iOS 屏幕亮度和闪光灯控制

##所用涉及框架：AVFoundation框架和ImageIO

读取屏幕亮度：[UIScreen mainScreen].brightness;
设置屏幕亮度：[[UIScreen mainScreen] setBrightness:0.5];

获取环境亮度主要代码：

- (void)getTorch {
    AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    AVCaptureDeviceInput *input = [[AVCaptureDeviceInput alloc]initWithDevice:device error:nil];
    AVCaptureVideoDataOutput *output = [[AVCaptureVideoDataOutput alloc] init];
    [output setSampleBufferDelegate:self queue:dispatch_get_main_queue()];
    self.session = [[AVCaptureSession alloc]init];
    [self.session setSessionPreset:AVCaptureSessionPresetHigh];
    if ([self.session canAddInput:input]) {
        [self.session addInput:input];
    }
    if ([self.session canAddOutput:output]) {
        [self.session addOutput:output];
    }
    [self.session startRunning];

}

- (void)captureOutput:(AVCaptureOutput*)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection*)connection {
    CFDictionaryRef metadataDict =CMCopyDictionaryOfAttachments(NULL,sampleBuffer, 
    kCMAttachmentMode_ShouldPropagate);
    NSDictionary *metadata = [[NSMutableDictionary alloc] initWithDictionary:
    (__bridgeNSDictionary*)metadataDict];

    CFRelease(metadataDict);
    NSDictionary *exifMetadata = [[metadata objectForKey:(NSString*)kCGImagePropertyExifDictionary] mutableCopy];
    float brightnessValue = [[exifMetadata objectForKey:(NSString*)kCGImagePropertyExifBrightnessValue] floatValue];
    NSLog(@"%f",brightnessValue);
    // 根据brightnessValue的值来打开和关闭闪光灯
    AVCaptureDevice*device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    BOOL result = [device hasTorch];// 判断设备是否有闪光灯
    if((brightnessValue <0) && result) {
    // 打开闪光灯
        [device lockForConfiguration:nil];
        [device setTorchMode:AVCaptureTorchModeOn];//开
        [device unlockForConfiguration];
    }else if((brightnessValue >0) && result) {
    // 关闭闪光灯
        [device lockForConfiguration:nil];        
        [device setTorchMode:AVCaptureTorchModeOff];//关
        [device unlockForConfiguration];    
    }
}