条目上的多个行为（Xamarin 表单）

Question

I've had a lot of trouble implementing multiple behaviors on a entry

I want to implement user input validation if the user type between integers, decimal numbers, date and text

Hope you can help or send examples

Answer 1

You are quite lucky, because Just recently I have implemented very similar thing:

 private FormQuestion formModel {get;set;}
 public Form()
    {
        InitializeComponent();
        //Hook up events, set layout of main Stack
        CreateViews(formModel);
        mainStack.Orientation = StackOrientation.Vertical;
        mainStack.VerticalOptions = LayoutOptions.FillAndExpand;
        mainStack.HorizontalOptions = LayoutOptions.FillAndExpand;
    }
    #endregion
    /// <summary>
    /// Method that returns Stack layout that contains control required for filling up the form if QuestionType is Text.
    /// This method creates StackLayout, Label and Entry then returns it.
    /// </summary>
    /// <param name="formQuestions"></param>
    /// <returns></returns>
    private StackLayout CreateQuestionStackText(FormQuestion formQuestions)
    {
        StackLayout stack = new StackLayout
        {
            Orientation = StackOrientation.Horizontal,
            VerticalOptions = LayoutOptions.Center,
            HorizontalOptions = LayoutOptions.FillAndExpand
        };
        Label label = new Label
        {
            HorizontalOptions = LayoutOptions.Start,
            VerticalOptions = LayoutOptions.CenterAndExpand
        };
        label.SetBinding(Label.TextProperty, "Text");
        Entry entry = new Entry
        {
            HorizontalOptions = LayoutOptions.EndAndExpand,
            VerticalOptions = LayoutOptions.CenterAndExpand,
            WidthRequest = 200
        };
        entry.SetBinding(Entry.TextProperty, "Value");
        stack.Children.Add(label);
        stack.Children.Add(entry);
        return stack;

