在WPF C＃中綁定可見性轉換器

Question

我有一個類型集合的依賴屬性，當它的回調基於我需要設置屏幕上某些控件的可見性的計數觸發時。

但控制仍然一直在崩潰。 根據代碼，一個控件始終可見。

XAML綁定是

   <TextBlock Text="106 search results for 'a'" Margin="5,0,100,0" Visibility="{Binding CountLabelVisibleReverse, Converter={StaticResource VisibilityConverter}}"/>
 <StackPanel Grid.Row="1" Orientation="Horizontal" Margin="0,0,90,0"  
                            Visibility="{Binding CountLabelVisible, Converter={StaticResource VisibilityConverter}}">
 <TextBlock Text="Sort By"  />
 <ComboBox Style="{StaticResource ComboBoxStyle1}" Width="100" x:Name="ComboBoxSorting" ItemsSource="{Binding SortBy}" />
   </StackPanel>

我的兩個屬性是

    public bool CountLabelVisible { get; set; }

    public bool CountLabelVisibleReverse { get; set; }

依賴屬性回調

   private static void ItemsCollectionChanged(DependencyObject obj, DependencyPropertyChangedEventArgs eventArgs)
    {
        var listingUserControl = (obj as ListingUserControl);

        var itemsResult = (eventArgs.NewValue as List<ItemsResult>);
        if (listingUserControl != null && itemsResult != null)
        {
            listingUserControl.CountLabelVisible = itemsResult.Count > 0;
            listingUserControl.CountLabelVisibleReverse =itemsResult.Count <= 0;
        }
    }

轉換器代碼是

 public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (parameter == null)
            return (bool)value == false ? Visibility.Collapsed : Visibility.Visible;

        return (bool)value ? Visibility.Collapsed : Visibility.Visible;
    }

Answer 1

您已經犯了綁定到對綁定有效的自動屬性的經典錯誤，但是在更改時不通知，這意味着綁定子系統無法檢測更改並更新綁定目標。

要解決此問題，請在viewmodel上實現INotifyPropertyChanged ，然后確保從屬性中通知屬性更改。

舉個例子，我在viewmodels的基類中有以下內容：

public abstract class BaseViewModel : INotifyPropertyChanged
{

    /// <summary>
    /// Helper method to set the value of a property and notify if the value has changed.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="newValue">The value to set the property to.</param>
    /// <param name="currentValue">The current value of the property.</param>
    /// <param name="notify">Flag indicating whether there should be notification if the value has changed.</param>
    /// <param name="notifications">The property names to notify that have been changed.</param>
    protected bool SetProperty<T>(ref T newValue, ref T currentValue, bool notify, params string[] notifications)
    {
        if (EqualityComparer<T>.Default.Equals(newValue, currentValue))
            return false;

        currentValue = newValue;
        if (notify && notifications.Length > 0)
            foreach (string propertyName in notifications)
                OnPropertyChanged(propertyName);

        return true;
    }

    /// <summary>
    /// Raises the <see cref="E:PropertyChanged"/> event.
    /// </summary>
    /// <param name="propertyName">The name of the property that changed.</param>
    protected void OnPropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

    /// <summary>
    /// Occurs when a property value changes.
    /// </summary>
    public event PropertyChangedEventHandler PropertyChanged;

}

然后在你的常規viewmodel中：

public class MyViewModel : BaseViewModel
{
    private bool _countLabelVisible;

    public bool CountLabelVisible
    {
        get { return _countLabelVisible; }
        set { SetProperty(ref value, ref _countLabelVisible, true, "CountLabelVisible", "CountLabelVisibleReverse"); }
    }

    public bool CountLabelVisibleReverse { get { return !_countLabelVisible; }} 
}

這樣，當CountLabelVisible被更改時，它也會通知屬性CountLabelVisibleReverse ，而屬性CountLabelVisibleReverse只包含一個getter - 因為它始終是CountLabelVisible的反轉。

因此，您可以按照自己的方式修復代碼，但實際情況是您不需要保留CountLabelVisibleReverse屬性，而是可以：

• 創建一個逆可見性轉換器作為單獨的轉換器
• 通過在綁定上傳遞可選參數來創建多功能可見性轉換器
• 堆疊多個轉換器，其中一個轉換器的輸出通過管道輸入到下一個轉換器的輸入

Answer 2

您綁定的布爾屬性是否在更改時通知視圖？ 是這樣的：

private bool countLabelVisible;
public bool CountLabelVisible
{
  get
  {
    return countLabelVisible;
  }
  set
  {
   if (countLabelVisible != value)
   {
      countLabelVisible = value;
      RaisePropertyChanged(() => CountLabelVisible);
   }
}

對於具有lambda的RaisePropertyChanged方法，您的viewodel應該從NotificationObject繼承

Answer 3

您需要通知更改：

public event PropertyChangedEventHandler PropertyChanged;
private bool _countLabelVisible = false;

private void RaisePropertyChanged(string propertyName)
{
    if (PropertyChanged != null)
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}


public bool CountLabelVisible 
{ 
    get
    {
        return _countLabelVisible;
    }
    set
    {
        _countLabelVisible = value;
        RaisePropertyChanged("CountLabelVisible");
    }
}

需要告知綁定“框架”綁定需要刷新，這就是Raise ...的意思。 這非常快速和骯臟（並且未經測試），但應該證明您需要做什么。

Answer 4

Bool到能見度轉換器類

    public class BoolToVisibilityConverter : IValueConverter
        {
            public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
            {
                return (bool)value ? Visibility.Visible : Visibility.Hidden;
            }

            public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
            {
                throw new NotImplementedException();
            }
        }

下面提到的Xaml更改顯示了可見性轉換器類的使用。 此處使用組框來顯示可見性。 更改單選按鈕選擇組框將顯示/隱藏。

 <Page x:Class="WpfApplication.MainWindow"
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            xmlns:vm="clr-namespace:WpfApplication" HorizontalAlignment="Left" VerticalAlignment="Top"
            Title="Customer"  Loaded="Page_Loaded">
        <Page.Resources>
            <vm:BoolToVisibilityConverter x:Key="converter" 
    </Page.Resources>
<RadioButton Grid.Column="0" x:Name="rdbCustomerDetail"  
    Content="Show Customer" 
    IsChecked="{Binding IsCustomerDetailChecked,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
            <GroupBox Header="Customer Details" Visibility="{Binding 

            Path=IsCustomerDetailChecked, 
            UpdateSourceTrigger=PropertyChanged, 
            Converter={StaticResource  converter}}">
    </GroupBox>

在ViewModel中，使用invokepropertychange只會獲得xaml上的可見性更改。

private Boolean isCustomerDetailChecked = false;
    public Boolean IsCustomerDetailChecked
    {
        get
        {
            return isCustomerDetailChecked;
        }
        set
        {
            isCustomerDetailChecked = value;
            InvokePropertyChanged("IsCustomerDetailChecked");
        }
    }