在 ViewModel 中訪問 XAML 對象

Question

如何訪問 ViewModel 中的 XAML 對象？ 我真的很困惑。 我想訪問<Controls:ModalContentPresenter>對象。 我怎么能以符合 MVVM 的方式實現這一點？ 在這個對象上我想調用一個方法ShowModalContent

<Controls:ModalContentPresenter x:Name="modalContent">
    <ScrollViewer Behaviors:AdvancedZooming.KeepInCenter="true" Visibility="{Binding LeerformularIsVisible}" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
        <Viewbox Stretch="Uniform">
            <Grid>
                <DataGrid BorderBrush="{x:Null}">
                    <DataGrid.ContextMenu>
                        <ContextMenu>
                            <MenuItem Command="{Binding AddFieldDefinitionCommand}" Header="Feld hinterlegen" Icon="pack://application:,,,/Images/Designer/field.png" />
                            <MenuItem Command="{Binding AddFunctionCommand}" Header="Funktion hinterlegen" Icon="pack://application:,,,/Images/Designer/FI_Taschenmesser_16x16.png" />
                            <MenuItem Command="{Binding RemoveFieldDefinitionCommand}" Header="Aktuelle Felddefinition entfernen" Icon="pack://application:,,,/Images/Designer/remove_field.png" />
                            <MenuItem Command="{Binding CutCommand}" Header="Ausschneiden" Icon="pack://application:,,,/Images/Zwischenablage/FI_Ausschneiden_16x16.png" />
                            <MenuItem Command="{Binding CopyCommand}" Header="Kopieren" Icon="pack://application:,,,/Images/Zwischenablage/FI_Kopieren_16x16.png" />
                            <MenuItem Command="{Binding PasteCommand}" Header="Einfügen" Icon="pack://application:,,,/Images/Zwischenablage/FI_Einfuegen_16x16.png" />
                        </ContextMenu>
                    </DataGrid.ContextMenu>

                </DataGrid>
            </Grid>

        </Viewbox>

    </ScrollViewer>

    <Controls:ModalContentPresenter.ModalContent>
        <StackPanel>
            <TextBlock>Test</TextBlock>
            <Button>Hide</Button>
        </StackPanel>
    </Controls:ModalContentPresenter.ModalContent>
</Controls:ModalContentPresenter>

ModalContentPresenter的代碼可以在這里找到：

using System;
using System.Collections;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;
using System.Windows.Markup;
using System.Windows.Media;

namespace Controls
{
    [ContentProperty("Content")]
    public class ModalContentPresenter : FrameworkElement
    {
        #region private fields
        private Panel layoutRoot;
        private ContentPresenter primaryContentPresenter;
        private ContentPresenter modalContentPresenter;
        private Border overlay;
        private object[] logicalChildren;
        private KeyboardNavigationMode cachedKeyboardNavigationMode;
        private static readonly TraversalRequest traversalDirection;
        #endregion

        #region dependency properties
        public static readonly DependencyProperty IsModalProperty = DependencyProperty.Register("IsModal", typeof(bool), typeof(ModalContentPresenter),
            new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, OnIsModalChanged));

        public static readonly DependencyProperty ContentProperty = DependencyProperty.Register("Content", typeof(object), typeof(ModalContentPresenter),
        new UIPropertyMetadata(null, OnContentChanged));

        public static readonly DependencyProperty ModalContentProperty = DependencyProperty.Register("ModalContent", typeof(object), typeof(ModalContentPresenter),
        new UIPropertyMetadata(null, OnModalContentChanged));

        public static readonly DependencyProperty OverlayBrushProperty = DependencyProperty.Register("OverlayBrush", typeof(Brush), typeof(ModalContentPresenter), 
        new UIPropertyMetadata(new SolidColorBrush(Color.FromArgb(204, 169, 169, 169)), OnOverlayBrushChanged));

        public bool IsModal
        {
            get { return (bool)GetValue(IsModalProperty); }
            set { SetValue(IsModalProperty, value); }
        }

        public object Content
        {
            get { return (object)GetValue(ContentProperty); }
            set { SetValue(ContentProperty, value); }
        }

        public object ModalContent
        {
            get { return (object)GetValue(ModalContentProperty); }
            set { SetValue(ModalContentProperty, value); }
        }

        public Brush OverlayBrush
        {
            get { return (Brush)GetValue(OverlayBrushProperty); }
            set { SetValue(OverlayBrushProperty, value); }
        } 
        #endregion

