WPF ContextMenu 在MVVM模式中綁定 Command及使用CommandParameter傳參

ContextMenu無論定義在.cs或.xaml文件中，都不繼承父級的DataContext，所以如果要綁定父級的DataContext，直接DataContext=“{Binding}”是行不通的

不能綁父級，但是能綁資源

第一步：定義一個中間類用來做資源對象

public class BindingProxy : Freezable
    {
        #region Overrides of Freezable

        protected override Freezable CreateInstanceCore()
        {
            return new BindingProxy();
        }

        #endregion

        public object Data
        {
            get { return (object)GetValue(DataProperty); }
            set { SetValue(DataProperty, value); }
        }

        public static readonly DependencyProperty DataProperty =
            DependencyProperty.Register("Data", typeof(object), typeof(BindingProxy), new UIPropertyMetadata(null));
    }

第二步：引用命名空間，在控件中定義資源

<UserControl.Resources>
        <libBinding:BindingProxy x:Key="BindingProxy" Data="{Binding}"/>
    </UserControl.Resources>

第三步：綁定ContextMenu、MenuItem

（Button.Command 和 ContextMenu.IsOpen 的綁定部分可以不關注，這兩個綁定是用來控制ContextMenu打開的）

<Button Command="{Binding Customfold}">
            <Button.ContextMenu>
                <ContextMenu DataContext="{Binding Data,Source={StaticResource BindingProxy}}"  
                             ItemsSource="{Binding ItemModelCollection}"
                             IsOpen="{Binding OpenCustomfold,Mode=OneWay}">
                    <ContextMenu.ItemContainerStyle>
                        <Style TargetType="MenuItem">
                            <Setter Property="Header" Value="{Binding ...}"/>
                            <Setter Property="Command" Value="{Binding ...}"/>
                            <Setter Property="CommandParameter" Value="{Binding ...}"/>
                        </Style>
                    </ContextMenu.ItemContainerStyle>
                </ContextMenu>
            </Button.ContextMenu>
            <Image .../>
        </Button>

第四步：傳遞參數

ContextMenu是它自身視覺樹的根節點，所以即使通過RelativeSource.FindAncestor也找不到要傳遞的參數。

解決：可以通過PlacementTarget解決。微軟對PlacementTarget的解釋是：獲取或設置UIElement，當它打開時相對於它確定ContextMenu的位置。應該可以理解為放置此ContextMenu的UIElement。

 CommandParameter="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ContextMenu}, Path=PlacementTarget}"  

如果要傳遞Item，如ListBox的SelectedItem：

 CommandParameter="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ContextMenu}, Path=PlacementTarget.SelectedItem}"  

參考：

http://www.thomaslevesque.com/2011/03/21/wpf-how-to-bind-to-data-when-the-datacontext-is-not-inherited/

http://stackoverflow.com/questions/1580753/wpf-contextmenu-with-itemssource-how-to-bind-to-command-in-each-item