wpf - 如何取消用户的 WPF TreeView 点击？

Question

我有一个带有 Treeview 控件的 WPF 应用程序。

当用户单击树上的节点时，页面上的其他 TextBox、ComboBox 等控件将填充适当的值。

然后，用户可以更改这些值并通过单击“保存”按钮保存他或她的更改。

但是，如果用户选择了不同的 Treeview 节点而不保存他或她的更改，我想显示一个警告并有机会取消该选择。

MessageBox: Continue and discard your unsaved changes? OK/Cancel http://img522.imageshack.us/img522/2897/discardsj3.gif

XAML...

<TreeView Name="TreeViewThings"
    ...
    TreeViewItem.Unselected="TreeViewThings_Unselected"
    TreeViewItem.Selected="TreeViewThings_Selected" >

视觉基本...

Sub TreeViewThings_Unselected(ByVal sender As System.Object, _
ByVal e As System.Windows.RoutedEventArgs)
Dim OldThing As Thing = DirectCast(e.OriginalSource.DataContext, Thing)
If CancelDueToUnsavedChanges(OldThing) Then
'这里放取消代码
万一
结束子

子 TreeViewThings_Selected(ByVal 发件人作为 System.Object，_
ByVal e As System.Windows.RoutedEventArgs)
将 NewThing 调暗为 Thing = DirectCast(e.OriginalSource.DataContext, Thing)
PopulateControlsFromThing(NewThing)
结束子

如何取消那些取消选择/选择事件？

更新:我问了一个后续问题......
How do I properly handle a PreviewMouseDown event with a MessageBox confirmation?

Answer 1

您实际上不能将逻辑放入 OnSelectedItemChanged 方法中，如果存在逻辑，则 Selected Item 实际上已经更改。

正如另一张海报所建议的，PreviewMouseDown 处理程序是实现逻辑的更好位置，但是，仍然需要完成大量的工作。

下面是我的 2 美分:

首先是我实现的 TreeView:

public class MyTreeView : TreeView
{
    static MyTreeView( )
    {
        DefaultStyleKeyProperty.OverrideMetadata(
            typeof(MyTreeView),
            new FrameworkPropertyMetadata(typeof(TreeView)));
    }

    // Register a routed event, note this event uses RoutingStrategy.Tunnel. per msdn docs
    // all "Preview" events should use tunneling.
    // http://msdn.microsoft.com/en-us/library/system.windows.routedevent.routingstrategy.aspx
    public static RoutedEvent PreviewSelectedItemChangedEvent = EventManager.RegisterRoutedEvent(
        "PreviewSelectedItemChanged",
        RoutingStrategy.Tunnel,
        typeof(CancelEventHandler),
        typeof(MyTreeView));

    // give CLR access to routed event
    public event CancelEventHandler PreviewSelectedItemChanged
    {
        add
        {
            AddHandler(PreviewSelectedItemChangedEvent, value);
        }
        remove
        {
            RemoveHandler(PreviewSelectedItemChangedEvent, value);
        }
    }

    // override PreviewMouseDown
    protected override void OnPreviewMouseDown(MouseButtonEventArgs e)
    {
        // determine which item is going to be selected based on the current mouse position
        object itemToBeSelected = this.GetObjectAtPoint<TreeViewItem>(e.GetPosition(this));

        // selection doesn't change if the target point is null (beyond the end of the list)
        // or if the item to be selected is already selected.
        if (itemToBeSelected != null && itemToBeSelected != SelectedItem)
        {
            bool shouldCancel;

            // call our new event
            OnPreviewSelectedItemChanged(out shouldCancel);
            if (shouldCancel)
            {
                // if we are canceling the selection, mark this event has handled and don't
                // propogate the event.
                e.Handled = true;
                return;
            }
        }

        // otherwise we want to continue normally
        base.OnPreviewMouseDown(e);
    }

    protected virtual void OnPreviewSelectedItemChanged(out bool shouldCancel)
    {
        CancelEventArgs e = new CancelEventArgs( );
        if (PreviewSelectedItemChangedEvent != null)
        {
            // Raise our event with our custom CancelRoutedEventArgs
            RaiseEvent(new CancelRoutedEventArgs(PreviewSelectedItemChangedEvent, e));
        }
        shouldCancel = e.Cancel;
    }
}

一些扩展方法来支持 TreeView 在鼠标下查找对象。

public static class ItemContainerExtensions
{
    // get the object that exists in the container at the specified point.
    public static object GetObjectAtPoint<ItemContainer>(this ItemsControl control, Point p)
        where ItemContainer : DependencyObject
    {
        // ItemContainer - can be ListViewItem, or TreeViewItem and so on(depends on control)
        ItemContainer obj = GetContainerAtPoint<ItemContainer>(control, p);
        if (obj == null)
            return null;

        // it is worth noting that the passed _control_ may not be the direct parent of the
        // container that exists at this point. This can be the case in a TreeView, where the
        // parent of a TreeViewItem may be either the TreeView or a intermediate TreeViewItem
        ItemsControl parentGenerator = obj.GetParentItemsControl( );

        // hopefully this isn't possible?
        if (parentGenerator == null)
            return null;

        return parentGenerator.ItemContainerGenerator.ItemFromContainer(obj);
    }

    // use the VisualTreeHelper to find the container at the specified point.
    public static ItemContainer GetContainerAtPoint<ItemContainer>(this ItemsControl control, Point p)
        where ItemContainer : DependencyObject
    {
        HitTestResult result = VisualTreeHelper.HitTest(control, p);
        DependencyObject obj = result.VisualHit;

        while (VisualTreeHelper.GetParent(obj) != null && !(obj is ItemContainer))
        {
            obj = VisualTreeHelper.GetParent(obj);
        }

        // Will return null if not found
        return obj as ItemContainer;
    }

    // walk up the visual tree looking for the nearest ItemsControl parent of the specified
    // depObject, returns null if one isn't found.
    public static ItemsControl GetParentItemsControl(this DependencyObject depObject)
    {
        DependencyObject obj = VisualTreeHelper.GetParent(depObject);
        while (VisualTreeHelper.GetParent(obj) != null && !(obj is ItemsControl))
        {
            obj = VisualTreeHelper.GetParent(obj);
        }

        // will return null if not found
        return obj as ItemsControl;
    }
}

最后但并非最不重要的是利用 RoutedEvent 子系统的自定义 EventArgs。

public class CancelRoutedEventArgs : RoutedEventArgs
{
    private readonly CancelEventArgs _CancelArgs;

    public CancelRoutedEventArgs(RoutedEvent @event, CancelEventArgs cancelArgs)
        : base(@event)
    {
        _CancelArgs = cancelArgs;
    }

    // override the InvokeEventHandler because we are going to pass it CancelEventArgs
    // not the normal RoutedEventArgs
    protected override void InvokeEventHandler(Delegate genericHandler, object genericTarget)
    {
        CancelEventHandler handler = (CancelEventHandler)genericHandler;
        handler(genericTarget, _CancelArgs);
    }

    // the result
    public bool Cancel
    {
        get
        {
            return _CancelArgs.Cancel;
        }
    }
}