在自定義用戶控件的DependencyProperty上進行綁定而不是在更改時更新

Question

我在自定義用戶控件上進行數據綁定時遇到了困難。我創建了一個示例項目來強調我的問題。我對WPF和MVVM完全陌生，所以忍受着我......

我創建了一個簡單的視圖，它使用數據綁定兩種方式。內置控制的數據綁定工作得很好。我的自定義控件不......我在我的控件的PropertyChangedCallback中放置了一個斷點。它在啓動時會被擊中一次，但不會再次。同時，我已經綁定到相同價值的標籤愉快地倒計時。

我錯過了什麼？我的例子項目如下：

主窗口：

<Window x:Class="WpfMVVMApp.MainWindow" 
     xmlns:local="clr-namespace:WpfMVVMApp" 
     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
     Title="MainWindow" Height="350" Width="525"> 
    <Grid> 
     <Grid.DataContext> 
      <local:CountdownViewModel /> 
     </Grid.DataContext> 
     <Label Name="custName" Content="{Binding Path=Countdown.ChargeTimeRemaining_Mins}" Height="45" VerticalAlignment="Top"></Label> 
     <local:UserControl1 MinutesRemaining="{Binding Path=Countdown.ChargeTimeRemaining_Mins}" Height="45"></local:UserControl1> 
    </Grid> 
</Window> 

這裏是我的模型：

namespace WpfMVVMApp 
{ 

    public class CountdownModel : INotifyPropertyChanged 
    { 
     private int chargeTimeRemaining_Mins; 
     public int ChargeTimeRemaining_Mins 
     { 
      get 
      { 
       return chargeTimeRemaining_Mins; 
      } 
      set 
      { 
       chargeTimeRemaining_Mins = value; 
       OnPropertyChanged("ChargeTimeRemaining_Mins"); 
      } 
     } 

     #region INotifyPropertyChanged Members 
     public event PropertyChangedEventHandler PropertyChanged; 
     private void OnPropertyChanged(string propertyName) 
     { 
      if (PropertyChanged != null) 
       PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); 
     } 
     #endregion 
    } 
} 

視圖模型：

namespace WpfMVVMApp 
{ 
    public class CountdownViewModel 
    { 
     public CountdownModel Countdown { get; set; } 

     DispatcherTimer timer; 
     private const int maxMins = 360; 

     public CountdownViewModel() 
     { 
      Countdown = new CountdownModel { ChargeTimeRemaining_Mins = 60 }; 

      // Setup timers 
      timer = new DispatcherTimer(); 
      timer.Tick += new EventHandler(this.SystemChargeTimerService); 
      timer.Interval = new TimeSpan(0, 0, 1); 
      timer.Start(); 
     } 

     private void SystemChargeTimerService(object sender, EventArgs e) 
     { 
      //convert to minutes remaining 
      // DEMO CODE - TODO: Remove 
      this.Countdown.ChargeTimeRemaining_Mins -= 1; 
     } 
    } 
} 

下面是我的用戶控制的XAML：

<UserControl x:Class="WpfMVVMApp.UserControl1" 
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      mc:Ignorable="d" 
      d:DesignHeight="300" d:DesignWidth="300"> 
    <Grid> 
     <Label Name="Readout"></Label> 
    </Grid> 
</UserControl> 

而這裏的用戶控件後面的代碼：

namespace WpfMVVMApp 
{ 
    public partial class UserControl1 : UserControl 
    { 
     #region Dependency Properties 
     public static readonly DependencyProperty MinutesRemainingProperty = 
        DependencyProperty.Register 
        (
         "MinutesRemaining", typeof(int), typeof(UserControl1), 
         new UIPropertyMetadata(10, new PropertyChangedCallback(minutesRemainChangedCallBack)) 
        ); 
     #endregion 

     public int MinutesRemaining 
     { 
      get 
      { 
       return (int)GetValue(MinutesRemainingProperty); 
      } 
      set 
      { 
       SetValue(MinutesRemainingProperty, value); 
      } 
     } 

     static void minutesRemainChangedCallBack(DependencyObject property, DependencyPropertyChangedEventArgs args) 
     { 
      UserControl1 _readout = (UserControl1)property; 
      _readout.MinutesRemaining = (int)args.NewValue; 

      _readout.Readout.Content = _readout.MinutesRemaining; 
     } 

     public UserControl1() 
     { 
      InitializeComponent(); 
     } 
    } 
} 

Answer 1

您更改回調更是打破了約束力。

作爲骨架：在你的窗口中你有UC.X="{Binding A}"然後在那個屬性改變（在UC）你有X=B;。這打破了約束，因爲在這兩種情況下，你設置了X。

整改，刪除更改回調，這增加了標籤：

Content="{Binding MinutesRemaining, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}}" 

Answer 2

我想你的代碼工作正常，我做出的唯一改變是去掉後面的代碼propertychangedcallback你已經和數據綁定標籤（讀數）到依賴項屬性。

用戶控件（XAML）

<UserControl x:Class="WpfApplication1.UserControl" 
     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
     xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
     mc:Ignorable="d" 
     d:DesignHeight="300" d:DesignWidth="300"> 
    <Grid> 
     <Label Name="Readout" Content="{Binding RelativeSource={RelativeSource 
          AncestorType=UserControl}, Path=MinutesRemaining}"/> 
    </Grid> 
</UserControl> 

用戶控件（代碼隱藏）

public partial class UserControl1 : UserControl 
{ 
    #region Dependency Properties 
    public static readonly DependencyProperty MinutesRemainingProperty = 
       DependencyProperty.Register 
       (
        "MinutesRemaining", typeof(int), typeof(UserControl1), 
        new UIPropertyMetadata(10) 
       ); 
    #endregion 

    public int MinutesRemaining 
    { 
     get 
     { 
      return (int)GetValue(MinutesRemainingProperty); 
     } 
     set 
     { 
      SetValue(MinutesRemainingProperty, value); 
     } 
    } 

    public UserControl1() 
    { 
     InitializeComponent(); 
    } 
}