Description

This re-lands my automatic focus highlight mode change that was reverted in #38866, and originally introduced in #37825.

The original caused a regression in the gallery transition performance benchmark. I tracked this down to adding and removing listeners (mostly removing) in the ObserverList that it used to track the listeners. I tried a number of things, but what it comes down to is that ObserverList is only efficient for small numbers of observers, and with the highlight mode change, it was getting hundreds of observers (even if they weren't being notified very often).

This adds a new HashedObserverList class that is based on a hash map that reference-counts the observer callbacks, and only notifies once for each callback. This can't be used in place of ObserverList in ChangeNotifier (the main use of ObserverList) because it makes things slower.

For small numbers of observers, the O(N) behavior of the remove method on ObserverList is not a problem, but because the constant for adding an item to a hash map is larger than the constant of appending an item to a small list, it would slow things down overall to use a HashMap for everything. It also would change the notification semantics, but internal Google tests indicate that that is not a problem, as most observers don't assume an order.

Also fixed two memory leaks that @ds84182 found in #38767.

Related Issues

#38860

Tests

• Added tests for ObserverList and HashedObserverList.

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Breaking Change

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