如何利用 AVFoundation 设计一个通用稳定的音视频框架？

承接上篇的 《AV Foundation开发秘籍——实践掌握iOS & OS X应用的视听处理技术 阅读指南》 今天这篇给大家讲解下如何利用AVFoundation设计一套通用稳定的音视频框架。

核心

AVCaptureSession开启捕获任务，配置AVCaptureDeviceInput定制捕获任务的输入源（多种摄像头），通过AVFoundation内各种Data output输出数据（元数据、视频帧、音频帧），AVAssetWriter开启写任务，将音视频数据归档为媒体文件。

实现功能

视频流预览、录像归档、捕获相片、切换摄像头、人脸检测、帧率配置、相机详细配置

框架源码

github.com/caixindong/…

具体设计

核心模块 XDCaptureService & XDVideoWritter

XDCaptureService是对外API的总入口，也是框架的核心类，主要做音视频输入输出配置工作和调度工作。 XDVideoWritter是音视频写模块，主要提供写数据和归档数据的基础操作，不对外暴露。 对外API设计：

@class XDCaptureService;


@protocol XDCaptureServiceDelegate <NSObject>

@optional
//service生命周期
- (void)captureServiceDidStartService:(XDCaptureService *)service;

- (void)captureService:(XDCaptureService *)service serviceDidFailWithError:(NSError *)error;

- (void)captureServiceDidStopService:(XDCaptureService *)service;

- (void)captureService:(XDCaptureService *)service getPreviewLayer:(AVCaptureVideoPreviewLayer *)previewLayer;

- (void)captureService:(XDCaptureService *)service outputSampleBuffer:(CMSampleBufferRef)sampleBuffer;

//录像相关
- (void)captureServiceRecorderDidStart:(XDCaptureService *)service ;

- (void)captureService:(XDCaptureService *)service recorderDidFailWithError:(NSError *)error;

- (void)captureServiceRecorderDidStop:(XDCaptureService *)service;

//照片捕获
- (void)captureService:(XDCaptureService *)service capturePhoto:(UIImage *)photo;

//人脸检测
- (void)captureService:(XDCaptureService *)service outputFaceDetectData:(NSArray <AVMetadataFaceObject*>*) faces;

//景深数据
- (void)captureService:(XDCaptureService *)service captureTrueDepth:(AVDepthData *)depthData API_AVAILABLE(ios(11.0));

@end

@protocol XDCaptureServicePreViewSource <NSObject>

- (AVCaptureVideoPreviewLayer *)preViewLayerSource;

@end

@interface XDCaptureService : NSObject

//是否录制音频，默认是NO
@property (nonatomic, assign) BOOL shouldRecordAudio;

//iOS原生人脸检测，默认是NO
@property (nonatomic, assign) BOOL openNativeFaceDetect;

//摄像头的方向，默认是AVCaptureDevicePositionFront（前置）
@property (nonatomic, assign) AVCaptureDevicePosition devicePosition;

//判断是否支持景深模式，当前只支持7p、8p、X的后置摄像头及X的前后摄像头,系统要求是iOS 11以上
@property (nonatomic, assign, readonly) BOOL depthSupported;

//是否开启景深模式，默认是NO
@property (nonatomic, assign) BOOL openDepth;

//只有以下指定的sessionPreset才有depth数据：AVCaptureSessionPresetPhoto、AVCaptureSessionPreset1280x720、AVCaptureSessionPreset640x480
@property (nonatomic, assign) AVCaptureSessionPreset sessionPreset;

//帧率，默认是30
@property (nonatomic, assign) int frameRate;

//录像的临时存储地址，建议每次录完视频做下重定向
@property (nonatomic, strong, readonly) NSURL *recordURL;

//如果设置preViewSource则内部不生成AVCaptureVideoPreviewLayer
@property (nonatomic, assign) id<XDCaptureServicePreViewSource> preViewSource;

@property (nonatomic, assign) id<XDCaptureServiceDelegate> delegate;

@property (nonatomic, assign, readonly) BOOL isRunning;


//视频编码设置（影响录制的视频的编码和大小）
@property (nonatomic, strong) NSDictionary *videoSetting;

