Xamarin set focus MVVM

Question

Using a Portable project in Xamarin.

The ScannerPage.xaml file in the Portable project directory

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         x:Class="StockCheckApp.Views.ScannerPage"
         xmlns:ViewModels="clr-namespace:StockCheckApp.ViewModels;assembly=StockCheckApp"
         FocusManager.FocusedElement="{Binding ElementName=}">
    <Label Text="{Binding MainText}" VerticalOptions="Center" HorizontalOptions="Center" />
    <ContentPage.BindingContext>
        <ViewModels:MainViewModel />
    </ContentPage.BindingContext>

    <StackLayout Orientation="Vertical">
        <Entry Text="{Binding UserInput, Mode=TwoWay}" />
        <Button Text="Check Stock" Command="{Binding PostCommand}"/>

        <ListView ItemsSource="{Binding StockList}" HasUnevenRows="True">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <StackLayout Orientation="Vertical" Padding="12,6">
                            <Label Text="{Binding St_Code}" FontSize="24" />
                            <Label Text="{Binding St_Desc}" />
                            <Label Text="{Binding Sl_Loc}" />
                            <Label Text="{Binding Sa_Datetime}" />
                            <Label Text="{Binding Sa_Qty}" />
                        </StackLayout>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </StackLayout> 
</ContentPage>

The MainViewModel.cs class inside the Portable directory.

namespace StockCheckApp.ViewModels
{
    public class MainViewModel : INotifyPropertyChanged
    {
        private List<Stock> _stockList;
        private string _userInput;
        public List<Stock> StockList
        {
            get { return _stockList; }
            set
            {
                _stockList = value;
                OnPropertyChanged();
            }
        }

        public string UserInput
        {
            get { return _userInput; }
            set
            {
                _userInput = value;
                OnPropertyChanged();
            }         
        }

        public MainViewModel()
        {
        }

        public Command PostCommand
        {
            get
            {
                return new Command(async () =>
                {
                    var stockService = new StockService();
                    StockList = await stockService.GetStockAsync(UserInput);
                });
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

What I need to do is set the <Entry Text="{Binding UserInput, Mode=TwoWay}" /> to be selected when the app starts and again after the button is clicked.

How is this achieved in my example?

Answer 1

几乎没有变化。 1.在xaml中删除

 <ContentPage.BindingContext>
    <ViewModels:MainViewModel />
  </ContentPage.BindingContext>

稍后您将需要一个参数来构建MainViewModel。

2.给你的盒子命名：

 <Entry x:Name="myBox" Text="{Binding UserInput, Mode=TwoWay}" />

1. 文件后面的ScannerPage代码

   public ScannerPage()    
   {
       InitializeComponent();
       BindingContext = new MainViewModel(this);
   
   }
   
   protected override void OnAppearing()
   {
           base.OnAppearing();
           myBox.Focus(); //select when apprears
   }        
   public Entry MyBox
   {
           get
           {
               return myBox;
           }        
   }
   

2. MainViewModel文件

   public class MainViewModel : INotifyPropertyChanged
   {
       //private List<Stock> _stockList;
       private string _userInput;
       //public List<Stock> StockList
       //{
       //    get { return _stockList; }
       //    set
       //    {
       //        _stockList = value;
       //        OnPropertyChanged();
       //    }
       //}
   ScannerPage page;
   public MainViewModel(ScannerPage parent)
   {
       page = parent;
   }
   public string UserInput
   {
       get { return _userInput; }
       set
       {
           _userInput = value;
           OnPropertyChanged();
       }
   }  
   public Command PostCommand
   {
       get
       {
           //return new Command(async () =>
           //{
           //    var stockService = new StockService();
           //    StockList = await stockService.GetStockAsync(UserInput);
           //});
           return new Command( () =>
           {
               page.MyBox.Focus(); //set focus when button clicked
           });
       }
   }
   public event PropertyChangedEventHandler PropertyChanged;
   protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
   {
       PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
   }
   

   }

为了能够运行它，我注释掉了一些代码。您可以保留所有代码。