Remote video stream black screen in iOS

[YYbrother]

The remote video stream is black, but there is sound.
I found that when I saved the answer locally, I only got the audio type of the ice candidate, but not the video type.I think this is the reason why there is no video intersection between the offer side and the answer side. But when I created the answer, the constraints of the audio and video have been set to true.Please help me find the problem, thank you