///相机专业设置，除非特定需求，一般不设置
//感光度（iOS8以上）
@property (nonatomic, assign, readonly) CGFloat deviceISO;
@property (nonatomic, assign, readonly) CGFloat deviceMinISO;
@property (nonatomic, assign, readonly) CGFloat deviceMaxISO;

//镜头光圈大小
@property (nonatomic, assign, readonly) CGFloat deviceAperture;

//曝光
@property (nonatomic, assign, readonly) BOOL supportsTapToExpose;
@property (nonatomic, assign) AVCaptureExposureMode exposureMode;
@property (nonatomic, assign) CGPoint exposurePoint;
@property (nonatomic, assign, readonly) CMTime deviceExposureDuration;

//聚焦
@property (nonatomic, assign, readonly) BOOL supportsTapToFocus;
@property (nonatomic, assign) AVCaptureFocusMode focusMode;
@property (nonatomic, assign) CGPoint focusPoint;

//白平衡
@property (nonatomic, assign) AVCaptureWhiteBalanceMode whiteBalanceMode;

//手电筒
@property (nonatomic, assign, readonly) BOOL hasTorch;
@property (nonatomic, assign) AVCaptureTorchMode torchMode;

//闪光灯
@property (nonatomic, assign, readonly) BOOL hasFlash;
@property (nonatomic, assign) AVCaptureFlashMode flashMode;

//相机权限判断
+ (BOOL)videoGranted;

//麦克风权限判断
+ (BOOL)audioGranted;

//切换摄像机
- (void)switchCamera;

//启动
- (void)startRunning;

//关闭
- (void)stopRunning;

//开始录像
- (void)startRecording;

//取消录像
- (void)cancleRecording;

//停止录像
- (void)stopRecording;

//拍照
- (void)capturePhoto;

@end
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CDG队列分流

因为在主线程启动音视频捕获及音视频读写会阻塞主线程，所以我们需要将这些任务派发到子线程中执行。我们选择GCD队列帮我们做这个视频。我们框架总共配置3个队列，分别是sessionQueue、writtingQueue、outputQueue，这些队列都是串行队列，因为音视频相关操作都是有顺序（时序）要求，保证当前队列只有一个操作的执行（配置、写数据、读数据）。sessionQueue主要负责音视频任务启动的调度，writtingQueue主要负责写数据的调度，保证数据帧能够准确归档到文件，outputQueue主要负责数据帧的输出。

@property (nonatomic, strong) dispatch_queue_t sessionQueue;
@property (nonatomic, strong) dispatch_queue_t writtingQueue;
@property (nonatomic, strong) dispatch_queue_t outputQueue;

 _sessionQueue = dispatch_queue_create("com.caixindong.captureservice.session", DISPATCH_QUEUE_SERIAL);
_writtingQueue = dispatch_queue_create("com.caixindong.captureservice.writting", DISPATCH_QUEUE_SERIAL);
_outputQueue = dispatch_queue_create("com.caixindong.captureservice.output", DISPATCH_QUEUE_SERIAL);
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音视频捕获

初始化捕获任务

sessionPreset指定了输出的视频帧的像素，例如640*480

@property (nonatomic, strong) AVCaptureSession *captureSession;
 _captureSession = [[AVCaptureSession alloc] init];
_captureSession.sessionPreset = _sessionPreset;
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配置捕获的输入

获取输入源设备，通过_cameraWithPosition可以获取摄像头的抽象表示，因为红外摄像头、双摄像头只能从较新API中获取，所以方法里已经做了兼容处理。并用输入源设备配置捕获输入AVCaptureDeviceInput

@property (nonatomic, strong) AVCaptureDeviceInput *videoInput;

- (BOOL)_setupVideoInputOutput:(NSError **) error {
    self.currentDevice = [self _cameraWithPosition:_devicePosition];
    
    self.videoInput = [[AVCaptureDeviceInput alloc] initWithDevice:_currentDevice error:error];
    if (_videoInput) {
        if ([_captureSession canAddInput:_videoInput]) {
            [_captureSession addInput:_videoInput];
        } else {
            *error = [NSError errorWithDomain:@"com.caixindong.captureservice.video" code:-2200 userInfo:@{NSLocalizedDescriptionKey:@"add video input fail"}];
            return NO;
        }
    } else {
        *error = [NSError errorWithDomain:@"com.caixindong.captureservice.video" code:-2201 userInfo:@{NSLocalizedDescriptionKey:@"video input is nil"}];
        return NO;
    }
    
