Question

新手在这里。我想显示来自ViewModel的提醒。

问题：当前上下文中不存在名称DisplayAlert

怎么做？以下是我的代码。

-- XAML
<Button x:Name="BtnLogin" Command="{Binding LoginCommand}" BackgroundColor="Green" TextColor="White" WidthRequest="150" HeightRequest="60"  Text="Login" />


--- ViewModel :

class LoginViewModel : ViewModelBase
    {


        private string _username;

        public string Username
        {
            get { return _username; }
            set
            {
                _username = value;
                OnPropertyChanged();
            }     
         }

        private string _password;

        public string Password
        {
            get { return _password; }
            set
            {
                _password = value;
                OnPropertyChanged();
            }
        }

        public LoginViewModel()
        {          

        }

        public Command LoginCommand
        {
            get
            {
                return new Command(ValidateUser);
            }

        }     

       async void ValidateUser()
        {
            if (_username.Length > 1 && _password.Length > 1)
            {
               //await DisplayAlert("Login", "Login Success", "Ok");
            //--Update:

                UserDialogs.Instance.Alert("Login Success", "Login", "OK");
            }
            else
            {
               // display invalid credential
            }
        }

更新有 a）Acr.UserDialogs V6.5.1 b）Acr.XamForms.UserDialog v5.0.0

我使用的旧版本是（b），因为我使用的是PCL。

我确实导入了它并更改代码以使用它如上所述。但是有错误的信息：使用Acr.UserDialogs;

Err Msg：

Java.Lang.NullPointerException: Exception of type 'Java.Lang.NullPointerException' was thrown.
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in <3fd174ff54b146228c505f23cf75ce71>:0 
  at Java.Interop.JniEnvironment+InstanceMethods.CallNonvirtualVoidMethod (Java.Interop.JniObjectReference instance, Java.Interop.JniObjectReference type, Java.Interop.JniMethodInfo method, Java.Interop.JniArgumentValue* args) [0x00089] in <bd30a18775d94dc8b6263aecd1ca9077>:0 
  at Java.Interop.JniPeerMembers+JniInstanceMethods.FinishCreateInstance (System.String constructorSignature, Java.Interop.IJavaPeerable self, Java.Interop.JniArgumentValue* parameters) [0x0004f] in <bd30a18775d94dc8b6263aecd1ca9077>:0 
  at Android.App.AlertDialog+Builder..ctor (Android.Content.Context context) [0x0007a] in <d855bac285f44dda8a0d8510b679b1e2>:0 
  at Acr.UserDialogs.Builders.AlertBuilder.Build (Android.App.Activity activity, Acr.UserDialogs.AlertConfig config) [0x0000d] in <addbf2648c204949b40c582bd49b7ddd>:0 
  at Acr.UserDialogs.UserDialogsImpl.Alert (Acr.UserDialogs.AlertConfig config) [0x00038] in <addbf2648c204949b40c582bd49b7ddd>:0 
  at Acr.UserDialogs.AbstractUserDialogs.Alert (System.String message, System.String title, System.String okText) [0x00024] in <ec0104dbfc974343b668f7b28f49a1ab>:0 
  at BookingNow.ViewModel.LoginViewModel.ValidateUser () [0x00026] in C:\Users\Edward\documents\visual studio 2017\Projects\BookingNow\BookingNow\BookingNow\ViewModel\LoginViewModel.cs:88 
  --- End of managed Java.Lang.NullPointerException stack trace ---
java.lang.NullPointerException
    at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:143)
    at android.app.AlertDialog$Builder.<init>(AlertDialog.java:360)
    at md5270abb39e60627f0f200893b490a1ade.ButtonRenderer_ButtonClickListener.n_onClick(Native Method)
    at md5270abb39e60627f0f200893b490a1ade.ButtonRenderer_ButtonClickListener.onClick(ButtonRenderer_ButtonClickListener.java:30)
    at android.view.View.performClick(View.java:4476)
    at android.view.View$PerformClick.run(View.java:18787)
    at android.os.Handler.handleCallback(Handler.java:730)
    at android.os.Handler.dispatchMessage(Handler.java:92)
    at android.os.Looper.loop(Looper.java:176)
    at android.app.ActivityThread.main(ActivityThread.java:5493)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:525)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1209)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1025)
    at dalvik.system.NativeStart.main(Native Method)

由于

Answer 1

创建界面：

public interface IMessageService
{
    Task ShowAsync(string message);
}

实现界面：

public class MessageService : IMessageService
{
    public async Task ShowAsync(string message)
    {
        await App.Current.MainPage.DisplayAlert("YourApp", message, "Ok");
    }
}

将依赖项服务注入App类：

public partial class App : Application
{
    public App()
    {
        //Add the next line
        DependencyService.Register<ViewModels.Services.IMessageService, Views.Services.MessageService>();
        InitializeComponent();
        MainPage = new MainPage();
    }
}

在您的ViewModel中将其初始化并使用：

public class YourViewModel 
{
    private readonly Services.IMessageService _messageService;

    public YourViewModel()
    {
        this._messageService = DependencyService.Get<Services.IMessageService>();

        //Show the dialog with next line
        _messageService.ShowAsync("Hi!!!");
    }
}

Answer 2

如果你想让它保持纯净，你应该避免以传统的方式使用警报，并找到一些方法来收集你可以通过切换属性来触发的输入。

然而，还有另一种更简单的方法。您可以使用ACR.UserDialogs。如果您还没有使用.NET Standard，则需要安装旧版本的NuGet软件包。请记住在共享项目和平台项目中安装它。它可能还需要一些初始化代码，具体取决于平台，请务必查看自述文件。

您现在可以使用：UserDialogs.Instance直接调用实例，然后使用方法显示警报或任何您需要的内容。为了让它更像MVVM，你也可以用它的界面对应注册这个实例，并将它注入你的视图模型。

Answer 3

您需要将页面传递给ViewModel

在您的视图中，使用＆＃39;此＆＃39;

将页面传递给ViewModel

public partial class ThePage : ContentPage
{

    ViewModel viewModel;

    public ThePage()
    {
        InitializeComponent();

        viewModel = new ViewModel(this);
}

然后在您的视图模型中

public class ViewModel
{

    Page page;
    public ViewModel(Page page)
   {
     this.page = page;

     //SOME CODE 
    }

   async void ValidateUser()
    {
        await page.DisplayAlert("Alert from View Model", "", "Ok");
    }
 }

如何从ViewModel显示警报

3 个答案: