We introduced the SKIP_SETUP sentinel value when converting
Field and its subclasses to ES6 class syntax, because super
must be called before any other code in a subclass
constructor, breaking the previous mechanism where subclasses
would set some properties before calling their superclass
constructor.
SKIP_SETUP was a singleton value of class Sentinel.
Recently, in PR #6639 @btw17 introduced the isSentinel type
predicate to improve the typing of Field. Unfortunately, there
were some aspects of this change that were not very elegant:
- isSentinel was declared as a static method on Field (rather
than on Sentinel itself).
- It only checks against the specific value SKIP_SETUP,
rather than checking if the argument is instanceof Sentinel
(though perhaps this is for efficiency?)
Additionally - as a result of the migration from ES6 to TS, and
predating PR #6639 - The signature for the Field constructor's
first argument was typed T|Sentinel, with subclass constructors
generally being <some type(s)>|Sentinel.
This creates a small problem when attempting to port Fields from
core to plugins, because Sentinel is not reexported by
core/utils.ts (and therefore not from core/blockly.ts either).
The latter problem could be solved simply by reexporting Sentinel,
or by having plugin constructors not accept SKIP_SETUP (though
this potentially makes them more difficult to subclass), but
neither is particularly desirable.
Instead, this PR proposes that we:
- Make Field.SKIP_SETUP a (unique) Symbol.
- Change the value argument to the Field constructor to accept
T|typeof Field.SKIP_SETUP - where typeof Field.SKIP_SETUP is
(like a literal type) a type that accepts just the single
value SKIP_SETUP.
- Remove the Sentinel class and core/utils/sentinel.ts.
Not treating this as a breaking change:
- Removes Field.isSentinel - though this addition has not yet
been published, so it can only break our own as-yet-unreleased
code in samples.
- Changes the type of Field.SKIP_SETUP and the first argument
of the Field constructor from Sentinel to typeof SKIP_SETUP
(a unique Symbol) - but given that Sentinel has never been
exported this should not break any actual external code.
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