分两部分来写

第一部分：前置相机的设置

代码如下：

- (void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];
    [self initCameraAndUI];
    if (self.session != nil && !self.session.isRunning) {
        [self.session startRunning];
    }
}
- (void)viewDidLoad {
    [super viewDidLoad];
}

#pragma mark initDevice && UI
- (void)initCameraAndUI{


    self.session = [[AVCaptureSession alloc]init];
    self.session.sessionPreset = AVCaptureSessionPreset640x480;
    NSArray *availableCameraDevices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];

    //前置摄像头
    for (AVCaptureDevice *device in availableCameraDevices) {

        if (device.position == AVCaptureDevicePositionFront) {
            self.frontDevice = device;
        }
    }

    //input
    AVCaptureDeviceInput *cameraInput = [AVCaptureDeviceInput deviceInputWithDevice:self.frontDevice error:nil];
    if ([self.session canAddInput:cameraInput]) {
        [self.session addInput:cameraInput];
    }

    //output
    AVCaptureVideoDataOutput *output = [[AVCaptureVideoDataOutput alloc]init];
    [output setSampleBufferDelegate:self queue:dispatch_queue_create("myQueue", NULL)];
    output.videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                            [NSNumber numberWithInt:kCVPixelFormatType_32BGRA], kCVPixelBufferPixelFormatTypeKey,
                            [NSNumber numberWithInt: 320], (id)kCVPixelBufferWidthKey,
                            [NSNumber numberWithInt: 240], (id)kCVPixelBufferHeightKey,
                            nil];
    if ([self.session canAddOutput:output]) {
        [self.session addOutput:output];
    }

    self.previewLayer = [[AVCaptureVideoPreviewLayer alloc]initWithSession:self.session];
    self.previewLayer.frame = self.cameraView.bounds;
    self.previewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;
    [self.cameraView.layer addSublayer:self.previewLayer];
}

- (void)viewDidDisappear:(BOOL)animated{
    [super viewDidDisappear:animated];

    if (self.session != nil && self.session.isRunning) {
        [self.session stopRunning];
    }
    
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];

    if (self.session != nil && self.session.isRunning) {
        [self.session stopRunning];
    }
}

第二部分：人脸检测

将前置相机捕获到的数据流转化为Image：

#pragma mark -- AVCaptureVideoDataOutputSampleBufferDelegate
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection{
    self.yuvImage = [GetYUVImage sampleBufferToImageWithSampleBuffer:sampleBuffer];
}

转化Image的方法：

+ (UIImage *)sampleBufferToImageWithSampleBuffer:(CMSampleBufferRef)sampleBuffer{

    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
    CVPixelBufferLockBaseAddress(imageBuffer, 0);

    void *baseAddress = CVPixelBufferGetBaseAddress(imageBuffer);

    size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
    size_t width = CVPixelBufferGetWidth(imageBuffer);
    size_t height = CVPixelBufferGetHeight(imageBuffer);

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    CGContextRef context = CGBitmapContextCreate(baseAddress, width, height, 8,
                                                 bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst);
    CGImageRef quartzImage = CGBitmapContextCreateImage(context);
    CVPixelBufferUnlockBaseAddress(imageBuffer,0);

    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);

    UIImage *image =  [UIImage imageWithCGImage:quartzImage scale:1.0 orientation:UIImageOrientationRight];

    CGImageRelease(quartzImage);

    return (image);
}

开始检测:

#pragma mark -开始人脸识别
- (void)beginDetectorFacewithImage:(UIImage *)imageA
{

    CIImage       *image    = [CIImage imageWithCGImage:imageA.CGImage];
    NSDictionary  *opts     = [NSDictionary dictionaryWithObject:CIDetectorAccuracyHigh forKey:CIDetectorAccuracy];
    CIDetector    *detector = [CIDetector detectorOfType:CIDetectorTypeFace context:nil options:opts];

    //得到人体面部数据
    NSArray* features = [detector featuresInImage:image];
    for (CIFaceFeature *f in features)
    {
        CGRect aRect = f.bounds;
        NSLog(@"%f, %f, %f, %f", aRect.origin.x, aRect.origin.y, aRect.size.width, aRect.size.height);

        //眼睛和嘴的位置
        if(f.hasLeftEyePosition) {
            NSString *leftEyeString = [NSString stringWithFormat:@"LeftEye---> %g %g\n", f.leftEyePosition.x, f.leftEyePosition.y];
            NSLog(@"%@\n", leftEyeString);
        }
        if(f.hasRightEyePosition) {
            NSString *rightEyeString = [NSString stringWithFormat:@"RightEye---> %g %g\n",  f.rightEyePosition.x, f.rightEyePosition.y];
            NSLog(@"%@\n", rightEyeString);
        }
        if(f.hasMouthPosition) {
            NSString *mouthString = [NSString stringWithFormat:@"Mouth---> %g %g\n",  f.mouthPosition.x, f.mouthPosition.y];
            NSLog(@"%@\n", mouthString);
        }
    }
}

大致步骤就是这么多，接下来可以尝试一下更详细的做法。