Question

这是我第一次使用C＃，以及.net。我试图了解MVVM以及它如何与我想要构建的屏幕一起工作。

单击其他按钮后，我在更改时，我的按钮文本没有更新。调试器显示即使OnPropertyChanged函数被调用，PropertyChanged也始终为null。

我查看了一些文档并查看了其他人提出类似问题的帖子，但这些解决方案都没有对我有用。我也在寻找解释，以便我能理解我做错了什么。

代码：

MonthViewPage.xaml

<?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="Mobile_Release_POC_4.MonthViewPage" 
             xmlns:local="clr-namespace:Mobile_Release_POC_4;assembly=Mobile_Release_POC_4"
             xmlns:telerikInput="clr-namespace:Telerik.XamarinForms.Input;assembly=Telerik.XamarinForms.Input"
             xmlns:sys="clr-namespace:System;assembly=mscorlib">

            <ContentPage.BindingContext>
                 <local:WeekViewDatesViewModel MiddleWeekViewDate='{x:Static sys:DateTime.Now}' />
            </ContentPage.BindingContext>

            <StackLayout>
               <Label Text="Monday, April 27, 2015"
               HorizontalOptions="Center"></Label>

               <Grid Padding="0" ColumnSpacing="-2">

                     <Grid.RowDefinitions>
                           <RowDefinition Height="75" />
                     </Grid.RowDefinitions>

                     <Grid.ColumnDefinitions >
                          <ColumnDefinition Width="50" />
                     </Grid.ColumnDefinitions>

                     <Button x:Name="monthViewDateButton1"
                          Text="{Binding MiddleWeekViewDate}"
                          FontSize="10"
                          Grid.Row="0"
                          Grid.Column="0"
                      />
                     <Button x:Name="monthViewDateButton2"
                     Text="Tue 4/28"
                     FontSize="10"
                     Grid.Row="0"
                     Grid.Column="1"
                     Clicked="OnButtonClicked"
                     />

               </Grid>


         </StackLayout>

MonthViewPage.xaml.cs

using Xamarin.Forms;

namespace Mobile_Release_POC_4
{
public partial class MonthViewPage : ContentPage
{
    public MonthViewPage ()
    {
        InitializeComponent();

    }


    void OnButtonClicked(object sender , EventArgs args){

        WeekViewDatesViewModel dc = new WeekViewDatesViewModel ();

        dc.MiddleWeekViewDate = new DateTime (2014, 2, 2);
    }
}

WeekViewDatesViewModel.cs

    namespace Mobile_Release_POC_4
{
    public class WeekViewDatesViewModel : INotifyPropertyChanged
    {
        DateTime middleWeekViewDate;


        public event PropertyChangedEventHandler PropertyChanged;


        public DateTime MiddleWeekViewDate
        {
            set
            {
                if (middleWeekViewDate != value)
                {
                    middleWeekViewDate = value;
                    OnPropertyChanged("MiddleWeekViewDate");
                }
            }
            get
            {
                return middleWeekViewDate;
            }
        }

        protected void OnPropertyChanged(string propertyName)
        {
            var handler = PropertyChanged;
            if (PropertyChanged != null)
            {
                PropertyChanged(this,
                    new PropertyChangedEventArgs(propertyName));
            }
        }


    }
}

谢谢

Answer 1

只创建一个ViewModel，其范围是页面的生命周期（至少）。

将您的代码更改为：

public partial class MonthViewPage : ContentPage
{

    public WeekViewDatesViewModel VM { get; set; } 

    public MonthViewPage ()
    {
        DataContext = VM = new  WeekViewDatesViewModel();
        InitializeComponent();
    }


    void OnButtonClicked(object sender , EventArgs args){

        VM.MiddleWeekViewDate = new DateTime (2014, 2, 2);
    }
}

PropertyChanged事件后的ViewModels属性始终为空

1 个答案: