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將只讀GUI屬性推回ViewModel

源碼算法與數(shù)據(jù)結(jié)構(gòu)

慕俠2389804 2019-07-17 09:38:45

將只讀GUI屬性推回ViewModel我想要寫一個視圖模型，它總是知道視圖中某些只讀依賴屬性的當前狀態(tài)。具體來說，我的GUI包含一個FlowDocumentPageViewer，它每次從FlowDocument中顯示一個頁面。FlowDocumentPageViewer公開了兩個名為CanGoToPreviousPage和CanGoToNextPage的只讀依賴項屬性。我希望我的ViewModel始終知道這兩個View屬性的值。我想我可以通過OneWayToSource數(shù)據(jù)庫實現(xiàn)這一點：<FlowDocumentPageViewer CanGoToNextPage="{Binding NextPageAvailable, Mode=OneWayToSource}" ...>如果允許這樣做，它將是完美的：每當FlowDocumentPageViewer的CanGoToNextPage屬性更改時，新的值將被向下推到ViewModel的NextPageAvailable屬性中，這正是我想要的。不幸的是，這不能編譯：我收到一個錯誤的說法“CanGoToPreviousPage”屬性是只讀的，不能從標記中設(shè)置。顯然只讀屬性不支持任何這是一種數(shù)據(jù)綁定，甚至不是針對該屬性的只讀數(shù)據(jù)。我可以將ViewModel的屬性變成DependencyProperties，并使OneWay綁定向相反的方向發(fā)展，但我并不熱衷于關(guān)注點沖突的分離(ViewModel需要引用視圖，MVVM數(shù)據(jù)庫應(yīng)該避免這種情況)。FlowDocumentPageViewer不公開CanGoToNextPageChanged事件，而且我也不知道從DependencyProperty獲取更改通知的任何好方法，除非創(chuàng)建另一個DependencyProperty來綁定它，這在這里似乎有點過分。如何將視圖只讀屬性的更改通知ViewModel？

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3 回答

慕姐4208626

TA貢獻1852條經(jīng)驗獲得超7個贊

我使用的是一種通用解決方案，它不僅適用于ActualWidth和ActualHL.8，而且還適用于至少在讀取模式下可以綁定到的任何數(shù)據(jù)。

如果ViewportWidth和ViewportHL.8是視圖模型的屬性，則標記如下

<Canvas>
    <u:DataPiping.DataPipes>
         <u:DataPipeCollection>
             <u:DataPipe Source="{Binding RelativeSource={RelativeSource AncestorType={x:Type Canvas}}, Path=ActualWidth}"
                         Target="{Binding Path=ViewportWidth, Mode=OneWayToSource}"/>
             <u:DataPipe Source="{Binding RelativeSource={RelativeSource AncestorType={x:Type Canvas}}, Path=ActualHeight}"
                         Target="{Binding Path=ViewportHeight, Mode=OneWayToSource}"/>
          </u:DataPipeCollection>
     </u:DataPiping.DataPipes><Canvas>

以下是自定義元素的源代碼

public class DataPiping{
    #region DataPipes (Attached DependencyProperty)

    public static readonly DependencyProperty DataPipesProperty =
        DependencyProperty.RegisterAttached("DataPipes",
        typeof(DataPipeCollection),
        typeof(DataPiping),
        new UIPropertyMetadata(null));

    public static void SetDataPipes(DependencyObject o, DataPipeCollection value)
    {
        o.SetValue(DataPipesProperty, value);
    }

    public static DataPipeCollection GetDataPipes(DependencyObject o)
    {
        return (DataPipeCollection)o.GetValue(DataPipesProperty);
    }

    #endregion}public class DataPipeCollection : FreezableCollection<DataPipe>{}public class DataPipe : Freezable{
    #region Source (DependencyProperty)

    public object Source
    {
        get { return (object)GetValue(SourceProperty); }
        set { SetValue(SourceProperty, value); }
    }
    public static readonly DependencyProperty SourceProperty =
        DependencyProperty.Register("Source", typeof(object), typeof(DataPipe),
        new FrameworkPropertyMetadata(null, new PropertyChangedCallback(OnSourceChanged)));

    private static void OnSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ((DataPipe)d).OnSourceChanged(e);
    }

    protected virtual void OnSourceChanged(DependencyPropertyChangedEventArgs e)
    {
        Target = e.NewValue;
    }

    #endregion

    #region Target (DependencyProperty)

    public object Target
    {
        get { return (object)GetValue(TargetProperty); }
        set { SetValue(TargetProperty, value); }
    }
    public static readonly DependencyProperty TargetProperty =
        DependencyProperty.Register("Target", typeof(object), typeof(DataPipe),
        new FrameworkPropertyMetadata(null));

    #endregion

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

反對回復(fù) 2019-07-17

青春有我

TA貢獻1784條經(jīng)驗獲得超8個贊

如果其他人感興趣，我在這里編寫了Kent的近似解：

class SizeObserver{
    #region " Observe "

    public static bool GetObserve(FrameworkElement elem)
    {
        return (bool)elem.GetValue(ObserveProperty);
    }

    public static void SetObserve(
      FrameworkElement elem, bool value)
    {
        elem.SetValue(ObserveProperty, value);
    }

    public static readonly DependencyProperty ObserveProperty =
        DependencyProperty.RegisterAttached("Observe", typeof(bool), typeof(SizeObserver),
        new UIPropertyMetadata(false, OnObserveChanged));

    static void OnObserveChanged(
      DependencyObject depObj, DependencyPropertyChangedEventArgs e)
    {
        FrameworkElement elem = depObj as FrameworkElement;
        if (elem == null)
            return;

        if (e.NewValue is bool == false)
            return;

        if ((bool)e.NewValue)
            elem.SizeChanged += OnSizeChanged;
        else
            elem.SizeChanged -= OnSizeChanged;
    }

    static void OnSizeChanged(object sender, RoutedEventArgs e)
    {
        if (!Object.ReferenceEquals(sender, e.OriginalSource))
            return;

        FrameworkElement elem = e.OriginalSource as FrameworkElement;
        if (elem != null)
        {
            SetObservedWidth(elem, elem.ActualWidth);
            SetObservedHeight(elem, elem.ActualHeight);
        }
    }

    #endregion

    #region " ObservedWidth "

    public static double GetObservedWidth(DependencyObject obj)
    {
        return (double)obj.GetValue(ObservedWidthProperty);
    }

    public static void SetObservedWidth(DependencyObject obj, double value)
    {
        obj.SetValue(ObservedWidthProperty, value);
    }

    // Using a DependencyProperty as the backing store for ObservedWidth.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty ObservedWidthProperty =
        DependencyProperty.RegisterAttached("ObservedWidth", typeof(double), typeof(SizeObserver), new UIPropertyMetadata(0.0));

    #endregion

    #region " ObservedHeight "

    public static double GetObservedHeight(DependencyObject obj)
    {
        return (double)obj.GetValue(ObservedHeightProperty);
    }

    public static void SetObservedHeight(DependencyObject obj, double value)
    {
        obj.SetValue(ObservedHeightProperty, value);
    }

    // Using a DependencyProperty as the backing store for ObservedHeight.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty ObservedHeightProperty =
        DependencyProperty.RegisterAttached("ObservedHeight", typeof(double), typeof(SizeObserver), new UIPropertyMetadata(0.0));

    #endregion}

在您的應(yīng)用程序中可以隨意使用它。效果很好。(謝謝肯特！)

反對回復(fù) 2019-07-17