iOS:导航后AVCaptureOutput停止工作

Question

我使用相机视图作为条形码扫描器，扫描条形码，并向另一个带有webview的视控制器启动一个segue。这是很好的工作，我可以导航回扫描仪从网页和扫描另一个条形码没有问题。但是，如果我离开带有摄像机的视图控制器并返回到它，则相机视图将加载但不再检测条形码。

@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

因此，我设法解决了这个问题，但没有给出任何真正的理解，为什么它工作。

我没有在viewDidLoad期间异步加载URL，而是将URL与segue一起传递，并从包含WebView的ViewController中加载它。

此外，变量声明被封装在花括号{}中，并删除了@synthesize mWebView。我不知道为什么会引起问题，所以任何可能的解释都会很感激