    //稳定帧率
    CMTime frameDuration = CMTimeMake(1, _frameRate);
    if ([_currentDevice lockForConfiguration:error]) {
        _currentDevice.activeVideoMaxFrameDuration = frameDuration;
        _currentDevice.activeVideoMinFrameDuration = frameDuration;
        [_currentDevice unlockForConfiguration];
    } else {
        *error = [NSError errorWithDomain:@"com.caixindong.captureservice.video" code:-2203 userInfo:@{NSLocalizedDescriptionKey:@"device lock fail(input)"}];
        
        return NO;
    }

……Other code
}

- (AVCaptureDevice *)_cameraWithPosition:(AVCaptureDevicePosition)position {
    if (@available(iOS 10.0, *)) {
        //AVCaptureDeviceTypeBuiltInWideAngleCamera默认广角摄像头，AVCaptureDeviceTypeBuiltInTelephotoCamera长焦摄像头，AVCaptureDeviceTypeBuiltInDualCamera后置双摄像头，AVCaptureDeviceTypeBuiltInTrueDepthCamera红外前置摄像头
        NSMutableArray *mulArr = [NSMutableArray arrayWithObjects:AVCaptureDeviceTypeBuiltInWideAngleCamera,AVCaptureDeviceTypeBuiltInTelephotoCamera,nil];
        if (@available(iOS 10.2, *)) {
            [mulArr addObject:AVCaptureDeviceTypeBuiltInDualCamera];
        }
        if (@available(iOS 11.1, *)) {
            [mulArr addObject:AVCaptureDeviceTypeBuiltInTrueDepthCamera];
        }
        AVCaptureDeviceDiscoverySession *discoverySession = [AVCaptureDeviceDiscoverySession discoverySessionWithDeviceTypes:[mulArr copy] mediaType:AVMediaTypeVideo position:position];
        return discoverySession.devices.firstObject;
    } else {
        NSArray *videoDevices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
        for (AVCaptureDevice *device in videoDevices) {
            if (device.position == position) {
                return device;
            }
        }
    }
    return nil;
}

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配置捕获输出

根据不同的功能需求，我们可以往捕获任务里添加不同的输出，例如捕获基础的视频帧数据，我们添加AVCaptureVideoDataOutput，捕获音频数据，我们添加AVCaptureAudioDataOutput，捕获人脸数据，我们添加AVCaptureMetadataOutput。因为音频的输出和视频输出的设置方式大同小异，所以这里只列出视频输出的关键代码，这里有几个关键的设计：

1、因为相机传感器问题，输出的视频流的方向会有90度偏转，所以我们需要通过获取与输出连接的videoConnection进行偏转配置； 2、视频帧（或者音频帧）都是以CMSampleBufferRef格式输出，视频帧可能经过多个业务处理，例如写文件或者抛到上层业务处理，所以处理数据前都对视频帧数据进行retatin操作，保证各个业务线处理的视频帧是独立的，具体可以看_processVideoData；

3、为了及时清理临时变量（对视频帧处理的各种操作可能需要较多内存空间），所以将外抛的帧处理用autorelease pool包裹起来，防止出现内存高峰；

@property (nonatomic, strong) AVCaptureVideoDataOutput *videoOutput;

