请问什么叫做“Top-down”?

[weixin_38073649]

无意中在微软网页上看到的，说又例如提高WPF的性能：

Build Your Tree Top-Down

When a node is added or removed from the logical tree, property invalidations are raised on the node's parent and all its children. As a result, a top-down construction pattern should always be followed to avoid the cost of unnecessary invalidations on nodes
that have already been validated. The following table shows the difference in execution speed between building a tree top-down versus bottom-up, where the tree is 150 levels deep with a single TextBlock and DockPanel at
each level.

那请问什么叫Top——Down？

引用：

https://docs.microsoft.com/en-us/dotnet/framework/wpf/advanced/optimizing-performance-layout-and-design


da jia hao!