ios7 UIImagePicker预览黑屏幕

When i try to load camera from my code, camera preview is black. If I wait for 10-20 seconds it will show real camera preview. I found several questions and some of them suggest that running some other code in background should be the reason for this. However I don't have any code running in background. How should I fix this?When i try to load camera from my code, camera