- (BOOL)_setupVideoInputOutput:(NSError **) error {
……Other code

self.videoOutput = [[AVCaptureVideoDataOutput alloc] init];
    _videoOutput.videoSettings = @{(id)kCVPixelBufferPixelFormatTypeKey: @(kCVPixelFormatType_32BGRA)};
    //对迟到的帧做丢帧处理
    _videoOutput.alwaysDiscardsLateVideoFrames = YES;
    
    dispatch_queue_t videoQueue = dispatch_queue_create("com.caixindong.captureservice.video", DISPATCH_QUEUE_SERIAL);
    //设置数据输出的delegate
    [_videoOutput setSampleBufferDelegate:self queue:videoQueue];
    
    if ([_captureSession canAddOutput:_videoOutput]) {
        [_captureSession addOutput:_videoOutput];
    } else {
        *error = [NSError errorWithDomain:@"com.caixindong.captureservice.video" code:-2204 userInfo:@{NSLocalizedDescriptionKey:@"device lock fail(output)"}];
        return NO;
    }
    
    self.videoConnection = [self.videoOutput connectionWithMediaType:AVMediaTypeVideo];
    //录制视频会有90度偏转，是因为相机传感器问题，所以在这里设置输出的视频流的方向
    if (_videoConnection.isVideoOrientationSupported) {
        _videoConnection.videoOrientation = AVCaptureVideoOrientationPortrait;
    }
    return YES;
}

#pragma mark - AVCaptureVideoDataOutputSampleBufferDelegate && AVCaptureAudioDataOutputSampleBufferDelegate
- (void)captureOutput:(AVCaptureOutput *)output didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
    //可以捕获到不同的线程
    if (connection == _videoConnection) {
        @synchronized(self) {
            [self _processVideoData:sampleBuffer];
        };
    } else if (connection == _audioConnection) {
        @synchronized(self) {
            [self _processAudioData:sampleBuffer];
        };
    }
}

#pragma mark - process Data
- (void)_processVideoData:(CMSampleBufferRef)sampleBuffer {
    //CFRetain的目的是为了每条业务线（写视频、抛帧）的sampleBuffer都是独立的
    if (_videoWriter && _videoWriter.isWriting) {
        CFRetain(sampleBuffer);
        dispatch_async(_writtingQueue, ^{
            [_videoWriter appendSampleBuffer:sampleBuffer];
            CFRelease(sampleBuffer);
        });
    }
    
    CFRetain(sampleBuffer);
    //及时清理临时变量，防止出现内存高峰
    dispatch_async(_outputQueue, ^{
        @autoreleasepool{
            if (self.delegate && [self.delegate respondsToSelector:@selector(captureService:outputSampleBuffer:)]) {
                [self.delegate captureService:self outputSampleBuffer:sampleBuffer];
            }
        }
        CFRelease(sampleBuffer);
    });
}
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配置图片数据输出

配置图片数据输出的目的是为了实现相片捕获功能，通过setOutputSettings，我们可以配置我们输出的图片格式。

@property (nonatomic, strong) AVCaptureStillImageOutput *imageOutput;

- (BOOL)_setupImageOutput:(NSError **)error {
    self.imageOutput = [[AVCaptureStillImageOutput alloc] init];
    NSDictionary *outputSetting = @{AVVideoCodecKey: AVVideoCodecJPEG};
    [_imageOutput setOutputSettings:outputSetting];
    if ([_captureSession canAddOutput:_imageOutput]) {
        [_captureSession addOutput:_imageOutput];
        return YES;
    } else {
        *error = [NSError errorWithDomain:@"com.caixindong.captureservice.image" code:-2205 userInfo:@{NSLocalizedDescriptionKey:@"device lock fail(output)"}];
        return NO;
    }
}

//拍照功能实现
- (void)capturePhoto {
    AVCaptureConnection *connection = [_imageOutput connectionWithMediaType:AVMediaTypeVideo];
    if (connection.isVideoOrientationSupported) {
        connection.videoOrientation = AVCaptureVideoOrientationPortrait;
    }
    
    __weak typeof(self) weakSelf = self;
    [_imageOutput captureStillImageAsynchronouslyFromConnection:connection completionHandler:^(CMSampleBufferRef  _Nullable imageDataSampleBuffer, NSError * _Nullable error) {
        __strong typeof(weakSelf) strongSelf = weakSelf;
        if (imageDataSampleBuffer != NULL) {
            NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageDataSampleBuffer];
            UIImage *image = [UIImage imageWithData:imageData];
            if (strongSelf.delegate && [strongSelf.delegate respondsToSelector:@selector(captureService:capturePhoto:)]) {
                [strongSelf.delegate captureService:strongSelf capturePhoto:image];
            }
        }
    }];
}
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配置人脸数据输出

