如何向 UserControl 的构造函数提供对象？ [复制]

Question

我有一个 UserControl 将属性绑定到控件，这似乎有效：

<UserControl x:Class="MyUserControl" Name="control">
<TextBox Text="{Binding SomeData, ElementName=control}" />

但是，获取此特定属性的信息需要设置另一个属性，该属性设置在主窗口中的InitializeComponent 之后。我的UserControl 的构造函数在InitializeComponent 中被调用，导致NullReferenceException：

public MainWindow() {
   InitializeComponent();           // <-- here my UserControl is instatiated and needs Settings

   Settings = new Settings();    
   MyUserControl.Settings = Settings;    // <-- only here do we set the Settings object (which is needed in UserControl's constructor!
}


public MyUserControl {
    public string SomeData { 
         get {
               return Settings.Get("someSetting");
         }
    }
}

我该如何解决这个问题？还是我的架构错了（刚开始使用 WPF 和数据绑定）。

Answer 1

您应该将MyUserControl 绑定到MainWindow.Settings 属性，该属性应该是DependencyProperty。然后将初始化从MyUserControl 的构造函数移动到MyUserControl.Loaded 事件处理程序。执行此处理程序时，绑定的Settings 值将可用。

另外定义一个PropertyChangedCallback。

MainWindow.xaml.cs

partial class MainWindow : Window
{
  public static readonly DependencyProperty SettingsProperty = DependencyProperty.Register(
    "Settings",
    typeof(Settings),
    typeof(MainWindow),
    new PropertyMetadata(default(Settings)));

  public Settings Settings
  {
    get => (Settings) GetValue(MainWindow.SettingsProperty);
    set => SetValue(MainWindow.SettingsProperty, value);
  }

  public MainWindow() 
  {
    InitializeComponent();
    this.Settings = new Settings();   
  }
}

MyUserControl.xaml.cs

partial class MyUserControl : UserControl
{
  public static readonly DependencyProperty SettingsProperty = DependencyProperty.Register(
    "Settings",
    typeof(Settings),
    typeof(MyUserControl),
    new PropertyMetadata(default(Settings), OnCurrentReadingChanged));

  public Settings Settings
  {
    get => (Settings) GetValue(MyUserControl.SettingsProperty);
    set => SetValue(MyUserControl.SettingsProperty, value);
  }

  public MyUserControl() 
  {
    InitializeComponent();
    this.Loaded += Initialize;   
  }

  private static void OnCurrentReadingChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
  {
    Settings settings = (d as MyUserControl).Settings;
    // TODO: Initialize using this.Settings
  }

  public Initialize(object sender, EventArgs e) 
  {
    // TODO: Initialize using this.Settings
  }
}

MyUserControl.xaml

<UserControl x:Class="MyUserControl" 
             Name="control" 
             Settings="{Binding RelativeSource={RelativeSource AncestorType=MainWindow}, Path=Settings}">
  <TextBox Text="{Binding SomeData, ElementName=control}" />
</UserControl>