将对象绑定到UserControl

debugcn 发表于 Dev

雅库布（Jakub F.）

我试图从我的主UserControl中控制子UserControls的状态。我正在考虑通过在父UC中创建DependencyProperty对象并在创建子UC时在XAML中引用该对象来实现这一目标。

问题是，我在Step.xaml.cs中获得了NullReferenceException（后面的子代码），所有内容都引用了Controller属性，首先是StepEnabled属性。当我省掉XAML绑定并让它在构造函数中实例化时，就可以了，尽管我当然不能控制任何东西，但是它的Controller属性始终为null ...代码如下：

ParentViewModel：

public class ParentVM : NotifyPropChanged
{
    ...

    public StepController Step1 = new StepController();
    public StepController Step2 = new StepController();
    public StepController Step3 = new StepController();

    public ParentVM()
    {
        ...

        GetReady();
    }

    public void GetReady()
    {
        Step1.Clear();
        Step2.Clear();
        Step3.Clear();

    }

StepController：

public class StepController
{
    public bool StepEnabled;
    public bool? WasSuccessful;

    public StepController()
    {
        Clear();
    }
    public void Clear()
    {
        StepEnabled = false;
        WasSuccessful = null;
    }
}

子代（后面的代码）：

public partial class Step : UserControl
{
    public static DependencyProperty ControllerProperty =
    DependencyProperty.Register("Controller", typeof(StepController), typeof(Step), new UIPropertyMetadata(null));

    public StepController Controller
    {
        get { return (StepController)GetValue(ControllerProperty); }
        set { SetValue(ControllerProperty, value); }
    }

    ...

    public bool StepEnabled
    {
        get { return (bool)Controller.StepEnabled; }
        set { Controller.StepEnabled = value; }
    }
    public bool IsDone
    {
        get
        {
            return (Controller.WasSuccessful != null);
        }
    }
    public bool? WasSuccessful
    {
        get { return Controller.WasSuccessful; }
        set { Controller.WasSuccessful = value; }
    }

    public Step()
    {
        InitializeComponent();
        this.DataContext = this;

        ...

        Controller = new StepController();

    }

}

ParentUC：

...
<local:Step Controller="{Binding Path=Step1, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"/>
...

ChildUC：

e.g.:
<TextBlock IsEnabled="{Binding StepEnabled, UpdateSourceTrigger=PropertyChanged}" />

我对WPF还是很陌生，仍然不确定诸如DataContext之类的很多东西，所以我想我缺少明显的东西了……感谢您的提前帮助！

Nebelkraehe

就像@Synapse已经提到您的DependencyProperty内部一样Step-UserControl应该为您的StepController课程实例化默认值：

public static readonly DependencyProperty ControllerProperty = DependencyProperty.Register(
        "Controller", typeof (StepController), typeof (Step), new PropertyMetadata(default(StepController)));

    public StepController Controller
    {
        get { return (StepController) GetValue(ControllerProperty); }
        set { SetValue(ControllerProperty, value); }
    }

您的ParentUC应该看起来像这样（例如，仅在此处设置DataContext）：

<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"
         xmlns:local="clr-namespace:StepControllerExample" x:Class="StepControllerExample.ParentUserControl"
         mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300">
<UserControl.DataContext>
    <local:ParentVM/>
</UserControl.DataContext>
<StackPanel HorizontalAlignment="Left" Height="100" VerticalAlignment="Top" Width="100">
    <local:Step Controller="{Binding Step1}"/>
    <local:Step Controller="{Binding Step2}"/>
    <local:Step Controller="{Binding Step3}"/>
</StackPanel>

您的子级UserControl XAML如下所示：

<UserControl x:Name="userControl" x:Class="StepControllerExample.Step"
         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">
<StackPanel Orientation="Vertical" d:LayoutOverrides="Height">
    <TextBlock x:Name="textBlock" TextWrapping="Wrap" Text="TextBlock"/>

    <CheckBox x:Name="checkBox" Content="CheckBox" IsChecked="{Binding Controller.StepEnabled, ElementName=userControl}"/>

</StackPanel>

您的孩子的UserControl代码背后是这样的：

public partial class Step : UserControl
{
    public Step()
    {
        InitializeComponent();
    }

    public static readonly DependencyProperty ControllerProperty = DependencyProperty.Register(
        "Controller", typeof (StepController), typeof (Step), new PropertyMetadata(default(StepController)));

    public StepController Controller
    {
        get { return (StepController) GetValue(ControllerProperty); }
        set { SetValue(ControllerProperty, value); }
    }
}

和你这样的StepController班级：

public class StepController : INotifyPropertyChanged
{
    private bool _stepEnabled;
    private bool? _wasSuccessful;

    public StepController()
    {
        Clear();
    }

    public bool StepEnabled
    {
        get { return _stepEnabled; }
        set
        {
            if (value == _stepEnabled) return;
            _stepEnabled = value;
            OnPropertyChanged();
        }
    }

    public bool? WasSuccessful
    {
        get { return _wasSuccessful; }
        set
        {
            if (value == _wasSuccessful) return;
            _wasSuccessful = value;
            OnPropertyChanged();
        }
    }

    public void Clear()
    {
        StepEnabled = false;
        WasSuccessful = null;
    }

    public event PropertyChangedEventHandler PropertyChanged;

    [NotifyPropertyChangedInvocator]
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

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