关键是配置人脸元数据输出AVCaptureMetadataOutput，并指定metadataObjectTypes为AVMetadataObjectTypeFace，捕获的人脸数据包含当前视频帧中所有的人脸，可以从数据中提取人脸的范围、位置、偏转角，但这个有个注意点，就是原始的人脸数据的坐标是相机坐标系，我们需要转化为屏幕坐标，这样才方便我们的业务处理，具体可以看人脸数据输出那一块。

@property (nonatomic, strong) AVCaptureMetadataOutput *metadataOutput;

-(void)captureOutput:(AVCaptureOutput *)output didOutputMetadataObjects:(NSArray<__kindof AVMetadataObject *> *)metadataObjects fromConnection:(AVCaptureConnection *)connection {
    NSMutableArray *transformedFaces = [NSMutableArray array];
    for (AVMetadataObject *face in metadataObjects) {
        @autoreleasepool{
            AVMetadataFaceObject *transformedFace = (AVMetadataFaceObject*)[self.previewLayer transformedMetadataObjectForMetadataObject:face];
            if (transformedFace) {
                [transformedFaces addObject:transformedFace];
            }
        };
    }
    @autoreleasepool{
        if (self.delegate && [self.delegate respondsToSelector:@selector(captureService:outputFaceDetectData:)]) {
            [self.delegate captureService:self outputFaceDetectData:[transformedFaces copy]];
        }
    };
}

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配置预览源

这里有两种方式，一种是外部已经通过实现预览数据源方法配置了数据源，另外一种是内部自己生成AVCaptureVideoPreviewLayer配置为预览源。

if (self.preViewSource && [self.preViewSource respondsToSelector:@selector(preViewLayerSource)]) {
        self.previewLayer = [self.preViewSource preViewLayerSource];
        [_previewLayer setSession:_captureSession];
        [_previewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
    } else {
        self.previewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:_captureSession];
        //充满整个屏幕
        [_previewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
        
        if (self.delegate && [self.delegate respondsToSelector:@selector(captureService:getPreviewLayer:)]) {
            [self.delegate captureService:self getPreviewLayer:_previewLayer];
        }
    }
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处理音视频前后台状态变化

iOS的音视频前后台机制较复杂，有各种生命周期变化，为了保证我们框架在正确状态下做正确的事，我们将数据帧的读和写的状态处理进行解耦，各位维护自己的通知状态变化处理。外层业务无需监听AVFoundation的通知手动处理视频流的状态变化。 读模块音视频通知配置：

//CaptureService和VideoWritter各自维护自己的生命周期，捕获视频流的状态与写入视频流的状态解耦分离，音视频状态变迁由captureservice内部管理，外层业务无需手动处理视频流变化
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_captureSessionNotification:) name:nil object:self.captureSession];
    
    //为了适配低于iOS 9的版本，在iOS 9以前，当session start 还没完成就退到后台，回到前台会捕获AVCaptureSessionRuntimeErrorNotification，这时需要手动重新启动session，iOS 9以后系统对此做了优化，系统退到后台后会将session start缓存起来，回到前台会自动调用缓存的session start，无需手动调用
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_enterForegroundNotification:) name:UIApplicationWillEnterForegroundNotification object:nil];

