iOS7後使用AVCapture出現 unsupported type found. Use -availableMetadataObjectTypes錯誤的解決

在集成掃描二維碼功能時候，我使用的是系統在iOS7.0 以後才支持的掃描二維碼功能類。剛開始建立代碼是這麼寫的

 1 -(void)setUpCamera
 2 {
 3     self.device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
 4     
 5     self.input = [AVCaptureDeviceInput deviceInputWithDevice:self.device error:nil];
 6     
 7     self.outPut = [[AVCaptureMetadataOutput alloc]init];
 8     self.outPut.metadataObjectTypes = @[AVMetadataObjectTypeQRCode];    
 9     [_outPut setMetadataObjectsDelegate:self queue:dispatch_get_main_queue()];
10     self.session = [[AVCaptureSession alloc]init];
11     [self.session setSessionPreset:AVCaptureSessionPresetHigh];
12     if ([self.session canAddInput:self.input])
13     {
14         [self.session addInput:self.input];
15         
16     }
17     
18     if ([self.session canAddOutput:self.outPut])
19     {
20         [self.session addOutput:self.outPut];
21     }
22     
23 
24     _preview =[AVCaptureVideoPreviewLayer layerWithSession:self.session];
25     _preview.videoGravity = AVLayerVideoGravityResizeAspectFill;
26     _preview.frame =CGRectMake(20,110,280,280);
27     [self.view.layer insertSublayer:self.preview atIndex:0];
28 
29     [self.session startRunning];
30 }

在其代理類裏面

-(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:@"二維碼相關內容" message:stringValue delegate:nil cancelButtonTitle:@"取消" otherButtonTitles:@"肯定", nil];
        [alertView show];
        
    }];
}

在真機上測試，系統是iOS7.1.1

運行時出現下面的錯誤：

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

解決不了，上網上搜索也沒有相關的解決方法，最後查找相關文檔而且查看手冊，仍是不可以解決問題，請教一大神，查看系統幫助文檔的 AVMetadataObjectTypes後改變了建立代碼塊-(void)setUpCamera以下；

-(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];
}

這樣問題就解決了

大神解釋官方文檔的意思大體是：

AVCaptureMetadataOutput類的對象(在本例裏是self.outPut),self.outPut的屬性metadataObjectTypes要在AVCaptureSession類對象（本例爲self.session)

[self.session addOutput:self.outPut]即添加了AVCaptureMetadataOutput的對象後，方可對其進行設置；