wpf，这个命令为什么换个位置就无效

[货郎大叔]

自定义控件CustomControl3：

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WpfApp1">
    <Style TargetType="{x:Type local:CustomControl3}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:CustomControl3}">
                    <Grid Background="{TemplateBinding Background}">
                        <Grid.RowDefinitions>
                            <RowDefinition/>
                            <RowDefinition/>
                        </Grid.RowDefinitions>
                        <TextBox Name="txt" Width="100" Height="30"/>
                        <Button Grid.Row="1" Command="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:CustomControl3}}, 
                            Path=XiongJunCommand}" CommandParameter="{Binding ElementName=txt,Path=Text}"/>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <local:CustomControl3/>
    </Grid>
</Window>

public class CustomControl3 : Control
{
    static CustomControl3()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(CustomControl3), new FrameworkPropertyMetadata(typeof(CustomControl3)));
    }
    public CustomControl3()
    {
        Loaded += CustomControl3_Loaded;
        XiongJunCommand = new RelayCommand<string>((param) => MessageBox.Show("你好" + param.ToString()));
    }
    private void CustomControl3_Loaded(object sender, RoutedEventArgs e)
    {
        // XiongJunCommand = new RelayCommand<string>((param) => MessageBox.Show("你好" + param.ToString()));
    }
    public RelayCommand<string> XiongJunCommand { get; set; } 
}

在上面的代码中，命令XiongJunCommand放在CustomControl3构造函数中初始化，就会有效果，如果放在CustomControl3_Loaded事件函数中初始化，就没有效果。
这是唱的哪一出啊？

[x8bits]

因为在xxx_Loaded时，WPF已经完成绑定（Binding）：解决方法是实现IPropertyChange接口，然后在XiongJunCommand的设置(setter)里面通知WPF属性已变，这样WPF才能刷新命令绑定。

[x8bits]

也可以把XiongJunCommand改成以DependencyProperty的方式实现