调用相机之前先检查权限

如果没有检查权限并作相应处理的话，同时用户没有开启权限，则用户使用app调用摄像头时会发生崩溃

// 相机权限
    AVAuthorizationStatus authStatus = [AVCaptureDevice authorizationStatusForMediaType:AVMediaTypeVideo];
    if (authStatus == AVAuthorizationStatusRestricted ||
        authStatus == AVAuthorizationStatusDenied) {
        
        
            // 无权限，引导用户开启权限
            NSURL *url = [NSURL URLWithString:UIApplicationOpenSettingsURLString];
            if ([[UIApplication sharedApplication] canOpenURL:url]) {
                UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"未获得授权使用摄像头" message:@"去“设置>隐私>相机”开启一下吧" preferredStyle:UIAlertControllerStyleAlert];
                
                UIAlertAction *confirmAction = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
                    [[UIApplication sharedApplication] openURL:url];
                }];
                [alertController addAction:confirmAction];
                
                [self presentViewController:alertController animated:YES completion:nil];
                
            }
        
    } else {
        self.captureSession = [[AVCaptureSession alloc] init];
        
        [self.captureSession beginConfiguration];
        
        AVCaptureDevice *captureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
        
        // 输入
        NSError *error = nil;
        AVCaptureDeviceInput *captureInput = [AVCaptureDeviceInput deviceInputWithDevice:captureDevice error:&error];
        
        if (error == nil) {
            if ([self.captureSession canAddInput:captureInput]) {
                [self.captureSession addInput:captureInput];
            } else {
                NSLog(@"Input Error %@", error);
            }
        }
        
        // 输出
        AVCaptureMetadataOutput *captureOutput = [[AVCaptureMetadataOutput alloc] init];
        [captureOutput setMetadataObjectsDelegate:self queue:dispatch_get_main_queue()];
        
        if ([self.captureSession canAddOutput:captureOutput]) {
            [self.captureSession addOutput:captureOutput];
            captureOutput.metadataObjectTypes = @[AVMetadataObjectTypeEAN13Code];
        }
        
        // 添加预览画面
        CALayer *layer = [AVCaptureVideoPreviewLayer layerWithSession:self.captureSession];
        layer.frame = self.view.layer.bounds;
        [self.view.layer addSublayer:layer];
        
        [self.captureSession commitConfiguration];
        
        [self.captureSession startRunning];
    }