How to get both the video output and full photo resolution image in AVFoundation Fram

[beof]

0
down vote
favorite
I use a capture session to coordinate data flow as the apple developer document described. I want to get the video data to verify if the image is good enough to take. If the condition is met, I want to immediately take the Full resolution photo.

However full photo resolution is not supported for video output. So I have to set the AVCapture sessionPreset to AVCaptureSessionPresetLow to capture video image data, then reconfigure the sessionPreset to AVCaptureSessionPresetPhoto when the condition is met. But the reconfiguration takes about two to three seconds, and does not meet the real-time requirements.

If I use two AVCaptureSessions, one for the video capture, the other for the capture the still image, then it seemed not work.

Does anybody have more ideas? Thank you.