ViewModels PropertyChanged事件后的属性始终为空

Question

这是我第一次使用C＃以及.net的一般体验。我试图理解MVVM以及它如何与我想构建的屏幕一起工作。

当我点击不同的按钮后改变它时，我的按钮文本没有更新。调试器显示即使OnPropertyChanged函数被调用，PropertyChanged也始终为空。

我已经看过一些文档，并看过其他人的帖子提问类似的问题，但没有任何解决方案为我工作。我也在寻找一个解释，所以我可以理解我做错了什么。

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

只有创建一个视图模型，其范围是页面的生命（至少）。

你的代码改成这样：

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); 
    } 
}