    }
    /// <summary>
    /// Method that returns Stack layout that contains control required for filling up the form if QuestionType is Number. 
    /// This method creates StackLayout, Label and Entry that enforces Numerical Keyboard then returns it.  
    /// </summary>
    /// <param name="formQuestions"></param>
    /// <returns></returns>
    private StackLayout CreateQuestionStackNumber(FormQuestion formQuestions)
    {
        StackLayout stack = new StackLayout
        {
            Orientation = StackOrientation.Horizontal,
            VerticalOptions = LayoutOptions.Center,
            HorizontalOptions = LayoutOptions.FillAndExpand
        };
        Label label = new Label
        {
            HorizontalOptions = LayoutOptions.Start,
            VerticalOptions = LayoutOptions.CenterAndExpand
        };
        label.SetBinding(Label.TextProperty, "Text");
        Entry entry = new Entry
        {
            HorizontalOptions = LayoutOptions.EndAndExpand,
            VerticalOptions = LayoutOptions.CenterAndExpand,
            Keyboard = Keyboard.Numeric,
            WidthRequest = 200
        };
        entry.SetBinding(Entry.TextProperty, "Value");
        stack.Children.Add(label);
        stack.Children.Add(entry);
        return stack;
    }
    /// <summary>
    /// Method that returns Stack layout that contains control required for filling up the form if QuestionType is DropDown
    /// This method creates StackLayout, Label and BindablePicker, which is custom Picker I've created to be able to get work with MVVM, then returns it.
    /// </summary>
    /// <param name="formQuestions"></param>
    /// <returns></returns>
    private StackLayout CreateQuestionStackDropDown(FormQuestion formQuestions)
    {
        StackLayout stack = new StackLayout
        {
            Orientation = StackOrientation.Horizontal,
            VerticalOptions = LayoutOptions.Center,
            HorizontalOptions = LayoutOptions.FillAndExpand
        };
        Label label = new Label 
        {
            HorizontalOptions = LayoutOptions.Start,
            VerticalOptions = LayoutOptions.CenterAndExpand
        };
        label.SetBinding(Label.TextProperty, "Text");
        BindablePicker picker = new BindablePicker();
        picker.HorizontalOptions = LayoutOptions.EndAndExpand;
        picker.BindingContext = this;
        var bind = this.BindingContext;
        var strings = formQuestions.DataSource.Values;
        var listOfStrings = strings.Split(';');
        List<string> result = new List<string>();
        foreach (var item in listOfStrings)
        {
            result.Add(item);
        }
        picker.ItemsSource = result;
        picker.SelectedIndexChanged += Picker_SelectedIndexChanged;
        //picker.SetBinding(BindablePicker.DisplayMemberPathProperty, new Binding( "Values", BindingMode.TwoWay, new ValuesToListOfStringsConverter(), null));
        picker.SetBinding(BindablePicker.SelectedItemProperty, "Value");
        picker.WidthRequest = 200;
        stack.Children.Add(label);
        stack.Children.Add(picker);
        return stack;
    }
    /// <summary>
    /// Method that returns Stack layout that contains control required for filling up the form if QuestionType is Data
    /// This method creates StackLayout, Label and DataPicker and returns it.
    /// </summary>
    /// <param name="formQuestions"></param>
    /// <returns></returns>
    private StackLayout CreateQuestionStackDate(FormQuestion formQuestions)
    {
        StackLayout stack = new StackLayout
        {
            Orientation = StackOrientation.Horizontal,
            VerticalOptions = LayoutOptions.Center,
            HorizontalOptions = LayoutOptions.FillAndExpand
        };
        Label label = new Label
        {
            HorizontalOptions = LayoutOptions.Start,
            VerticalOptions = LayoutOptions.CenterAndExpand
        };
        label.SetBinding(Label.TextProperty, "Text");
        DatePicker datePicker = new DatePicker();
        datePicker.HorizontalOptions = LayoutOptions.EndAndExpand;
        datePicker.SetBinding(DatePicker.DateProperty, "Value");
        stack.Children.Add(label);
        stack.Children.Add(datePicker);
        return stack;
    }
    /// <summary>
    /// Little hack, that unfortunately I had to implement. Because DropDown contains list of DataSource Properties that is essentially array of objects, I had to work around with pulling data of selected item and only
    /// get Values property value. This couldnt be bound correctly, because Values are expecting to be string, but SelectedItem property of picker is an actuall full Object of DataSource. 
    /// I Could probably work around it by Implementing Converter, but Converter will have more code, and I couldnt be bothered. 
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void Picker_SelectedIndexChanged(object sender, EventArgs e)
    {
        //Cast sender object to BindablePicker (since sender is BindablePicker)
        var picker = (BindablePicker)sender;
        ///Get SelectedItem Value and cast it to DataSource (since SelectedItem is DataSource)
        var value = (string)picker.SelectedItem;
        // Assign Values value back to My Collection
        ((FormQuestion)BindingContext).Value = value;
    }
    /// <summary>
    /// Determine which stack should be created, depending on QuestionType
    /// </summary>
    /// <param name="formModel"></param>
    private void CreateViews(FormQuestion formModel)
    {
        StackLayout newStack = new StackLayout();
                switch (formModel.QuestionType.Name)
                {
                    case "text":
                    newStack.Children.Add(CreateQuestionStackText(formModel));
                        break;
                    case "number":
                    newStack.Children.Add(CreateQuestionStackNumber(formModel));
                        break;
                    case "dropDown":
                    newStack.Children.Add(CreateQuestionStackDropDown(formModel));
                        break;
                    case "date":
                    newStack.Children.Add(CreateQuestionStackDate(formModel));
                        break;
                    default:
                    newStack.Children.Add(CreateQuestionStackText(formModel));
                    break;
                }
        this.mainStack.Children.Add(newStack);
    }
    #endregion

    public class FormQuestion
{
    public int FormQuestionId { get; set; }
    public string Text { get; set; }
    public string Value { get; set; }
    public int QuestionTypeId { get; set; }
    public virtual QuestionType QuestionType { get; set; }
    public int FormId { get; set; }
    public Form Form { get; set; }
    public DataSource DataSource { get; set; }
}
   public class QuestionType
{
    public int QuestionTypeId { get; set; }

    public string Name { get; set; }
}

This would be your Xaml.

            <StackLayout VerticalOptions="FillAndExpand" HorizontalOptions="FillAndExpand">
    <Frame BorderColor="Gray" CornerRadius="2" VerticalOptions="FillAndExpand" HorizontalOptions="FillAndExpand" >
    <StackLayout Orientation="Vertical" VerticalOptions="FillAndExpand" HorizontalOptions="FillAndExpand" x:Name="mainStack">
    </StackLayout>
    </Frame>
    </StackLayout>

You will need to modify slightly to fit your purpose, but I have provided you my implementation so you have basic grip and concept of what you should do. also bear in mind this was done to use MVVM. I also stripped out some methods that are irrelavant. BindablePicker is a Extended Version of Picker due to standard Xamarin Forms picker does not have bindable properties. Main purpose of my code was to dynamically create different type of entry, picker, datepicker etc depending on Form type that comes from back end and bind them to ViewModel inside ListView.