iOS：AVCaptureOutput在導航後停止工作

Question

我將相機視圖用作掃描條形碼並使用webview啓動其他視圖控制器的條形碼掃描器。這工作正常，我可以從webview導航到掃描儀並掃描另一個條形碼，而不會出現問題。但是，如果我用相機遠離視圖控制器並返回到視圖控制器，攝像機視圖將加載，但不再檢測條形碼。

@implementation ProductScanViewController 

NSString *loadUrl;  
AVCaptureSession *_captureSession; 
AVCaptureDevice *_videoDevice; 
AVCaptureDeviceInput *_videoInput; 
AVCaptureVideoPreviewLayer *_previewLayer; 
BOOL _running; 
AVCaptureMetadataOutput *_metadataOutput; 
@synthesize mWebView; 


- (void)viewDidLoad { 
    [super viewDidLoad]; 
NSURL *url = [NSURL URLWithString:loadUrl]; 
NSURLRequest *request = [NSURLRequest requestWithURL:url]; 
NSOperationQueue *queue = [[NSOperationQueue alloc]init]; 
[NSURLConnection sendAsynchronousRequest:request queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) 
{if([data length] > 0 && error == nil)[mWebView loadRequest:request];   else if (error != nil) NSLog(@"Error: %", error);} 
]; 

    [self setupCaptureSession]; 
} 

- (void)setupCaptureSession { 

    // 1 
    if (_captureSession){   
     return; 
    } 

    // 2 
    _videoDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo]; 
    if (!_videoDevice) { 
     return; 
    } 

    // 3 
    _captureSession = [[AVCaptureSession alloc] init]; 

    // 4 
    _videoInput = [[AVCaptureDeviceInput alloc] initWithDevice:_videoDevice error:nil]; 

    // 5 
    if ([_captureSession canAddInput:_videoInput]) { 
     [_captureSession addInput:_videoInput]; 
    } 

    // 6 
    _previewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:_captureSession]; 
    _previewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill; 

    // capture and process the metadata 
    _metadataOutput = [[AVCaptureMetadataOutput alloc] init]; 
    dispatch_queue_t metadataQueue = 
    dispatch_queue_create("com.1337labz.featurebuild.metadata", 0); 
    [_metadataOutput setMetadataObjectsDelegate:self 
             queue:metadataQueue]; 
    if ([_captureSession canAddOutput:_metadataOutput]) { 
     [_captureSession addOutput:_metadataOutput]; 
    } 
} 

#pragma mark - Delegate functions 

- (void)captureOutput:(AVCaptureOutput *)captureOutput 
    didOutputMetadataObjects:(NSArray *)metadataObjects 
     fromConnection:(AVCaptureConnection *)connection { 

    [metadataObjects enumerateObjectsUsingBlock:^(AVMetadataObject *obj, 
           NSUInteger idx, 
           BOOL *stop) { 
     if ([obj isKindOfClass: [AVMetadataMachineReadableCodeObject class]]) { 
      //NSLog(@"Capture Output started"); 

      // 3 
      AVMetadataMachineReadableCodeObject *code = (AVMetadataMachineReadableCodeObject*) 
      [_previewLayer transformedMetadataObjectForMetadataObject:obj]; 

      // 4 
      Barcode * barcode = [Barcode processMetadataObject:code]; 

      for (NSString * str in self.allowedBarcodeTypes) { 
       if([barcode.getBarcodeType isEqualToString:str]){ 
        [self validBarcodeFound:(barcode)]; 
        return; 
       } 
      } 
     } 
    }]; 
} 

- (void) validBarcodeFound:(Barcode *)barcode{ 
NSLog(@"Found Barcode"); 

[self stopRunning]; 
[self.foundBarcodes addObject:barcode]; 
//[self showBarcodeAlert:barcode]; 
NSString *alertMessage = @""; 
alertMessage = [alertMessage stringByAppendingString:[barcode getBarcodeType]]; 
NSLog([barcode getBarcodeData]); 
NSLog(alertMessage); 

NSLog([NSString stringWithFormat:@"%@", barcode.getBarcodeData]); 
if ([barcode.getBarcodeType isEqualToString:@"org.iso.QRCode"]) 
{ 
if ([[NSString stringWithFormat:@"%lu",(unsigned long)[self.foundBarcodes count]-1] length] > 0){ 

    NSString *input = [barcode getBarcodeData]; 
    [NSString stringWithFormat:@"%lu",(unsigned long)[self.foundBarcodes count]-1]; 
    NSLog(input); 


    if ([input length] >= 13) 
    { 
     input = [input substringToIndex:12]; 
    } 
    loadUrl = [[@"http://www.mywebsite.co.uk/" stringByAppendingString:input] stringByAppendingString:@"?utm_source=iphone"]; 
    NSLog(loadUrl);   
    dispatch_sync(dispatch_get_main_queue(), ^{ 
     [self performSegueWithIdentifier:@"toWebView" sender:self]; 
    }); 
} 


} 

Answer 1

所以我設法解決這個問題，但不能真正理解它爲什麼會起作用。

而不是在viewDidLoad期間異步加載的URL，我通過與segue的URL並從包含WebView的ViewController中加載它。

除此之外，變量聲明被封裝在大括號{}中，並且@synthesize mWebView被刪除。我不知道爲什麼這會導致問題，所以任何可能的解釋，將不勝感激