Question:

iOS 10: “[App] if we're in the real pre-commit handler we can't actually add any new fences due to CA restriction”

Answer:

in your Xcode:
Click on your active scheme name right next to the Stop button
Click on Edit Scheme....
in Run (Debug) select the Arguments tab
in Environment Variables click +
add variable: OS_ACTIVITY_MODE = disable

References:

1. Xcode8取消运行时打印的数据
2. iOS 10: “[App] if we're in the real pre-commit handler we can't actually add any new fences due to CA restriction”