微信在扫描二维码时会根据环境的明暗程度提示用户是否打开闪关灯，这样的细节还是不错的。那我们应该怎么实现呢？

我的思路是利用摄像头获取环境光感参数，小于一定值后提示用户打开闪光灯。不过微信、支付宝这些可能也结合摄像头的图像来做分析吧。

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

@interface LightSensitiveVC () <AVCaptureVideoDataOutputSampleBufferDelegate>

@property (nonatomic, strong) AVCaptureSession *session;

@property (weak, nonatomic) IBOutlet UIButton *torchSwitchBtn;

@end

@implementation LightSensitiveVC

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    self.navigationItem.title = @"光感";
    // 设置光感
    [self lightSensitiveSetting];
}

#pragma mark - 设置光感
- (void)lightSensitiveSetting
{
    // 获取硬件设备
    AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    // 创建输入流
    AVCaptureDeviceInput *input = [[AVCaptureDeviceInput alloc]initWithDevice:device error:nil];
    // 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];
    }
    // 启动会话
    [self.session startRunning];
}

#pragma mark- AVCaptureVideoDataOutputSampleBufferDelegate的方法
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
{
    // 获取环境光感参数
    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];
    // 根据brightnessValue的值来现实隐藏闪光灯开关按钮
    AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    BOOL result = [device hasTorch];// 判断设备是否有闪光灯
    if (result && device.torchMode == AVCaptureTorchModeOff) {
        if (brightnessValue < 0 && _torchSwitchBtn.hidden == YES) {
            [UIView animateWithDuration:0.4 animations:^{
                _torchSwitchBtn.hidden = NO;
            }];
        } else if(brightnessValue > 0 && _torchSwitchBtn.hidden == NO) {
            [UIView animateWithDuration:0.4 animations:^{
                _torchSwitchBtn.hidden = YES;
            }];
        }
    }
}

- (IBAction)torchSwitchAction:(UIButton *)sender
{
    AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    BOOL result = [device hasTorch];
    if (result) {
        if (sender.selected == NO) {
            // 打开闪光灯
            [device lockForConfiguration:nil];
            [device setTorchMode: AVCaptureTorchModeOn];
            [device unlockForConfiguration];
            [sender setTitle:@"关闭照明" forState:UIControlStateNormal];
            sender.selected = YES;
        } else if(sender.selected == YES) {
            // 关闭闪光灯
            [device lockForConfiguration:nil];
            [device setTorchMode: AVCaptureTorchModeOff];
            [device unlockForConfiguration];
            [sender setTitle:@"环境较暗 开启照明" forState:UIControlStateNormal];
            sender.selected = NO;
        }
    }
}

实现AVCaptureVideoDataOutputSampleBufferDelegate的代理方法，参数brightnessValue就是环境的光感参数，范围大概在 -5 ~ 12 之间,参数数值越大，代表环境越亮。

项目代码：https://github.com/thunder0513/LightSensitive