        #region routed events
        public static readonly RoutedEvent PreviewModalContentShownEvent = EventManager.RegisterRoutedEvent("PreviewModalContentShown", RoutingStrategy.Tunnel,
        typeof(RoutedEventArgs), typeof(ModalContentPresenter));

        public static readonly RoutedEvent ModalContentShownEvent = EventManager.RegisterRoutedEvent("ModalContentShown", RoutingStrategy.Bubble,
        typeof(RoutedEventArgs), typeof(ModalContentPresenter));

        public static readonly RoutedEvent PreviewModalContentHiddenEvent = EventManager.RegisterRoutedEvent("PreviewModalContentHidden", RoutingStrategy.Tunnel, 
        typeof(RoutedEventArgs), typeof(ModalContentPresenter));

        public static readonly RoutedEvent ModalContentHiddenEvent = EventManager.RegisterRoutedEvent("ModalContentHidden",  RoutingStrategy.Bubble, 
        typeof(RoutedEventArgs), typeof(ModalContentPresenter));

        public event RoutedEventHandler PreviewModalContentShown
        {
            add { AddHandler(PreviewModalContentShownEvent, value); }
            remove { RemoveHandler(PreviewModalContentShownEvent, value); }
        }

        public event RoutedEventHandler ModalContentShown
        {
            add { AddHandler(ModalContentShownEvent, value); }
            remove { RemoveHandler(ModalContentShownEvent, value); }
        }

        public event RoutedEventHandler PreviewModalContentHidden
        {
            add { AddHandler(PreviewModalContentHiddenEvent, value); }
            remove { RemoveHandler(PreviewModalContentHiddenEvent, value); }
        }

        public event RoutedEventHandler ModalContentHidden
        {
            add { AddHandler(ModalContentHiddenEvent, value); }
            remove { RemoveHandler(ModalContentHiddenEvent, value); }
        }
        #endregion

        #region ModalContentPresenter implementation
        static ModalContentPresenter()
        {
            traversalDirection = new TraversalRequest(FocusNavigationDirection.First);
        }

        public ModalContentPresenter()
        {
            layoutRoot = new ModalContentPresenterPanel();
            primaryContentPresenter = new ContentPresenter();
            modalContentPresenter = new ContentPresenter();
            overlay = new Border();

            AddVisualChild(layoutRoot);

            logicalChildren = new object[2];

            overlay.Background = OverlayBrush;
            overlay.Child = modalContentPresenter;
            overlay.Visibility = Visibility.Hidden;

            layoutRoot.Children.Add(primaryContentPresenter);
            layoutRoot.Children.Add(overlay);
        }

        public void ShowModalContent()
        {
            if (!IsModal)
                IsModal = true;
        }

        public void HideModalContent()
        {
            if (IsModal)
                IsModal = false;
        }

        private void RaiseModalContentShownEvents()
        {
            RoutedEventArgs args = new RoutedEventArgs(PreviewModalContentShownEvent);
            OnPreviewModalContentShown(args);
            if (!args.Handled)
            {
                args = new RoutedEventArgs(ModalContentShownEvent);
                OnModalContentShown(args);
            }
        }

        private void RaiseModalContentHiddenEvents()
        {
            RoutedEventArgs args = new RoutedEventArgs(PreviewModalContentHiddenEvent);
            OnPreviewModalContentHidden(args);
            if (!args.Handled)
            {
                args = new RoutedEventArgs(ModalContentHiddenEvent);
                OnModalContentHidden(args);
            }
        }

        protected virtual void OnPreviewModalContentShown(RoutedEventArgs e)
        {
            RaiseEvent(e);
        }

        protected virtual void OnModalContentShown(RoutedEventArgs e)
        {
            RaiseEvent(e);
        }

        protected virtual void OnPreviewModalContentHidden(RoutedEventArgs e)
        {
            RaiseEvent(e);
        }

        protected virtual void OnModalContentHidden(RoutedEventArgs e)
        {
            RaiseEvent(e);
        }
        #endregion

