Use cached value in derived atom watcher #3
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Avoids recalculating the previous value using selector if the cache already contains the correct value. Uses the same code as -deref to establish validity of cache value.
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This change avoids recalculating
old-valueusing theselectorfn ifcachealready contains the correct value. For expensive selector functions this can have a big impact on performance.It uses the same code as is used in
-derefto establish validity of the cached value.I saw there was previously code which attempted to use the cache in watchers but it was removed in df32b5a with a comment that it caused race conditions in some situations.
I think there were two issues with the previous code that aren't present in this change:
srccachevalue matched theold-source-valuewhich could result in an incorrect cache value being used.