iOS7后使用AVCapture出现 unsupported type found. Use -availableMetadataObjectTypes错误的解决

在集成扫描二维码功能时候，我使用的是系统在iOS7.0 之后才支持的扫描二维码功能类。刚开始创建代码是这么写的

-(void)setUpCamera
{
    self.device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    
    self.input = [AVCaptureDeviceInput deviceInputWithDevice:self.device error:nil];
    
    self.outPut = [[AVCaptureMetadataOutput alloc]init];
    self.outPut.metadataObjectTypes = @[AVMetadataObjectTypeQRCode];    
    [_outPut setMetadataObjectsDelegate:self queue:dispatch_get_main_queue()];
    self.session = [[AVCaptureSession alloc]init];
    [self.session setSessionPreset:AVCaptureSessionPresetHigh];
    if ([self.session canAddInput:self.input])
    {
        [self.session addInput:self.input];
        
    }
    
    if ([self.session canAddOutput:self.outPut])
    {
        [self.session addOutput:self.outPut];
    }
    

    _preview =[AVCaptureVideoPreviewLayer layerWithSession:self.session];
    _preview.videoGravity = AVLayerVideoGravityResizeAspectFill;
    _preview.frame =CGRectMake(20,110,280,280);
    [self.view.layer insertSublayer:self.preview atIndex:0];

    [self.session startRunning];
}

在其代理类里面

-(void)captureOutput:(AVCaptureOutput *)captureOutput didOutputMetadataObjects:(NSArray *)metadataObjects fromConnection:(AVCaptureConnection *)connection
{
    NSString *stringValue;
    
    if ([metadataObjects count] > 0 ) {
        AVMetadataMachineReadableCodeObject *metadataObject = [metadataObjects objectAtIndex:0];
        stringValue = metadataObject.stringValue;
    }
    [self.session stopRunning];
    [self dismissViewControllerAnimated:YES completion:^{
        [timer invalidate];
        UIAlertView *alertView = [[UIAlertView alloc]initWithTitle:@"二维码相关内容" message:stringValue delegate:nil cancelButtonTitle:@"取消" otherButtonTitles:@"确定", nil];
        [alertView show];
        
    }];
}

在真机上测试，系统是iOS7.1.1

运行时出现下面的错误：

*** Terminating app due to uncaught exception ‘NSInvalidArgumentException’, reason: ‘*** -[AVCaptureMetadataOutput setMetadataObjectTypes:] – unsupported type found. Use -availableMetadataObjectTypes.’

解决不了，上网上搜索也没有相关的解决方法，最后查找相关文档并且查看手册，还是不能够解决问题，请教一大神，查看系统帮助文档的 AVMetadataObjectTypes后改变了创建代码块-(void)setUpCamera如下；

-(void)setUpCamera
{
    self.device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    
    self.input = [AVCaptureDeviceInput deviceInputWithDevice:self.device error:nil];
    
    self.outPut = [[AVCaptureMetadataOutput alloc]init];
//    self.outPut.metadataObjectTypes = @[AVMetadataObjectTypeQRCode];
//    self.outPut.metadataObjectTypes = @[AVMetadataObjectTypeQRCode];
    
    [_outPut setMetadataObjectsDelegate:self queue:dispatch_get_main_queue()];
    self.session = [[AVCaptureSession alloc]init];
    [self.session setSessionPreset:AVCaptureSessionPresetHigh];
    if ([self.session canAddInput:self.input])
    {
        [self.session addInput:self.input];
        
    }
    
    if ([self.session canAddOutput:self.outPut])
    {
        [self.session addOutput:self.outPut];
    }
    self.outPut.metadataObjectTypes = [NSArray arrayWithObject:AVMetadataObjectTypeQRCode];

    _preview =[AVCaptureVideoPreviewLayer layerWithSession:self.session];
    _preview.videoGravity = AVLayerVideoGravityResizeAspectFill;
    _preview.frame =CGRectMake(20,110,280,280);
    [self.view.layer insertSublayer:self.preview atIndex:0];

    [self.session startRunning];
}

这样问题就解决了

大神解释官方文档的意思大致是：

AVCaptureMetadataOutput类的对象(在本例里是self.outPut),self.outPut的属性metadataObjectTypes要在AVCaptureSession类对象（本例为self.session)

[self.session addOutput:self.outPut]即添加了AVCaptureMetadataOutput的对象后，方可对其进行设置；