The original effect design for Angular had one "bucket" of effects, which
are scheduled on the microtask queue. This approach got us pretty far, but
as developers have built more complex reactive systems, we've hit the
limitations of this design.
This commit changes the nature of effects significantly. In particular,
effects created in components have a completely new scheduling system, which
executes them as a part of the change detection cycle. This results in
behavior similar to that of nested effects in other reactive frameworks. The
scheduling behavior here uses the "mark for traversal" flag
(`HasChildViewsToRefresh`). This has really nice behavior:
* if the component is dirty already, effects run following preorder hooks
(ngOnInit, etc).
* if the component isn't dirty, it doesn't get change detected only because
of the dirty effect.
This is not a breaking change, since `effect()` is in developer preview (and
it remains so).
As a part of this redesigned `effect()` behavior, the `allowSignalWrites`
flag was removed. Effects no longer prohibit writing to signals at all. This
decision was taken in response to feedback / observations of usage patterns,
which showed the benefit of the restriction did not justify the DX cost.
The new effect timing is not yet enabled - a future PR will flip the flag.
PR Close#56501
4e890cc5ac