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c# - Xamarin 使用 INotifyPropertyChanged 绑定 Entry 和 Label 以及 MVVM

问题描述

我正在通过 Android 项目的 MainActivity.cs 中的意图获得一个字符串。我正在使用共享项目。该字符串是一个条形码，如果条形码被扫描，则由意图更新。我想通过数据绑定在 .xaml 文件的条目或标签中显示条形码字符串。

问题是，如果我手动单击条目，条形码只会在条目和标签中更新。当意图中的字符串发生变化时，如何确保更新标签或条目？

代码：

MainActivity.cs：

public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
{
    public static MainActivity Instance;
    myBroadcastReceiver receiver;
    BarcodeModel barcodeModel;

    protected override void OnCreate(Bundle savedInstanceState)
    {
        TabLayoutResource = Resource.Layout.Tabbar;
        ToolbarResource = Resource.Layout.Toolbar;

        base.OnCreate(savedInstanceState);

        // Barcode
        MainActivity.Instance = this;
        receiver = new myBroadcastReceiver();
        barcodeModel = new BarcodeModel();

        Xamarin.Essentials.Platform.Init(this, savedInstanceState);
        global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
        LoadApplication(new App());
    }
    public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
    {
        Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);

        base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
    }

    protected override void OnResume()
    {
        base.OnResume();
        //  Register the broadcast receiver dynamically
        RegisterReceiver(receiver, new IntentFilter(Resources.GetString(Resource.String.activity_intent_filter_action)));
    }

    protected override void OnPause()
    {
        base.OnPause();
        UnregisterReceiver(receiver);
    }

    public void DisplayResult(Intent intent)
    {
        //  Output the scanned barcode to ViewModel
        barcodeModel.decodedData = intent.GetStringExtra(Resources.GetString(Resource.String.datawedge_intent_key_data));


    }
}

//  Broadcast receiver to receive scanned data
[BroadcastReceiver(Enabled = true)]
public class myBroadcastReceiver : BroadcastReceiver
{
    public override void OnReceive(Context context, Intent intent)
    {
        String action = intent.Action;
        if (action.Equals(MainActivity.Instance.Resources.GetString(Resource.String.activity_intent_filter_action)))
        {
            //  A barcode has been scanned
            MainActivity.Instance.RunOnUiThread(() => MainActivity.Instance.DisplayResult(intent));
        }
    }

}

}

视图模型：

 public class BarcodeModel : INotifyPropertyChanged
{
    private string data;
    public event PropertyChangedEventHandler PropertyChanged;

    public BarcodeModel()
    {

    }

    protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
    public string decodedData
    {
        get { return data; }
        set { data = value; OnPropertyChanged(); }
    }

内容页：

        <Label Text="{Binding decodedData}"
                   HorizontalOptions="StartAndExpand"
                   VerticalOptions="Center"
                   TextColor="Accent"/>

        <Entry Keyboard="Text"
                   Placeholder="No Object selected"
                   VerticalOptions="Center"
                   HorizontalOptions="StartAndExpand"
                   x:Name="eintrag"
                   Text="{Binding decodedData}"/>

.xaml.cs 中的 BindingContext：BindingContext = new BarcodeModel();

标签： c#xamarinmvvmdata-bindinginotifypropertychanged

解决方案

你在Android项目中改变的模型和你的bindingContext不是一个模型，所以它不起作用。

解决方案：

使用messagesCenter将模型从您的 Android 项目发送到您的共享项目并在那里更新模型：

在 Android 项目中，每次扫描 barode 时发送消息：

public void DisplayResult(Intent intent)
{
    //  Output the scanned barcode to ViewModel
    barcodeModel.decodedData = intent.GetStringExtra(Resources.GetString(Resource.String.datawedge_intent_key_data));
    barcodeModel.decodedData = "test";

    MessagingCenter.Send<object, BarcodeModel>(new object(), "barCodeScanned", barcodeModel);
}

在共享项目中，订阅该消息并更新模型：

public partial class MainPage : ContentPage
{
    BarcodeModel barCodeModel;

    public MainPage()
    {
        InitializeComponent();

        barCodeModel = new BarcodeModel();
        barCodeModel.decodedData = "defaultValue";
        this.BindingContext = barCodeModel;

        MessagingCenter.Subscribe<object, BarcodeModel>(new object(), "barCodeScanned", (sender, arg) =>
        {
            BarcodeModel tempbarCodeModel = arg as BarcodeModel;
            barCodeModel.decodedData = tempbarCodeModel.decodedData;
        });
    }
}

我在这里上传了我的测试项目，请随时问我任何问题。

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