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 > ) {
        AVMetadataMachineReadableCodeObject *object = [metadataObjects lastObject];
        NSLog(@"%@", object.stringValue);

        [self.session stopRunning];
        [self.layer removeFromSuperlayer];
    } else {
        NSLog(@"获取数据失败");
    }
}