iOS延迟我无法弄清楚使用AVFoundation扫描条形码

Question

我有一个时间问题让我很困惑，因为我使用委托(delegate)将数据发送回 HomeView 进行处理并重定向到问题所在的 ScanResults 但即使我

[self performSegueWithIdentifier:@"ScanResultsSegue" sender:self];

直接到它我仍然看到一个巨大的延迟，直到 ScanResultsView 出现。这是 NSLog。最后一行显示延迟

2014-05-24 16:20:55.640 selfcheckin[2748:60b] StartReading Called
2014-05-24 16:21:03.519 selfcheckin[2748:3603] We have a QR -> performSelectorOnMainThread
2014-05-24 16:21:03.522 selfcheckin[2748:3603] IsMainThread: NO
2014-05-24 16:21:03.549 selfcheckin[2748:3603] Barcode: {"stop_id":"3","event_id":"1"}
2014-05-24 16:21:03.555 selfcheckin[2748:3603] didScan Delegate called!
2014-05-24 16:21:03.576 selfcheckin[2748:3603] Parsing Json Dictionary
2014-05-24 16:21:03.584 selfcheckin[2748:3603] Event_id: 1
2014-05-24 16:21:03.590 selfcheckin[2748:3603] Stop_id: 3
2014-05-24 16:21:03.603 selfcheckin[2748:3603] Account_id: 4
2014-05-24 16:21:04.782 selfcheckin[2748:60b] StopReading Called
2014-05-24 16:21:14.721 selfcheckin[2748:3603] Made it to Success!!!

转换到 ScanResultsPage 需要 10 秒.......我无法在我的代码中找到导致它的原因。需要那么久吗？？相关部分

- (BOOL)startReading {
    NSError *error;

    AVCaptureDevice *captureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:captureDevice error:&error];
    if (!input) {
        NSLog(@"%@", [error localizedDescription]);
        return NO;
    }

    _captureSession = [[AVCaptureSession alloc] init];
    [_captureSession addInput:input];

    AVCaptureMetadataOutput *captureMetadataOutput = [[AVCaptureMetadataOutput alloc] init];
    [_captureSession addOutput:captureMetadataOutput];

    dispatch_queue_t dispatchQueue;
    dispatchQueue = dispatch_queue_create("myQueue", NULL);
    [captureMetadataOutput setMetadataObjectsDelegate:self queue:dispatchQueue];
    [captureMetadataOutput setMetadataObjectTypes:[NSArray arrayWithObject:AVMetadataObjectTypeQRCode]];

    _videoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:_captureSession];
    [_videoPreviewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
    [_videoPreviewLayer setFrame:_viewPreview.layer.bounds];
    [_viewPreview.layer addSublayer:_videoPreviewLayer];

    [_captureSession startRunning];
    NSLog(@"%@",@"StartReading Called");
    return YES;
}

-(void)stopReading{
    [_captureSession stopRunning];
    _captureSession = nil;
    [_videoPreviewLayer removeFromSuperlayer];
    NSLog(@"%@",@"StopReading Called");
}

-(void)captureOutput:(AVCaptureOutput *)captureOutput didOutputMetadataObjects:(NSArray *)metadataObjects fromConnection:(AVCaptureConnection *)connection{
    if (metadataObjects != nil && [metadataObjects count] > 0) {
        AVMetadataMachineReadableCodeObject *metadataObj = [metadataObjects objectAtIndex:0];
        if ([[metadataObj type] isEqualToString:AVMetadataObjectTypeQRCode]) {
            NSLog(@"We have a QR -> performSelectorOnMainThread");
            NSLog(@"IsMainThread: %@", ([NSThread isMainThread]) ? @"YES" : @"NO");
            [self performSelectorOnMainThread:@selector(stopReading) withObject:nil waitUntilDone:NO];
             _isReading = NO;
            NSLog(@"Barcode: %@",[(AVMetadataMachineReadableCodeObject *)metadataObj stringValue]);
            //NSLog(@"Now performSegueWithIdentifier");
            //[self performSegueWithIdentifier:@"ScanResultsSegue" sender:self];
            NSLog(@"didScan Delegate called!");
            [self.scandelegate didScan:self barcodedata:[(AVMetadataMachineReadableCodeObject *)metadataObj stringValue]];
        }
    }
}

Answer 1

此类问题几乎总是由尝试与后台线程上的界面对话引起的。与接口(interface)的所有通信必须在主线程上。

总的来说，您的代码在线程方面非常不连贯。您可以从日志记录中看到这一点，它显示了两个不同的线程(主线程和后台线程)交错。另请注意，这是荒谬且危险的:

[self performSelectorOnMainThread:@selector(stopReading) withObject:nil waitUntilDone:NO];
_isReading = NO;

您现在正在后台线程中访问 _isReading，因此线程之间可能会对此 ivar 产生争用。而且，你在说谎；您仍在阅读，因为stopReading尚未被调用(如日志记录所示):waitUntilDone为NO，因此代码立即继续，并且 stopReading 稍后在主线程上调用。一切都乱了套，杂乱无章。

您知道 captureOutput: 正在后台线程上调用，因此此时您需要开始非常非常小心。如果您没有理由处于后台线程，最安全的做法是立即进入主线程 (dispatch_async)，以便该方法的其余部分在主线程上执行。如果您确实有理由进入后台线程，那么至少要确保您不会做线程不安全的事情，就像您现在所做的那样。