Question

在我的xaml中，我使用q转换器检查AlarmStatus true或false。当用户编辑现有警报时，它可以正常工作，但是当用户创建新警报时，属性AlarmStatus不存在，并且未设置背景颜色。如果属性AlarmStatus存在，是否可以签入xaml？如果没有，则应将颜色设置为红色。

public class AlarmModel
{
    [PrimaryKey, AutoIncrement]
    public int Id { get; set; }
    [MaxLength(255)]
    public string Title { get; set; }

    [MaxLength(255)]
    public string Content { get; set; }

    public DateTime TimeCreate { get; set; }

    public DateTime AlarmTime { get; set; }
    public bool AlarmStatus { get; set; }

}

[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class AlarmView : TabbedPage
{
    private AlarmModel CurrentAlarm;

    //if user create a new alarm then no argument pass in constructor
    public AlarmView (AlarmModel alarm = null)
    {
        InitializeComponent ();
        if (alarm != null)
        {
            this.CurrentAlarm = alarm;                         
        }

        BindingContext = CurrentAlarm;

    }
}

<StackLayout VerticalOptions="FillAndExpand">
            <Entry  x:Name="titleInput" Text="{Binding Title}" HeightRequest="50" FontSize="20"/>               
            <Editor x:Name="contentInput" Text="{Binding Content}" VerticalOptions="FillAndExpand" />
            <Button Text="Start" BackgroundColor="{Binding AlarmStatus,Converter={StaticResource ColorConverter}}" Clicked="Button_Clicked"/>
</StackLayout>

class ColorConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if(value==null)return Color.Black;

        bool b = ((bool)value);
        return b ? Color.Green : Color.Red;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Answer 1

将AlarmModel.AlarmStatus属性更新为可为空的布尔值，即bool?，以允许您描述的三种状态。

public class AlarmModel {
    [PrimaryKey, AutoIncrement]
    public int Id { get; set; }
    [MaxLength(255)]
    public string Title { get; set; }

    [MaxLength(255)]
    public string Content { get; set; }

    public DateTime TimeCreate { get; set; }

    public DateTime AlarmTime { get; set; }
    public bool? AlarmStatus { get; set; } //<-- needs to be nullable for 3 states
}

转换器预计会bool?

public class ColorConverter : IValueConverter {
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
        var status = (bool?) value;
        if (status == null) return Color.Black;                
        return status.Value ? Color.Green : Color.Red;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

您还需要确保在创建新警报时有一个要绑定的模型。

[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class AlarmView : TabbedPage {
    private AlarmModel CurrentAlarm;

    //if user create a new alarm then no argument pass in constructor
    public AlarmView (AlarmModel alarm = null) {
        InitializeComponent ();
        if (alarm != null) {
            this.CurrentAlarm = alarm;                         
        }
        BindingContext = CurrentAlarm ?? new AlarmModel(); //Must have a model to bind to.
    }
}

您没有分配绑定上下文，这意味着视图无法绑定到任何内容。如果没有提供模型，则必须为要绑定的视图创建一个模型并获得所需的行为。

Answer 2

每按一次按钮实现警报状态的更改。真 - 绿色，假 - 红色，不存在 - 黑色。

<强> XML：

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:ButtonRendererDemo;assembly=ButtonRendererDemo"
             x:Class="ButtonRendererDemo.ValueConverterPage">

    <ContentPage.Resources>
        <ResourceDictionary>
            <local:ColorConverter x:Key="colorConvert"></local:ColorConverter>
        </ResourceDictionary>
    </ContentPage.Resources>

    <ContentPage.Content>
        <StackLayout>
            <Button Text="Click me" BackgroundColor="{Binding AlarmStatus, Converter={StaticResource colorConvert}}" Clicked="Button_Clicked"/>
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

代码背后：

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace ButtonRendererDemo
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class ValueConverterPage : ContentPage
    {
        public ValueConverterPage()
        {
            InitializeComponent();
            BindingContext = this;
        }

        bool? alarmStatus = null;
        public bool? AlarmStatus
        {
            get
            {
                return alarmStatus;
            }
            set
            {
                alarmStatus = value;
                OnPropertyChanged("AlarmStatus");
            }
        }

        private void Button_Clicked(object sender, EventArgs e)
        {
            switch (AlarmStatus)
            {
                case true:
                    AlarmStatus = false;
                    break;
                case false:
                    AlarmStatus = null;
                    break;
                case null:
                    AlarmStatus = true;
                    break;
            }
        }
    }

    public class ColorConverter : IValueConverter
    {

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            switch (value as bool?)
            {
                case true:
                    return Color.Green;
                case false:
                    return Color.Red;
                case null:
                default:
                    return Color.Black;
            }
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }

    }
}

Xamarin，如果存在可绑定属性，如何检入xaml？

2 个答案: