// 判断设备是否有摄像头
- (BOOL) isCameraAvailable{
return [UIImagePickerController isSourceTypeAvailable:UIImagePickerControllerSourceTypeCamera];
}

// 前面的摄像头是否可用
- (BOOL) isFrontCameraAvailable{
return [UIImagePickerControllerisCameraDeviceAvailable:UIImagePickerControllerCameraDeviceFront];
}

// 后面的摄像头是否可用
- (BOOL) isRearCameraAvailable{
return [UIImagePickerControllerisCameraDeviceAvailable:UIImagePickerControllerCameraDeviceRear];
}

- (AVCaptureDevice *)cameraWithPosition : (AVCaptureDevicePosition) position
{
NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
for (AVCaptureDevice *device in devices )
if ( device.position == position )
return device;

return nil ;
}

- (void) swapFrontAndBackCameras {
//check for available cameras!
if (![self hasMultipleCameras]) return;

//assumes session is running
NSArray *inputs = self.captureSession.inputs; //should only be one value!
for ( AVCaptureDeviceInput *captureDeviceInput in inputs ) {
AVCaptureDevice *device = captureDeviceInput.device ;
if ( [device hasMediaType:AVMediaTypeVideo ] ) {
AVCaptureDevicePosition position = device.position ;
AVCaptureDevice *newCamera = nil ;
AVCaptureDeviceInput *newInput = nil ;

if (position == AVCaptureDevicePositionFront)
newCamera = [self cameraWithPosition:AVCaptureDevicePositionBack];
else
newCamera = [self cameraWithPosition:AVCaptureDevicePositionFront];

[self initializeCaptureDevice:newCamera];
newInput = [AVCaptureDeviceInput deviceInputWithDevice:newCamera error:nil];

// beginConfiguration ensures that pending changes are not applied immediately
[self.captureSession beginConfiguration ];

[self.captureSession removeInput:captureDeviceInput]; //remove current
[self.captureSession addInput:newInput]; //add new

// Changes take effect once the outermost commitConfiguration is invoked.
[self.captureSession commitConfiguration];
break ;
}
}
}