如何向用户控件的构造函数提供对象？

Question

我有一个UserControl，它将一个属性绑定到一个控件，这似乎很有效：

<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，该属性在主窗口中的InitializeComponent之后设置。不过，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？或者我的体系结构是错误的(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。

或者定义一个CCD_ 12。

主窗口.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>