Gives an example of how to initialize an video capture session

AVCaptureSessionSetup.m

// MEModel.h
@interface MEModel : NSObject

@property (nonatomic, strong) AVCaptureSession *session;
@property (nonatomic, strong) AVCaptureDevice *frontCamera;
@property (nonatomic, strong) AVCaptureDevice *backCamera;
@property (nonatomic, strong) AVCaptureDeviceInput *inputDevice;
@property (nonatomic, strong) AVCaptureMovieFileOutput *videoFileOutput;
@property (nonatomic, strong) AVCaptureVideoPreviewLayer *previewLayer; // Once the session is setup, this can just easily be added to any view. i.e. [self.view.layer  addSublayer:[[MEModel sharedInstance] previewLayer];

@end

// MEModel.m
#import "MEModel.h"

@implementation MEModel

+ (instancetype)sharedInstance
{
    static MEModel *instance = nil;
    
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[MEModel alloc] init];
    });
    
    return instance;
}

- (instancetype)init
{
    self = [super init];
    if (self) {
        [self initializeCaptureSession];
    }
    return self;
}

#pragma mark -
#pragma mark AVFoundation Setup
- (void)initializeCaptureSession
{
    self.session = [[AVCaptureSession alloc] init];
    
    [self initializeCameraReferences];
    
    // Video
    self.videoFileOutput = [[AVCaptureMovieFileOutput alloc] init];
    [self.session addOutput:self.videoFileOutput];
    
    self.previewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:self.session];
    [self.previewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
    
    [self.session startRunning];
    [self beginRecordingWithDevice:self.frontCamera];
}

- (void)initializeCameraReferences
{
    NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
    
    for (AVCaptureDevice *device in devices)
    {
        if (device.position == AVCaptureDevicePositionBack) {
            self.backCamera = device;
        }
        else if (device.position == AVCaptureDevicePositionFront) {
            self.frontCamera = device;
        }
    }
}

- (void)beginRecordingWithDevice:(AVCaptureDevice *)device
{
    [self.session stopRunning];
    
    if (self.inputDevice) {
        [self.session removeInput:self.inputDevice];
    }
    
    NSError *error = nil;
    self.inputDevice = [AVCaptureDeviceInput deviceInputWithDevice:device error:&error];
    if (error) {
        NSLog(@"Error: %@", error.debugDescription);
        return;
    }
    
    [self.session addInput:self.inputDevice];
    
    self.session.sessionPreset = AVCaptureSessionPreset640x480; // Replace with whichever preset you want
    [self.session startRunning];
}

- (void)toggleCameras
{
    BOOL isBackFacing = (self.inputDevice.device == self.backCamera);
    [self.session stopRunning];
    
    if (isBackFacing) {
        [self beginRecordingWithDevice:self.frontCamera];
    }
    else {
        [self beginRecordingWithDevice:self.backCamera];
    }
}

@end