#pragma mark - CaptureSession Notification
- (void)_captureSessionNotification:(NSNotification *)notification {
    NSLog(@"_captureSessionNotification:%@",notification.name);
    if ([notification.name isEqualToString:AVCaptureSessionDidStartRunningNotification]) {
        if (!_firstStartRunning) {
            NSLog(@"session start running");
            _firstStartRunning = YES;
            if (self.delegate && [self.delegate respondsToSelector:@selector(captureServiceDidStartService:)]) {
                [self.delegate captureServiceDidStartService:self];
            }
        } else {
            NSLog(@"session resunme running");
        }
    } else if ([notification.name isEqualToString:AVCaptureSessionDidStopRunningNotification]) {
        if (!_isRunning) {
            NSLog(@"session stop running");
            if (self.delegate && [self.delegate respondsToSelector:@selector(captureServiceDidStopService:)]) {
                [self.delegate captureServiceDidStopService:self];
            }
        } else {
            NSLog(@"interupte session stop running");
        }
    } else if ([notification.name isEqualToString:AVCaptureSessionWasInterruptedNotification]) {
        NSLog(@"session was interupted, userInfo: %@",notification.userInfo);
    } else if ([notification.name isEqualToString:AVCaptureSessionInterruptionEndedNotification]) {
        NSLog(@"session interupted end");
    } else if ([notification.name isEqualToString:AVCaptureSessionRuntimeErrorNotification]) {
        NSError *error = notification.userInfo[AVCaptureSessionErrorKey];
        if (error.code == AVErrorDeviceIsNotAvailableInBackground) {
            NSLog(@"session runtime error : AVErrorDeviceIsNotAvailableInBackground");
            _startSessionOnEnteringForeground = YES;
        } else {
            if (self.delegate && [self.delegate respondsToSelector:@selector(captureService:serviceDidFailWithError:)]) {
                [self.delegate captureService:self serviceDidFailWithError:error];
            }
        }
    } else {
        NSLog(@"handel other notification : %@",notification.name);
    }
}

#pragma mark - UIApplicationWillEnterForegroundNotification
- (void)_enterForegroundNotification:(NSNotification *)notification {
    if (_startSessionOnEnteringForeground == YES) {
        NSLog(@"为了适配低于iOS 9的版本，在iOS 9以前，当session start 还没完成就退到后台，回到前台会捕获AVCaptureSessionRuntimeErrorNotification，这时需要手动重新启动session，iOS 9以后系统对此做了优化，系统退到后台后会将session start缓存起来，回到前台会自动调用缓存的session start，无需手动调用");
        _startSessionOnEnteringForeground = NO;
        [self startRunning];
    }
}
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写模块音视频通知配置：

//写模块注册通知，只负责写相关的状态处理
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_assetWritterInterruptedNotification:) name:AVCaptureSessionWasInterruptedNotification object:nil];

- (void)_assetWritterInterruptedNotification:(NSNotification *)notification {
    NSLog(@"assetWritterInterruptedNotification");
    [self cancleWriting];
}
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启动捕获 & 关闭捕获

异步启动，防止阻塞主线程。串行队列中执行启动和关闭，保证不会出现启动到一半就关闭这种异常case。

- (void)startRunning {
    dispatch_async(_sessionQueue, ^{
        NSError *error = nil;
        BOOL result =  [self _setupSession:&error];
        if (result) {
            _isRunning = YES;
            [_captureSession startRunning];
        }else{
            if (self.delegate && [self.delegate respondsToSelector:@selector(captureService:serviceDidFailWithError:)]) {
                [self.delegate captureService:self serviceDidFailWithError:error];
            }
        }
    });
}

- (void)stopRunning {
    dispatch_async(_sessionQueue, ^{
        _isRunning = NO;
        NSError *error = nil;
        [self _clearVideoFile:&error];
        if (error) {
            if (self.delegate && [self.delegate respondsToSelector:@selector(captureService:serviceDidFailWithError:)]) {
                [self.delegate captureService:self serviceDidFailWithError:error];
            }
        }
        [_captureSession stopRunning];
    });
}

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切换摄像头

切换不仅仅是切换device，同时还要将旧的捕获input移除，添加新的device input。切换摄像头时，videoConnection会变化，所以需要重新获取。

- (void)switchCamera {
    if (_openDepth) {
        return;
    }
    
    NSError *error;
    AVCaptureDevice *videoDevice = [self _inactiveCamera];
    AVCaptureDeviceInput *videoInput = [AVCaptureDeviceInput deviceInputWithDevice:videoDevice error:&error];
    
    if (videoInput) {
        [_captureSession beginConfiguration];
        
        [_captureSession removeInput:self.videoInput];
        
        if ([self.captureSession canAddInput:videoInput]) {
            [self.captureSession addInput:videoInput];
            self.videoInput = videoInput;
            //切换摄像头videoConnection会变化，所以需要重新获取
            self.videoConnection = [self.videoOutput connectionWithMediaType:AVMediaTypeVideo];
            if (_videoConnection.isVideoOrientationSupported) {
                _videoConnection.videoOrientation = AVCaptureVideoOrientationPortrait;
            }
        } else {
            [self.captureSession addInput:self.videoInput];
        }
        
