[WPF源码分析]ContentControl依赖项属性的双向绑定，two-way binding view's DependencyProperty and ViewModel's variable

/// <summary>
        ///     The DependencyProperty for the Content property.
        ///     Flags:              None
        ///     Default Value:      null
        /// </summary>
        [CommonDependencyProperty]
        public static readonly DependencyProperty ContentProperty =
                DependencyProperty.Register(
                        "Content",
                        typeof(object),
                        typeof(ContentControl),
                        new FrameworkPropertyMetadata(
                                (object)null,
                                new PropertyChangedCallback(OnContentChanged)));
 
        /// <summary>
        ///     Content is the data used to generate the child elements of this control.
        /// </summary>
        [Bindable(true), CustomCategory("Content")]
        public object Content
        {
            get { return GetValue(ContentProperty); }
            set { SetValue(ContentProperty, value); }
        }
 
        /// <summary>
        ///     Called when ContentProperty is invalidated on "d."
        /// </summary>
        private static void OnContentChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            // 根据需要实现自己的方法 
            ContentControl ctrl = (ContentControl) d;
            ctrl.SetValue(HasContentPropertyKey, (e.NewValue != null) ? BooleanBoxes.TrueBox : BooleanBoxes.FalseBox);
 
            ctrl.OnContentChanged(e.OldValue, e.NewValue);
        }