// 1.获取硬件设备AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];// 2.创建输入流AVCaptureDeviceInput *input = [[AVCaptureDeviceInput alloc]initWithDevice:device error:nil];// 3.创建设备输出流AVCaptureVideoDataOutput *output = [[AVCaptureVideoDataOutput alloc] init];[output setSampleBufferDelegate:self queue:dispatch_get_main_queue()];// AVCaptureSession属性self.session = [[AVCaptureSession alloc]init];// 设置为高质量采集率
    [self.session setSessionPreset:AVCaptureSessionPresetHigh];// 添加会话输入和输出if ([self.session canAddInput:input]) {[self.session addInput:input];}if ([self.session canAddOutput:output]) {[self.session addOutput:output];}// 9.启动会话[self.session startRunning];

#pragma mark - AVCaptureVideoDataOutputSampleBufferDelegate- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection{CFDictionaryRef metadataDict = CMCopyDictionaryOfAttachments(NULL,sampleBuffer, kCMAttachmentMode_ShouldPropagate);NSDictionary *metadata = [[NSMutableDictionary alloc] initWithDictionary:(__bridge NSDictionary*)metadataDict];CFRelease(metadataDict);NSDictionary *exifMetadata = [[metadata objectForKey:(NSString *)kCGImagePropertyExifDictionary] mutableCopy];float brightnessValue = [[exifMetadata objectForKey:(NSString *)kCGImagePropertyExifBrightnessValue] floatValue];// brightnessValue 值代表光线强度，值越小代表光线越暗if (brightnessValue <= -2 && !_isAutoOpen) {_isAutoOpen = YES;[self.torchBtn setSelected:YES];[self turnTorchOn:YES];}
}

// 打开/关闭手电筒
- (void)turnTorchOn:(BOOL)on{if ([self.device hasTorch] && [self.device hasFlash]){[self.device lockForConfiguration:nil];if (on) {[self.device setTorchMode:AVCaptureTorchModeOn];[self.device setFlashMode:AVCaptureFlashModeOn];} else {[self.device setTorchMode:AVCaptureTorchModeOff];[self.device setFlashMode:AVCaptureFlashModeOff];}[self.device unlockForConfiguration];} else {[SSAlertView showWithTitle:@"提示" message:@"当前设备没有闪光灯，不能提供手电筒功能" commitTitle:@"我知道了" cancelTitle:nil commitAction:nil cancelBlock:nil];}
}

转自：http://www.cenzhijun.top/2017/07/自动打开闪光灯/