Xamarin.Forms中View和ViewModel之间的通信

Question

i have a problem with ZXing that correctly scan qr codes in my xamarin app, but it does multiple times and it sends to other viewmodels more than one time. To resolve this problem i use a counter and it works good. So i thought to use messenger provided by MvvmLight to send, from view, a message to viewmodel to reset the counter every time my view start the OnAppearing method. Is it a good way? Can i do it better in another way?

This is my OnAppearing method in my view:

protected override void OnAppearing()
        {
        base.OnAppearing();
        var messageScanActivation = new MessageResetScan();
        Messenger.Default.Send(messageScanActivation);
        Console.WriteLine("i sent reset message");
    }

This is my viewmodel:

   public ScannerViewModel(INavigationService navigationService, 
  IScanCreatorService scanCreatorService) : base(navigationService)
    {
        _scanCreatorService = scanCreatorService;
        title = "Scan";
        _countScan = 0;
        OnBarcodeScannedCommand = new Command(OnBarcodeScanned);
        _isScanning = true;
        Messenger.Default.Register<MessageResetScan>(this, ResetScan);
    }



    private void OnBarcodeScanned()
    {
        _countScan++;
        _isAnalyzing = false;
        Device.BeginInvokeOnMainThread(() =>
            {
                if (_countScan == 1)
                {
                    Debug.WriteLine("RisultatoScansione: " + Result.Text);
                    UpdateHistoryScans(Result.Text);
                    Debug.WriteLine("currentKey: " + _navigationService.CurrentPageKey);
                    DisplayScanAcquiredPrompt();
                    _navigationService.GoBack();
                }           

            });
        _isAnalyzing = true;

    }

    private void ResetScan(MessageResetScan message)
    {
        Console.WriteLine("I received reset message! CountScan: " + _countScan);
        _countScan = 0;
    }

Answer 1

Messaging is one way, another MVVM friendly way is to listen for viewmodel data changes in the view and then trigger code behind.

Using David's excellent behaviors library and when the data comes in from the rest service the view triggers the code behind when the data changes.

<ContentPage.Behaviors>
<b:DataChangedBehavior Binding="{Binding Position}" ComparisonCondition="NotEqual" Value="{x:Null}">
  <b:InvokeMethodAction TargetObject="{Binding Source={x:Reference GeoposView}}"
                        MethodName="MapRefresh" />
</b:DataChangedBehavior>

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