如何在WPF UserControl中通过ReactiveUI使用带参数的多命令

问题原因与解决方案 #

问题1 参数无法传递的根因 #

1. 你在WidgetUserControl内部的两个Button仅绑定了Command属性，没有显式绑定Button的CommandParameter属性到你定义的用户控件依赖属性，导致命令触发时没有携带参数，ReactiveCommand收到空值时会转为int值类型的默认值0，这就是你看到参数始终为0的原因。
2. 你定义的OnCommandParameterCallback回调存在逻辑错误：无论是CommandParameter还是Command2Parameter触发变更，都会强制给CommandParameter赋值，会覆盖原有值，这个回调没有存在必要，可以直接删除。

问题2 命令与参数的关联规则 #

WPF不存在Command和CommandParameter自动关联的命名约定，不会自动将CommandX和CommandXParameter做绑定匹配。所有命令的参数都需要你手动为触发命令的控件（如Button）显式绑定对应的参数属性，多命令场景下也需要每个命令单独配置绑定。

修复代码示例 #

修改WidgetUserControl的XAML #

<UserControl x:Class="WpfApp1.WidgetUserControl"
             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:WpfApp1"
             mc:Ignorable="d" 
             d:DesignHeight="450" d:DesignWidth="800">
    <Grid>
    <Border BorderBrush="Aqua" BorderThickness="2">
        <StackPanel Orientation="Horizontal" Margin="5">
            <Button Content="Button1"  
                Command="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=UserControl}, Path=Command}"
                CommandParameter="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=UserControl}, Path=CommandParameter}"/>

            <Button Content="Button2" 
                Command="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=UserControl}, Path=Command2}"
                <!-- 需要两个命令传相同参数就绑定CommandParameter，需要不同就绑定Command2Parameter -->
                CommandParameter="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=UserControl}, Path=CommandParameter}"/>
        </StackPanel>
    </Border>
    </Grid>
</UserControl>

修改WidgetUserControl的后台代码 #

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

    public static readonly DependencyProperty CommandProperty = DependencyProperty.Register("Command", typeof(ICommand), typeof(WidgetUserControl));
    public static readonly DependencyProperty CommandParameterProperty = DependencyProperty.Register("CommandParameter", typeof(object), typeof(WidgetUserControl));

    public static readonly DependencyProperty Command2Property = DependencyProperty.Register("Command2", typeof(ICommand), typeof(WidgetUserControl));
    public static readonly DependencyProperty Command2ParameterProperty = DependencyProperty.Register("Command2Parameter", typeof(object), typeof(WidgetUserControl));

    public ICommand Command
    {
        get => (ICommand)GetValue(CommandProperty);
        set => SetValue(CommandProperty, value);
    }

    public object CommandParameter
    {
        get => GetValue(CommandParameterProperty);
        set => SetValue(CommandParameterProperty, value);
    }

    public ICommand Command2
    {
        get => (ICommand)GetValue(Command2Property);
        set => SetValue(Command2Property, value);
    }

    public object Command2Parameter
    {
        get => GetValue(Command2ParameterProperty);
        set => SetValue(Command2ParameterProperty, value);
    }
}

如果需要给Command2传独立参数，只需要在MainWindow中给WidgetUserControl添加Command2Parameter="你的参数"属性赋值，同时将第二个Button的CommandParameter绑定到Command2Parameter即可。

内容的提问来源于stack exchange，提问作者canice