Switching from class to class sometimes is slow

On the memory diff tab, after taking a snapshot, pretend we have classes A and B.

Selecting class A performs computation to calculate the set of retaining paths for to A. Then selecting class B, does the same thing for all the retaining paths of class B. Selecting class A again should be fast, since we have already performed this work. But it appears to take equally as long to compute the set of retaining paths. I suspect we are not caching this computation.

从 memory diff 标签页的实现开始，跟踪选择类时触发的 retaining path 计算。在获取快照后重现 A–B–A 选择序列，并比较重复计算所需的时间。当返回类 A 时能够避免重新计算其 retaining paths，同时保留显示的结果，即表示完成。

由索引模型根据 Issue 内容生成。