这样当远端或者本地的图像尺寸发生改变时，会就有回调产生。
这样我们就可以在尺寸变化后调整view的布局，避免造成图像拉伸。

产生这个回调的流程是：
1、webrtc解码图像后，需要渲染，会调用VideoRendererAdapter的以下代码

class VideoRendererAdapter
    : public rtc::VideoSinkInterface<cricket::VideoFrame> {
 public:
  VideoRendererAdapter(RTCVideoRendererAdapter* adapter) {
    adapter_ = adapter;
    size_ = CGSizeZero;
  }

  void OnFrame(const cricket::VideoFrame& nativeVideoFrame) override {
    RTCVideoFrame* videoFrame = [[RTCVideoFrame alloc]
        initWithVideoBuffer:nativeVideoFrame.video_frame_buffer()
                   rotation:nativeVideoFrame.rotation()
                timeStampNs:nativeVideoFrame.timestamp_us() *
                                rtc::kNumNanosecsPerMicrosec];
    CGSize current_size = (videoFrame.rotation % 180 == 0)
                              ? CGSizeMake(videoFrame.width, videoFrame.height)
                              : CGSizeMake(videoFrame.height, videoFrame.width);

    if (!CGSizeEqualToSize(size_, current_size)) {
      size_ = current_size;
      [adapter_.videoRenderer setSize:size_];
    }
    [adapter_.videoRenderer renderFrame:videoFrame];
  }

 private:
  __weak RTCVideoRendererAdapter *adapter_;
  CGSize size_;
};
}

2、然后会调用RTCEAGLVideoView中的setSize

- (void)setSize:(CGSize)size {
  __weak RTCEAGLVideoView *weakSelf = self;
  dispatch_async(dispatch_get_main_queue(), ^{
    RTCEAGLVideoView *strongSelf = weakSelf;
    [strongSelf.delegate videoView:strongSelf didChangeVideoSize:size];
  });
}

3、setSize中就会调用RTCEAGLVideoViewDelegate的didChangeVideoSize方法