        [self.captureSession commitConfiguration];
    }
    
    _devicePosition = _devicePosition == AVCaptureDevicePositionFront?AVCaptureDevicePositionBack:AVCaptureDevicePositionFront;
}

- (AVCaptureDevice *)_inactiveCamera {
    AVCaptureDevice *device = nil;
    if (_devicePosition == AVCaptureDevicePositionBack) {
        device = [self _cameraWithPosition:AVCaptureDevicePositionFront];
    } else {
        device = [self _cameraWithPosition:AVCaptureDevicePositionBack];
    }
    return device;
}
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录像功能

通过videoSetting配置录制完的视频的编码格式，框架默认的编码格式是H.264，H.264是一种高效的视频编码格式，之后再出篇文章讲下这种编码格式，现阶段你只需要知道这是一种常用的编码格式，想了解更多编码格式，可以看下AVVideoCodecType里面的内容。在XDCaptureService的startRecording方法中，我们初始化我们的写模块XDVideoWritter，XDVideoWritter根据videoSetting配置对应的编码格式。

- (void)startRecording {
    dispatch_async(_writtingQueue, ^{
        @synchronized(self) {
            NSString *videoFilePath = [_videoDir stringByAppendingPathComponent:[NSString stringWithFormat:@"Record-%llu.mp4",mach_absolute_time()]];
            
            _recordURL = [[NSURL alloc] initFileURLWithPath:videoFilePath];
            
            if (_recordURL) {
                _videoWriter = [[XDVideoWritter alloc] initWithURL:_recordURL VideoSettings:_videoSetting audioSetting:_audioSetting];
                _videoWriter.delegate = self;
                [_videoWriter startWriting];
                if (self.delegate && [self.delegate respondsToSelector:@selector(captureServiceRecorderDidStart:)]) {
                    [self.delegate captureServiceRecorderDidStart:self];
                }
            } else {
                NSLog(@"No record URL");
            }
        }
    });
}

//XDVideoWritter.m
- (void)startWriting {
    if (_assetWriter) {
        _assetWriter = nil;
    }
    NSError *error = nil;
    
    NSString *fileType = AVFileTypeMPEG4;
    _assetWriter = [[AVAssetWriter alloc] initWithURL:_outputURL fileType:fileType error:&error];
    
    if (!_assetWriter || error) {
        if (self.delegate && [self.delegate respondsToSelector:@selector(videoWritter:didFailWithError:)]){
            [self.delegate videoWritter:self didFailWithError:error];
        }
    }
    
    if (_videoSetting) {
        _videoInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo outputSettings:_videoSetting];
        
        _videoInput.expectsMediaDataInRealTime = YES;
        
        if ([_assetWriter canAddInput:_videoInput]) {
            [_assetWriter addInput:_videoInput];
        } else {
            NSError *error = [NSError errorWithDomain:@"com.caixindong.captureservice.writter" code:-2210 userInfo:@{NSLocalizedDescriptionKey:@"VideoWritter unable to add video input"}];
            if (self.delegate && [self.delegate respondsToSelector:@selector(videoWritter:didFailWithError:)]) {
                [self.delegate videoWritter:self didFailWithError:error];
            }
            return;
        }
    } else {
        NSLog(@"warning: no video setting");
    }
    
    if (_audioSetting) {
        _audioInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio outputSettings:_audioSetting];
        
