ComboBox DataContex 未更新放置在 ContentControl (UWP) 内的属性？

Question

我在ContentTemplate 的ContentControl 中使用了ComboBox。最初，我在后面的代码中将DataContext (Profession1) 与ContentControl 的内容属性绑定。

然后我在运行时更改ComboBox 的值。之后，在运行时将ContentControl 的DataContext 更改为Profession2。在这种情况下，ComboBox DataContext 不会更改，之前的 DataContext 的 SelectedItem 值也设置为 null。

我认为这是 MSControl 中的一个问题。请任何建议您的想法来解决此问题。

请参考以下代码：

查看：

    <Page.Resources>
        <local:MainPageViewModel x:Key="datacontent"></local:MainPageViewModel>
    </Page.Resources>
    <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
          Margin="10">
        <ContentControl x:Name="contentControl" >
            <ContentControl.ContentTemplate>
                <DataTemplate>
                    <ComboBox x:Name="comboBox"
                              ItemsSource="{Binding Professions}"
                              SelectedItem="{Binding Profession, Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"                                     
                              HorizontalAlignment="Stretch"></ComboBox>
                </DataTemplate>
            </ContentControl.ContentTemplate>
        </ContentControl>
         <Button Click="Button_Click_2" Content="Change DataContext" Width="100" Height="50"></Button>
     </Grid>
</Page>

模型和视图模型：

  public class MainPageViewModel
    {
        public MainPageViewModel()
        {            
            Profession1 = new Person();
            Profession2 = new Person();                      
          private Person profession1;

        public Person Profession1
        {
            get { return profession1; }
            set { this.profession1 = value; }
        }

        private Person profession2;

        public Person Profession2
        {
            get { return profession2; }
            set { this.profession2 = value; }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        private void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

    }

    public class Person : INotifyPropertyChanged
    {
        public Person()
        {
            _professions = new List<string>();
            _professions.Add("Lawyer");
            _professions.Add("Politician");
            _professions.Add("Other");         
        }

        private string _profession;
        public string Profession
        {
            get
            {
                if (string.IsNullOrWhiteSpace(_profession))
                {
                    // _profession = _professions.LastOrDefault();
                }
                return _profession;
            }
            set
            {
                if (_profession != value)
                {
                    _profession = value;
                    NotifyPropertyChanged("Profession");
                }
            }
        }      

        private List<string> _professions;

        public List<string> Professions
        {
            get
            {
                return _professions;
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        private void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }   

代码背后：

public sealed partial class MainPage : Page
    {
        Binding binding;
        public MainPage()
        {
            this.InitializeComponent();
            var datacontent = (this.Resources["datacontent"] as MainPageViewModel);
            this.UpdateBinding1(this.contentControl, datacontent);
        }         

        public void UpdateBinding1(FrameworkElement contentcontrol, object datacontext)
        {
            binding = new Binding();
            binding.Path = new PropertyPath("Profession1");
            binding.Mode = BindingMode.TwoWay;
            binding.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;
            binding.Source = datacontext;  // view model
            BindingOperations.SetBinding(contentcontrol, ContentControl.ContentProperty, binding);
        }

        public void UpdateBinding(FrameworkElement contentcontrol, object datacontext)
        {
            binding = new Binding();
            binding.Path = new PropertyPath("Profession2");
            binding.Mode = BindingMode.TwoWay;
            binding.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;
            binding.Source = datacontext;  // view model
            BindingOperations.SetBinding(contentcontrol, ContentControl.ContentProperty, binding);
        }

       private void Button_Click_2(object sender, RoutedEventArgs e)
        {
            var datacontext = (this.Resources["datacontent"] as MainPageViewModel);
            this.UpdateBinding(this.contentControl,datacontext);
        }
}

Answer 1

之后，在运行时将 ContentControl 的 DataContext 更改为 Profession2。

绑定路径确实从Profession1 更新为Profession2。并且更新了ComboBox 的ItemsSource。但是由于您将Profession1 和Profession2 定义为相同的值，因此看起来绑定源没有更新。请将它们设置为不同的值以检查结果。例如，更新您的MainViewModel 如下：

public class MainPageViewModel
{
    List<string> Profession1list=new List<string>() { "Lawyer", "Politician", "Other" };
    List<string> Profession2list = new List<string>() { "Lawyer2", "Politician2", "Other2" };
    public MainPageViewModel()
    {
        Profession1 = new Person(Profession1list);
        Profession2 = new Person(Profession2list);
    }
      private Person profession1;

    public Person Profession1
    {
        get { return profession1; }
        set { this.profession1 = value; }
    }

    private Person profession2;

    public Person Profession2
    {
        get { return profession2; }
        set { this.profession2 = value; }
    }

   ...

}

public class Person : INotifyPropertyChanged
{
    public Person(List<string> Professionlist)
    {
        _professions = new List<string>();
        foreach(string item in Professionlist)
        {
            _professions.Add(item);
        }

        //_professions.Add("Lawyer");
        //_professions.Add("Politician");
        //_professions.Add("Other");
    }     

    private List<string> _professions;

    public List<string> Professions
    {
        get
        {
            return _professions;
        }
    }
  ...

}

在这种情况下，ComboBox DataContext 没有改变，SelectedItem 值之前的 DataContext 也设置为 null。

由于ComboBox的ItemsSource是通过重新绑定更新的，所以SelectedItem被清除了。

我的测试环境是 OS build 15063。更多细节请参考Data binding overview。如果您详细说明您用于此功能的场景，我们可能会为此提供更好的实践。例如，避免更新绑定路径以满足需求，而是通过其他方式。