检查 AVAuthorizationStatus ，然后处理相应的情况

Check AVAuthorizationStatus for camera availability and then handle cases accordingly

AVAuthorizationStatus status = [AVCaptureDevice authorizationStatusForMediaType:AVMediaTypeVideo];
if(status == AVAuthorizationStatusAuthorized) {
  // authorized
} else if(status == AVAuthorizationStatusDenied){
  // denied
} else if(status == AVAuthorizationStatusRestricted){
  // restricted
} else if(status == AVAuthorizationStatusNotDetermined){
  // not determined
  [AVCaptureDevice requestAccessForMediaType:AVMediaTypeVideo completionHandler:^(BOOL granted) {
    if(granted){
  NSLog(@"Granted access");
} else {
  NSLog(@"Not granted access");
}
  }];
}