        #region property changed callbacks
        private static void OnIsModalChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            ModalContentPresenter control = (ModalContentPresenter)d;

            if ((bool)e.NewValue == true)
            {
                control.cachedKeyboardNavigationMode = KeyboardNavigation.GetTabNavigation(control.primaryContentPresenter);
                KeyboardNavigation.SetTabNavigation(control.primaryContentPresenter, KeyboardNavigationMode.None);

                control.overlay.Visibility = Visibility.Visible;
                control.overlay.MoveFocus(traversalDirection);

                control.RaiseModalContentShownEvents();
            }
            else
            {
                control.overlay.Visibility = Visibility.Hidden;

                KeyboardNavigation.SetTabNavigation(control.primaryContentPresenter, control.cachedKeyboardNavigationMode);
                control.primaryContentPresenter.MoveFocus(traversalDirection);

                control.RaiseModalContentHiddenEvents();
            }
        }

        private static void OnContentChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            ModalContentPresenter control = (ModalContentPresenter)d;

            if (e.OldValue != null)
                control.RemoveLogicalChild(e.OldValue);

            control.primaryContentPresenter.Content = e.NewValue;
            control.AddLogicalChild(e.NewValue);
            control.logicalChildren[0] = e.NewValue;
        }

        private static void OnModalContentChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            ModalContentPresenter control = (ModalContentPresenter)d;

            if (e.OldValue != null)
                control.RemoveLogicalChild(e.OldValue);

            control.modalContentPresenter.Content = e.NewValue;
            control.AddLogicalChild(e.NewValue);
            control.logicalChildren[1] = e.NewValue;
        }

        private static void OnOverlayBrushChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            ModalContentPresenter control = (ModalContentPresenter)d;
            control.overlay.Background = (Brush)e.NewValue;
        }
        #endregion

        #region FrameworkElement overrides
        protected override Visual GetVisualChild(int index)
        {
            if (index < 0 || index > 1)
                throw new ArgumentOutOfRangeException("index");

            return layoutRoot;
        }

        protected override int VisualChildrenCount
        {
            get { return 1; }
        }

        protected override IEnumerator LogicalChildren
        {
            get { return logicalChildren.GetEnumerator(); }
        }

        protected override Size ArrangeOverride(Size finalSize)
        {
            layoutRoot.Arrange(new Rect(finalSize));
            return finalSize;
        }

        protected override Size MeasureOverride(Size availableSize)
        {
            layoutRoot.Measure(availableSize);
            return layoutRoot.DesiredSize;
        }
        #endregion

        #region layout panel
        class ModalContentPresenterPanel : Panel
        {
            protected override Size MeasureOverride(Size availableSize)
            {
                Size resultSize = new Size(0, 0);

                foreach (UIElement child in Children)
                {
                    child.Measure(availableSize);
                    resultSize.Width = Math.Max(resultSize.Width, child.DesiredSize.Width);
                    resultSize.Height = Math.Max(resultSize.Height, child.DesiredSize.Height);
                }

                return resultSize;
            }

            protected override Size ArrangeOverride(Size finalSize)
            {
                foreach (UIElement child in InternalChildren)
                {
                    child.Arrange(new Rect(finalSize));
                }

                return finalSize;
            }
        }
        #endregion
    }
}

Answer 1

您不應該這樣做（至少，這絕對不是 MVVM 方式）。
改用IsModal屬性和數據綁定：

<Controls:ModalContentPresenter x:Name="modalContent" IsModal="{Binding IsModal}">