        _audioInput.expectsMediaDataInRealTime = YES;
        
        if ([_assetWriter canAddInput:_audioInput]) {
            [_assetWriter addInput:_audioInput];
        } else {
            NSError *error = [NSError errorWithDomain:@"com.caixindong.captureservice.writter" code:-2211 userInfo:@{NSLocalizedDescriptionKey:@"VideoWritter unable to add audio input"}];
            if (self.delegate && [self.delegate respondsToSelector:@selector(videoWritter:didFailWithError:)]) {
                [self.delegate videoWritter:self didFailWithError:error];
            }
            return;
        }
    } else {
        NSLog(@"warning: no audio setting");
    }
    
    if ([_assetWriter startWriting]) {
        self.isWriting = YES;
    } else {
        NSError *error = [NSError errorWithDomain:@"com.xindong.captureservice.writter" code:-2212 userInfo:@{NSLocalizedDescriptionKey: [NSString stringWithFormat: @"VideoWritter startWriting fail error: %@",_assetWriter.error.localizedDescription]}];
        if (self.delegate && [self.delegate respondsToSelector:@selector(videoWritter:didFailWithError:)]) {
            [self.delegate videoWritter:self didFailWithError:error];
        }
    }
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(_assetWritterInterruptedNotification:) name:AVCaptureSessionWasInterruptedNotification object:nil];
}

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录像的原理就是在抛数据的同时调用XDVideoWritter的appendSampleBuffer方法将数据写入一个临时文件中，当调用stopRecording，也就是调用到XDVideoWritter的stopWriting方法停止写数据，将临时文件归档为MP4文件。

- (void)stopRecording {
    dispatch_async(_writtingQueue, ^{
        @synchronized(self) {
            if (_videoWriter) {
                [_videoWriter stopWriting];
            }
        }
    });
}

//XDVideoWritter.m
- (void)appendSampleBuffer:(CMSampleBufferRef)sampleBuffer {
    CMFormatDescriptionRef formatDesc = CMSampleBufferGetFormatDescription(sampleBuffer);
    
    CMMediaType mediaType = CMFormatDescriptionGetMediaType(formatDesc);
    
    if (mediaType == kCMMediaType_Video) {
        CMTime timestamp = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
        
        if (self.firstSample) {
            [_assetWriter startSessionAtSourceTime:timestamp];
            self.firstSample = NO;
        }
        
        if (_videoInput.readyForMoreMediaData) {
            if (![_videoInput appendSampleBuffer:sampleBuffer]) {
                NSError *error = [NSError errorWithDomain:@"com.caixindong.captureservice.writter" code:-2213 userInfo:@{NSLocalizedDescriptionKey:[NSString stringWithFormat: @"VideoWritter appending video sample buffer fail error:%@",_assetWriter.error.localizedDescription]}];
                if (self.delegate && [self.delegate respondsToSelector:@selector(videoWritter:didFailWithError:)]) {
                    [self.delegate videoWritter:self didFailWithError:error];
                }
            }
        }
    } else if (!self.firstSample && mediaType == kCMMediaType_Audio) {
        if (_audioInput.readyForMoreMediaData) {
            if (![_audioInput appendSampleBuffer:sampleBuffer]) {
                NSError *error = [NSError errorWithDomain:@"com.caixindong.captureservice.writter" code:-2214 userInfo:@{NSLocalizedDescriptionKey:[NSString stringWithFormat:@"VideoWritter appending audio sample buffer fail error: %@",_assetWriter.error]}];
                if (self.delegate && [self.delegate respondsToSelector:@selector(videoWritter:didFailWithError:)]) {
                    [self.delegate videoWritter:self didFailWithError:error];
                }
            }
        }
    }
}

- (void)stopWriting {
    if (_assetWriter.status == AVAssetWriterStatusWriting) {
        self.isWriting = NO;
        [_assetWriter finishWritingWithCompletionHandler:^{
            if (_assetWriter.status == AVAssetWriterStatusCompleted) {
                if (self.delegate && [self.delegate respondsToSelector:@selector(videoWritter:completeWriting:)]) {
                    [self.delegate videoWritter:self completeWriting:nil];
                }
            } else {
                if (self.delegate && [self.delegate respondsToSelector:@selector(videoWritter:completeWriting:)]) {
                    [self.delegate videoWritter:self completeWriting:_assetWriter.error];
                }
            }
        }];
    } else {
        NSLog(@"warning : stop writing with unsuitable state : %ld",_assetWriter.status);
    }
    [[NSNotificationCenter defaultCenter] removeObserver:self name:AVCaptureSessionWasInterruptedNotification object:nil];
}

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