WPF 只读附加属性的赋值与绑定

附加只读属性是通过获取非公开的DependencyPropertyKey字段的DependencyProperty属性。

　　　　 internal static readonly DependencyPropertyKey ClickCountReadOnlyPropertyKey = DependencyProperty.RegisterAttachedReadOnly("ClickCountReadOnly", typeof(int), typeof(CheckBoxAttch), new PropertyMetadata(0));

        public static readonly DependencyProperty ClickCountReadOnlyProperty = ClickCountReadOnlyPropertyKey.DependencyProperty;
       
        public static int GetClickCountReadOnly(DependencyObject dependencyObject)
        {
            return (int)dependencyObject.GetValue(ClickCountReadOnlyProperty);
        }

因为是附加属性，所有包装器是以方法的形式存在。

其中，在注册只读属性时注册名要与公开的附加属性，公开方法一致，符合约定。

附加属性是可读，可写。附加只读属性是不可写入。

当然这是有条件的，在外部获取，在拿不到非公开的dependency property key时是不可写入。换句说，如果想要赋值必须通过非公开的只读key。

以下列代码为例子

假定检测一个点击checkbox的数与是否选择有关。

点击数 1 是选择

点击数 2 是非选择

点击数 0 是默认值

xaml：

  <StackPanel>
        <CheckBox VerticalContentAlignment="Center" Content="{Binding ElementName=tb1, Path=Text}" ContentStringFormat="Click:{0}"  x:Name="cb1" FontSize="55"  VerticalAlignment="Top" local:CheckBoxAttch.AutoCount="True"/>
        <CheckBox VerticalContentAlignment="Center" Content="{Binding ElementName=tb2, Path=Text}" ContentStringFormat="Click:{0}"  x:Name="cb2" FontSize="55"  VerticalAlignment="Top" local:CheckBoxAttch.AutoCount="True"/>
        <TextBlock x:Name="tb1" VerticalAlignment="Top" HorizontalAlignment="Right" Text="{Binding  ElementName=cb1, Path=(local:CheckBoxAttch.ClickCountReadOnly)}"/>
        <TextBlock x:Name="tb2" VerticalAlignment="Top" HorizontalAlignment="Right" Text="{Binding  ElementName=cb2, Path=(local:CheckBoxAttch.ClickCountReadOnly)}"/>
    </StackPanel>

code：

　　 class CheckBoxAttch
    {
        public static bool GetAutoCount(DependencyObject obj)
        {
            return (bool)obj.GetValue(AutoCountProperty);
        }

        public static void SetAutoCount(DependencyObject obj, bool value)
        {
            obj.SetValue(AutoCountProperty, value);
        }

       
        public static readonly DependencyProperty AutoCountProperty =
            DependencyProperty.RegisterAttached("AutoCount", typeof(bool), typeof(CheckBoxAttch), new PropertyMetadata(false, new PropertyChangedCallback(OnAutoValueChanged)));

        private static void OnAutoValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is CheckBox box)
            {
                if ((bool)e.NewValue)
                {

                    box.PreviewMouseLeftButtonUp -= Picker_PreviewMouseDown;
                    box.PreviewMouseLeftButtonUp += Picker_PreviewMouseDown;
                }
                else
                {
                    box.PreviewMouseLeftButtonDown -= Picker_PreviewMouseDown;
                }
            }
        }

        private static void Picker_PreviewMouseDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
        {
            var box = (CheckBox)sender;
            var count = (int)box.GetValue(ClickCountReadOnlyProperty);
            if (box.IsChecked.HasValue)
            {
                if (count != 1)
                {
                    box.SetValue(ClickCountReadOnlyPropertyKey, 1);
                }
                else
                {
                    if (count == 1)
                    {
                        box.SetValue(ClickCountReadOnlyPropertyKey, 2);
                    }
                }
            }

        }

        internal static readonly DependencyPropertyKey ClickCountReadOnlyPropertyKey = DependencyProperty.RegisterAttachedReadOnly("ClickCountReadOnly", typeof(int), typeof(CheckBoxAttch), new PropertyMetadata(0));

        public static readonly DependencyProperty ClickCountReadOnlyProperty = ClickCountReadOnlyPropertyKey.DependencyProperty;
       
        public static int GetClickCountReadOnly(DependencyObject dependencyObject)
        {
            return (int)dependencyObject.GetValue(ClickCountReadOnlyProperty);
        }
    }