其中綁定表達式中的IsModal是視圖模型中的綁定數據屬性。

Answer 2

在 MVVM 中，任何來回傳輸的數據都應該通過視圖模型上的數據綁定屬性來完成。 這幾乎包括ContentPresenter所有屬性 - 只需添加一個綁定。 但是，如果您想以對 MVVM 友好的方式調用 XAML 對象的方法，則可以使用Action 。

假設我想通過視圖模型將焦點設置在文本框上（人為的示例，我知道還有其他 MVVM 方法可以做到這一點 - 只是想要一個需要視圖子對象的示例）。

首先，在 XAML 中為應該獲得焦點的文本框命名，即，

<TextBox x:Name="textBox1"/>

在您的視圖模型上，為 Action 添加一個屬性：

public Action FocusAction {get;set;}

在您的視圖加載之前或加載時，獲取您的 DataContext（即您的視圖模型）並將 Action 添加到后面的代碼（視圖的 .cs 文件）：

ViewModel vm = (ViewModel)this.DataContext;

if ( vm.FocusAction == null )
    vm.FocusAction= new Action(() => this.textBox1.Focus());

例如，您可以實現 IsLoaded 事件並在那里執行。 您也可以在 InitializeComponent 之后在構造函數中執行此操作，只要它創建了您的視圖模型實例或作為參數傳入。 關鍵是視圖模型已經實例化並分配給視圖的數據上下文。

然后在您的視圖模型中，無論您想將焦點添加到該文本框的何處，請調用：

FocusAction();

由於我剛剛在上面展示的內容要求您的視圖將 DataContext 轉換為特定的視圖模型，因此我所做的是為我需要的各種操作創建一個接口，如下所示：

interface IFocusable
{
     Action FocusAction {get;set;}
}

然后我讓我的視圖模型實現那個接口。 完成后，在我看來的代碼隱藏中，我可以這樣說：

if(this.DataContext is IFocusable)
    ((IFocusable)this.DataContext).FocusAction = new Action(() => this.textBox1.Focus());

我認為這使得它更符合 MVVM，因為它沒有與特定的視圖模型緊密耦合，如果視圖模型是可以使用它的視圖模型類型，視圖只知道它可以添加一個動作。

此處提供更多詳細信息和另一個示例： http : //jkshay.com/closure-a-wpf-window-using-mvvm-and-minimal-code-behind/

Answer 3

您可以為接受 ContentPage 作為其參數的視圖模型創建一個構造函數。

public class ViewModelClass 
{
public ViewModelClass(ContentPage p=null)
{...}
}

然后在 Contentpage 后台代碼腳本中設置綁定上下文，將 contentpage 的引用傳遞給 viewmodel

public class ContentPageClass : ContentPage
{
public ContentPageClass()
{
BindingContext = new ViewModelClass(p:this); 
}
}

Answer 4

我知道已經有幾年了，但我只是面臨同樣的情況，所以這是我的答案，以防有人發現它有用......在您的 xaml 文件中，請參考 DataContext 中的 ViewModel 類：

<Window.DataContext>
    <local:YourViewModel x:Name="yourViewModel"/>
</Window.DataContext>

在您的 ViewModel 類中，使用您的 xaml 文件作為參數和一個私有成員來創建一個公共函數來保存它：

private MainWindow mainWindow;

public void OnViewInitialized(MainWindow mainWindow)
{
    this.mainWindow = mainWindow;
}

在您后面的代碼中，使用函數傳遞您在 xaml 中定義的“yourViewModel”：

public MainWindow()
{
    InitializeComponent();
    yourViewModel.OnViewInitialized(this);
}

就是這樣 :) 現在您可以使用 mainWindow 成員從 ViewModel 訪問所有 xaml 元素，只需為它們命名即可。

mainWindow.textBox1