如何通过按钮将焦点设置到嵌套UserControl内的元素？

需要实现点击主窗口按钮，直接将焦点定位到多层嵌套UserControl中的TextBox或PasswordBox，要求完全在View层面实现，不依赖ViewModel处理视图细节。当前尝试的FocusManager.FocusedElement绑定方式无法生效。

解决方案：通过依赖属性在UserControl中暴露焦点触发能力 #

这种方式完全在View层处理，无需ViewModel参与，同时保证了UserControl的封装性。

1. 修改UserView，添加焦点触发依赖属性 #

在UserView.xaml.cs中添加两个依赖属性，用于触发用户名输入框和密码框的焦点：

public partial class UserView : UserControl
{
    // 用户名焦点触发属性
    public static readonly DependencyProperty IsUserNameFocusedProperty =
        DependencyProperty.Register(nameof(IsUserNameFocused), typeof(bool), typeof(UserView), 
            new PropertyMetadata(false, OnIsUserNameFocusedChanged));

    // 密码框焦点触发属性
    public static readonly DependencyProperty IsPasswordFocusedProperty =
        DependencyProperty.Register(nameof(IsPasswordFocused), typeof(bool), typeof(UserView), 
            new PropertyMetadata(false, OnIsPasswordFocusedChanged));

    public bool IsUserNameFocused
    {
        get => (bool)GetValue(IsUserNameFocusedProperty);
        set => SetValue(IsUserNameFocusedProperty, value);
    }

    public bool IsPasswordFocused
    {
        get => (bool)GetValue(IsPasswordFocusedProperty);
        set => SetValue(IsPasswordFocusedProperty, value);
    }

    private static void OnIsUserNameFocusedChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if ((bool)e.NewValue && d is UserView view)
        {
            view.txtUserName.Focus();
            // 重置属性，避免重复点击时无法触发
            view.IsUserNameFocused = false;
        }
    }

    private static void OnIsPasswordFocusedChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if ((bool)e.NewValue && d is UserView view)
        {
            view.pwdPassword.Focus();
            view.IsPasswordFocused = false;
        }
    }

    public UserView()
    {
        InitializeComponent();
    }
}

2. 在MainWindow中用行为绑定按钮点击事件 #

使用WPF行为库（Microsoft.Xaml.Behaviors.Wpf，.NET Framework可使用System.Windows.Interactivity），通过声明式XAML实现按钮点击触发焦点：

首先在MainWindow的XAML中添加行为命名空间：

xmlns:i="http://schemas.microsoft.com/xaml/behaviors"

然后修改按钮的XAML代码：

<Window x:Class="Project.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:Local.Views="clr-namespace:Project.Views"
        xmlns:i="http://schemas.microsoft.com/xaml/behaviors">
  <DockPanel>
    <Local.Views:AllUsersView x:Name="vwAllUsers" DataContext="{Binding Users}" />
    <UniformGrid Columns="3">
      <Button Content="Enter User 1">
        <i:Interaction.Triggers>
          <i:EventTrigger EventName="Click">
            <i:SetPropertyAction TargetObject="{Binding ElementName=vwAllUsers, Path=vwUser1}" 
                                PropertyName="IsUserNameFocused" Value="True" />
          </i:EventTrigger>
        </i:Interaction.Triggers>
      </Button>
      <Button Content="Enter User 2">
        <i:Interaction.Triggers>
          <i:EventTrigger EventName="Click">
            <i:SetPropertyAction TargetObject="{Binding ElementName=vwAllUsers, Path=vwUser2}" 
                                PropertyName="IsUserNameFocused" Value="True" />
          </i:EventTrigger>
        </i:Interaction.Triggers>
      </Button>
      <Button Content="Enter User 3">
        <i:Interaction.Triggers>
          <i:EventTrigger EventName="Click">
            <i:SetPropertyAction TargetObject="{Binding ElementName=vwAllUsers, Path=vwUser3}" 
                                PropertyName="IsUserNameFocused" Value="True" />
          </i:EventTrigger>
        </i:Interaction.Triggers>
      </Button>
      <Button Content="Enter Password 1">
        <i:Interaction.Triggers>
          <i:EventTrigger EventName="Click">
            <i:SetPropertyAction TargetObject="{Binding ElementName=vwAllUsers, Path=vwUser1}" 
                                PropertyName="IsPasswordFocused" Value="True" />
          </i:EventTrigger>
        </i:Interaction.Triggers>
      </Button>
      <Button Content="Enter Password 2">
        <i:Interaction.Triggers>
          <i:EventTrigger EventName="Click">
            <i:SetPropertyAction TargetObject="{Binding ElementName=vwAllUsers, Path=vwUser2}" 
                                PropertyName="IsPasswordFocused" Value="True" />
          </i:EventTrigger>
        </i:Interaction.Triggers>
      </Button>
      <Button Content="Enter Password 3">
        <i:Interaction.Triggers>
          <i:EventTrigger EventName="Click">
            <i:SetPropertyAction TargetObject="{Binding ElementName=vwAllUsers, Path=vwUser3}" 
                                PropertyName="IsPasswordFocused" Value="True" />
          </i:EventTrigger>
        </i:Interaction.Triggers>
      </Button>
    </UniformGrid>
  </DockPanel>
</Window>

3. 替代方案：纯代码隐藏实现 #

如果不想引入行为库，可以直接在MainWindow的代码隐藏中处理按钮点击事件：

首先给按钮添加Click事件绑定：

<Window x:Class="Project.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:Local.Views="clr-namespace:Project.Views">
  <DockPanel>
    <Local.Views:AllUsersView x:Name="vwAllUsers" DataContext="{Binding Users}" />
    <UniformGrid Columns="3">
      <Button Content="Enter User 1" Click="FocusUser1Name" />
      <Button Content="Enter User 2" Click="FocusUser2Name" />
      <Button Content="Enter User 3" Click="FocusUser3Name" />
      <Button Content="Enter Password 1" Click="FocusUser1Password" />
      <Button Content="Enter Password 2" Click="FocusUser2Password" />
      <Button Content="Enter Password 3" Click="FocusUser3Password" />
    </UniformGrid>
  </DockPanel>
</Window>

然后在MainWindow.xaml.cs中实现事件处理方法：

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void FocusUser1Name(object sender, RoutedEventArgs e)
    {
        var userNameBox = vwAllUsers.vwUser1.FindName("txtUserName") as TextBox;
        userNameBox?.Focus();
    }

    private void FocusUser2Name(object sender, RoutedEventArgs e)
    {
        var userNameBox = vwAllUsers.vwUser2.FindName("txtUserName") as TextBox;
        userNameBox?.Focus();
    }

    private void FocusUser3Name(object sender, RoutedEventArgs e)
    {
        var userNameBox = vwAllUsers.vwUser3.FindName("txtUserName") as TextBox;
        userNameBox?.Focus();
    }

    private void FocusUser1Password(object sender, RoutedEventArgs e)
    {
        var passwordBox = vwAllUsers.vwUser1.FindName("pwdPassword") as PasswordBox;
        passwordBox?.Focus();
    }

    private void FocusUser2Password(object sender, RoutedEventArgs e)
    {
        var passwordBox = vwAllUsers.vwUser2.FindName("pwdPassword") as PasswordBox;
        passwordBox?.Focus();
    }

    private void FocusUser3Password(object sender, RoutedEventArgs e)
    {
        var passwordBox = vwAllUsers.vwUser3.FindName("pwdPassword") as PasswordBox;
        passwordBox?.Focus();
    }
}

内容的提问来源于stack exchange，提问作者Derrick Moeller