iOS自动打开闪光灯

#import <AVFoundation/AVFoundation.h>
#import <ImageIO/ImageIO.h>

// 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];
    }
}