以后要往这方面发展啦，好好梳理一下知识 (｡◕ˇ∀ˇ◕)

1.设备授权

- (void)judgeDeviceAuth{
    
    switch ([AVCaptureDevice authorizationStatusForMediaType:AVMediaTypeVideo]) {
        case AVAuthorizationStatusAuthorized:
            NSLog(@"已授权");
            [self initAVCapture];
            break;
            //未进行授权选择<例如: 没有选择退出APP, 再次打开提示>
        case AVAuthorizationStatusNotDetermined:
        {
            [AVCaptureDevice requestAccessForMediaType:AVMediaTypeVideo completionHandler:^(BOOL granted) {
                if (granted) {
                    NSLog(@"已授权");
                    [self initAVCapture];
                    
                }else{
                    NSLog(@"用户拒绝授权摄像头的使用权,返回上一页.请打开\n设置-->隐私/通用等权限设置");
                }
            }];
        }
            break;
            
        default:{
            NSLog(@"用户拒绝授权摄像头的使用权,返回上一页.请打开\n设置-->隐私/通用等权限设置");
        }
            break;
    }
    
}

2.初始化AVCapture

- (void)initAVCapture{
    
    _captureSession = [[AVCaptureSession alloc] init];
    //设置录像的分辨率
    if ([_captureSession canSetSessionPreset:AVCaptureSessionPreset640x480]) {
        [_captureSession canSetSessionPreset:AVCaptureSessionPreset640x480];
    }
    
    [_captureSession beginConfiguration];

    //设置摄像头为后置摄像头
    _videoDevice = [self deviceWithMediaType:AVMediaTypeVideo preferringPosition:AVCaptureDevicePositionBack];

    //视频的输入输出
    [self videoIO];

    //音频的输入
    [self audioIO];

    //创建预览层
    [self previewLayer];

    [_captureSession commitConfiguration];
    
    [_captureSession startRunning];
    
    
}

3.音视频的输入和输出

- (void)videoIO{
    /*---------------input-------------------------*/
    NSError *videoError;

    _videoInput = [[AVCaptureDeviceInput alloc] initWithDevice:_videoDevice error:&videoError];
    if (videoError) {
        NSLog(@"---- 取得摄像头设备时出错 ------ %@",videoError);
        return;
    }
    // 将视频输入对象添加到会话中
    if ([_captureSession canAddInput:_videoInput]) {
        [_captureSession addInput:_videoInput];
    }
    
    /*---------------output------------------------*/

    _movieOutput = [[AVCaptureMovieFileOutput alloc] init];
    
    if ([_captureSession canAddOutput:_movieOutput]) {
        [_captureSession addOutput:_movieOutput];
        AVCaptureConnection *captureConnection = [_movieOutput connectionWithMediaType:AVMediaTypeVideo];
        // 视频稳定设置
        if ([captureConnection isVideoStabilizationSupported]) {
            captureConnection.preferredVideoStabilizationMode = AVCaptureVideoStabilizationModeAuto;
        }
        captureConnection.videoScaleAndCropFactor = captureConnection.videoMaxScaleAndCropFactor;
    }
}

- (void)audioIO{
    
    /*-------------input---------*/
    NSError *audioError;
    // 添加一个音频输入设备
    _audioDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeAudio];
    //  音频输入对象
    _audioInput = [[AVCaptureDeviceInput alloc] initWithDevice:_audioDevice error:&audioError];
    if (audioError) {
        NSLog(@"取得录音设备时出错 ------ %@",audioError);
        return;
    }
    // 将音频输入对象添加到会话中
    if ([_captureSession canAddInput:_audioInput]) {
        [_captureSession addInput:_audioInput];
    }
}

4.创建预览层

- (void)previewLayer{
    [self.view layoutIfNeeded];
    
