C# DependencyObject.HasExpression方法代码示例

        internal static void UnlinkContainerFromItem(DependencyObject container, object item, IGeneratorHost host)
        {
            // When a container is removed from the tree, its future takes one of
            // two forms:
            //      a) [normal mode] the container becomes eligible for GC
            //      b) [recycling mode] the container joins the recycled list, and
            //          possibly re-enters the tree at some point, usually with a
            //          different item.
            //
            // As Dev10 bug 452669 and some "subtle issues" that arose in the
            // container recycling work illustrate, it's important that the container
            // and its subtree sever their connection to the data item.  Otherwise
            // you can get aliasing - a dead container reacting to the same item as a live
            // container.  Even without aliasing, it's a perf waste for a dead container
            // to continue reacting to its former data item.
            //
            // On the other hand, it's a perf waste to spend too much effort cleaning
            // up the container and its subtree, since they will often just get GC'd
            // in the near future.
            //
            // WPF initially did a full cleanup of the container, removing all properties
            // that were set in PrepareContainerForItem.  This avoided aliasing, but
            // was deemed too expensive, especially for scrolling.  For Windows OS Bug
            // 1445288, all this cleanup work was removed.  This sped up scrolling, but
            // introduced the problems cited in Dev10 452669 and the recycling "subtle
            // issues".  A compromise is needed.
            //
            // The compromise is tell the container to attach to a sentinel item
            // BindingExpressionBase.DisconnectedItem.  We allow this to propagate into the
            // conainer's subtree through properties like DataContext and
            // ContentControl.Content that are normally set by PrepareItemForContainer.
            // A Binding that sees the sentinel as the data item will disconnect its
            // event listeners from the former data item, but will not change its
            // own value or invalidate its target property.  This avoids the cost
            // of re-measuring most of the subtree.

            container.ClearValue(ItemForItemContainerProperty);

            // TreeView virtualization requires that we call ClearContainer before setting
            // the DataContext to "Disconnected".  This gives the TreeViewItems a chance
            // to save "Item values" in the look-aside table, before that table is
            // discarded.   (See Dev10 628778)
            host.ClearContainerForItem(container, item);

            if (container != item)
            {
                DependencyProperty dp = FrameworkElement.DataContextProperty;

                #if DEBUG
                // Some ancient code at this point handled the case when DataContext
                // was set via an Expression (presumably a binding).  I don't think
                // this actually happens any more.  Just in case...
                EntryIndex entryIndex = container.LookupEntry(dp.GlobalIndex);
                Debug.Assert(!container.HasExpression(entryIndex, dp), "DataContext set by expression (unexpectedly)");
                #endif

                container.SetValue(dp, BindingExpressionBase.DisconnectedItem);
            }
        }

        // establish the link from the container to the corresponding item
        internal static void LinkContainerToItem(DependencyObject container, object item)
        {
            // always set the ItemForItemContainer property
            container.ClearValue(ItemForItemContainerProperty);
            container.SetValue(ItemForItemContainerProperty, item);

            // for non-direct items, set the DataContext property
            if (container != item)
            {
                #if DEBUG
                // Some ancient code at this point handled the case when DataContext
                // was set via an Expression (presumably a binding).  I don't think
                // this actually happens any more.  Just in case...
                DependencyProperty dp = FrameworkElement.DataContextProperty;
                EntryIndex entryIndex = container.LookupEntry(dp.GlobalIndex);
                Debug.Assert(!container.HasExpression(entryIndex, dp), "DataContext set by expression (unexpectedly)");
                #endif

                container.SetValue(FrameworkElement.DataContextProperty, item);
            }
        }