iOS：二维码的扫描

iOS 中二维码的扫描借用#import <AVFoundation/AVFoundation.h> 实现，会用到<AVCaptureMetadataOutputObjectsDelegate>这个代理，并会调用相机进行扫描，

具体实现步骤如下：

// 1.创建捕捉会话
    AVCaptureSession *session = [[AVCaptureSession alloc] init];
    
    // 2.添加一个输入源
    AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:device error:nil];
    [session addInput:input];
    self.session = session;
    
    // 3.添加一个输出源
    AVCaptureMetadataOutput *output = [[AVCaptureMetadataOutput alloc] init];
    [output setMetadataObjectsDelegate:self queue:dispatch_get_main_queue()];
    [session addOutput:output];
    [output setMetadataObjectTypes:@[AVMetadataObjectTypeQRCode]];
    
    // 4.添加一个预览图层
    AVCaptureVideoPreviewLayer *layer = [AVCaptureVideoPreviewLayer layerWithSession:session];
    layer.frame = self.view.bounds;
    [self.view.layer addSublayer:layer];
    self.layer = layer;
    
    // 5.开启扫描
    [session startRunning];

#pragma mark - 实现输出源的代理方法
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputMetadataObjects:(NSArray *)metadataObjects fromConnection:(AVCaptureConnection *)connection
{
    if (metadataObjects.count > 0) {
        AVMetadataMachineReadableCodeObject *object = [metadataObjects lastObject];
        NSLog(@"%@", object.stringValue);
        
        [self.session stopRunning];
        [self.layer removeFromSuperlayer];
    } else {
        NSLog(@"获取数据失败");
    }
}