    // 通过会话 (AVCaptureSession) 创建预览层
    _captureVideoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:_captureSession];
    _captureVideoPreviewLayer.frame = self.view.layer.bounds;
    //有时候需要拍摄完整屏幕大小的时候可以修改这个
    //    _captureVideoPreviewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;
    // 如果预览图层和视频方向不一致,可以修改这个
    _captureVideoPreviewLayer.connection.videoOrientation = [_movieOutput connectionWithMediaType:AVMediaTypeVideo].videoOrientation;
    _captureVideoPreviewLayer.position = CGPointMake(self.view.frame.size.width*0.5,self.view.frame.size.height*0.5);
    
    // 显示在视图表面的图层
    CALayer *layer = self.view.layer;
    layer.masksToBounds = true;
    [self.view layoutIfNeeded];
    [layer addSublayer:_captureVideoPreviewLayer];
    
}

5.获取摄像头

- (AVCaptureDevice *)deviceWithMediaType:(NSString *)mediaType preferringPosition:(AVCaptureDevicePosition)position
{
    NSArray *devices = [AVCaptureDevice devicesWithMediaType:mediaType];
    AVCaptureDevice *captureDevice = devices.firstObject;
    
    for ( AVCaptureDevice *device in devices ) {
        if ( device.position == position ) {
            captureDevice = device;
            break;
        }
    }
    
    return captureDevice;
}

6.摄像头切换

- (void)changeCamera {
    UIButton * changeBtn = [UIButton buttonWithType:UIButtonTypeCustom];
    changeBtn.backgroundColor = [UIColor yellowColor];
    changeBtn.frame = CGRectMake(0, 20, 100, 100);
    [changeBtn addTarget:self action:@selector(changeCanmera:) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:changeBtn];
}

- (void)changeCanmera:(id)sender {
    
    switch (_videoDevice.position) {
        case AVCaptureDevicePositionBack:
            _videoDevice = [self deviceWithMediaType:AVMediaTypeVideo preferringPosition:AVCaptureDevicePositionFront];
            break;
        case AVCaptureDevicePositionFront:
            _videoDevice = [self deviceWithMediaType:AVMediaTypeVideo preferringPosition:AVCaptureDevicePositionBack];
            break;
        default:
            return;
            break;
    }
    
    
    [self changeDevicePropertySafety:^(AVCaptureDevice *captureDevice) {
        NSError *error;
        AVCaptureDeviceInput *newVideoInput = [[AVCaptureDeviceInput alloc] initWithDevice:_videoDevice error:&error];
        
        if (newVideoInput != nil) {
            //必选先 remove 才能询问 canAdd
            [_captureSession removeInput:_videoInput];
            if ([_captureSession canAddInput:newVideoInput]) {
                [_captureSession addInput:newVideoInput];
                _videoInput = newVideoInput;
            }else{
                [_captureSession addInput:_videoInput];
            }
            
        } else if (error) {
            NSLog(@"切换前/后摄像头失败, error = %@", error);
        }
    }];
    
    
    
}

7.更改设备属性前一定要锁上

-(void)changeDevicePropertySafety:(void (^)(AVCaptureDevice *captureDevice))propertyChange{
    //也可以直接用_videoDevice,但是下面这种更好
    AVCaptureDevice *captureDevice= [_videoInput device];
    NSError *error;
    //注意改变设备属性前一定要首先调用lockForConfiguration:调用完之后使用unlockForConfiguration方法解锁,意义是---进行修改期间,先锁定,防止多处同时修改
    BOOL lockAcquired = [captureDevice lockForConfiguration:&error];
    if (!lockAcquired) {
        NSLog(@"锁定设备过程error，错误信息：%@",error.localizedDescription);
    }else{
        
        //调整设备前后要调用beginConfiguration/commitConfiguration
        [_captureSession beginConfiguration];
        propertyChange(captureDevice);
        [captureDevice unlockForConfiguration];
        [_captureSession commitConfiguration];
    }
}