WPF mvvm绑定，获取/设置值：阻止更新值的问题

Question

I'm working in a WPF mvvm environment.

I have some binded vars and data from cs file to xaml.

One is different from others: it is the index of the selected tab in my tabsCollection. When the user has more than one tab opened and has got mods to save, I show him a dialog. If he cliks "ok", he proceed with the change of the tab, if he clicks "cancel", the tab must remain the same.

this is my code:

private int p_SelectedDocumentIndex;
public int SelectedDocumentIndex{ get { return p_SelectedDocumentIndex; }
    set {
        if (tabsCollection.Count() > 1 && CanSave() == true)
        {
            if (dm.ShowMessage1(ServiceContainer.GetService<DevExpress.Mvvm.IDialogService>("confirmYesNo")))
            {
                p_SelectedDocumentIndex = value;
                base.RaisePropertiesChanged("SelectedDocumentIndex");
            }
            //else {
            //    CODE FOR NOT CHANGE THE VALUE 
            //}
        }
        else {
            p_SelectedDocumentIndex = value;
            base.RaisePropertiesChanged("SelectedDocumentIndex");
        }
    }
 }

So, the question is: how can I not apply the change in the "set" section? (like an undo, I think)

This is simpliest way to do it, but, if this approach is incorrect, how can I do?

Previous failed attempts:

1)
p_SelectedDocumentIndex = p_SelectedDocumentIndex
base.RaisePropertiesChanged("SelectedDocumentIndex");

2)
base.RaisePropertiesChanged("SelectedDocumentIndex");

3)
nothing in the else branch

Answer 1

Dispatcher.CurrentDispatcher.BeginInvoke(new Action(() => SelectedDocumentIndex= p_SelectedDocumentIndex ), DispatcherPriority.Send);

该调用安排将UI状态恢复到操作开始之前的状态

Answer 2

I solved it. I've took the solution from here:

http://blog.alner.net/archive/2010/04/25/cancelling-selection-change-in-a-bound-wpf-combo-box.aspx

    public int SelectedDocumentIndex{ get { return p_SelectedDocumentIndex; }
        set {
            // Store the current value so that we can 
            // change it back if needed.
            var origValue = p_SelectedDocumentIndex;

            // If the value hasn't changed, don't do anything.
            if (value == p_SelectedDocumentIndex)
                return;

            // Note that we actually change the value for now.
            // This is necessary because WPF seems to query the 
            //  value after the change. The combo box
            // likes to know that the value did change.
            p_SelectedDocumentIndex = value;
            if (tabsCollection.Count() > 1 && CanSave() == true)
            {
                if (!dm.ShowMessage1(ServiceContainer.GetService<DevExpress.Mvvm.IDialogService>("confirmYesNo")))
                {
                    Debug.WriteLine("Selection Cancelled.");

                    // change the value back, but do so after the 
                    // UI has finished it's current context operation.
                    Application.Current.Dispatcher.BeginInvoke(
                            new Action(() =>
                            {
                                Debug.WriteLine("Dispatcher BeginInvoke " + "Setting CurrentPersonCancellable.");

                                // Do this against the underlying value so 
                                //  that we don't invoke the cancellation question again.
                                p_SelectedDocumentIndex = origValue;
                                DocumentPanel p = tabsCollection.ElementAt(p_SelectedDocumentIndex);
                                p.IsActive = true;
                                base.RaisePropertiesChanged("SelectedDocumentIndex");
                            }),
                            System.Windows.Threading.DispatcherPriority.ContextIdle,
                            null
                        );

                    // Exit early. 
                    return;
                }
            }
            // Normal path. Selection applied. 
            // Raise PropertyChanged on the field.
            Debug.WriteLine("Selection applied.");
            base.RaisePropertiesChanged("SelectedDocumentIndex");
        }
    }

Answer 3

If your VM class is derived from DependencyObject then you can change your property to a DependecyProperty with a coerce callback which enables "undo" as follows:

    public int SelectedDocumentIndex
    {
        get { return (int)GetValue(SelectedDocumentIndexProperty); }
        set { SetValue(SelectedDocumentIndexProperty, value); }
    }
    public static readonly DependencyProperty SelectedDocumentIndexProperty =
        DependencyProperty.Register("SelectedDocumentIndex", typeof(int), typeof(MyViewModel), new PropertyMetadata(0,
            (d, e) =>
            {
                //Callback after value is changed
                var vm = (MyViewModel)d;
                var val = (int)e.NewValue;
            }, (d, v) =>
            {
                //Coerce before value is changed
                var vm = (MyViewModel)d;
                var val = (int)v;
                if (vm.tabsCollection.Count() > 1 && vm.CanSave() == true)
                {
                    if (vm.dm.ShowMessage1(ServiceContainer.GetService<DevExpress.Mvvm.IDialogService>("confirmYesNo")))
                    {
                        //no coerce is needed
                        return v;
                    }
                    else
                    {
                        //should coerce to the previous value
                        return VM.SelectedDocumentIndex;
                    }
                }
                else
                {
                    //no coerce is needed
                    return v;
                }
            }));