React Event System: Refactor ElementListenerMap for upgrading #18308
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This PR refactors the
ElementListenerMapproperties of the existing modern event system, React Flare responder system and the legacy event systems. They all useElementListenerMapto register an event type against a container. Unfortunately, they don't have a way of co-ordinating when event listeners should be "upgraded" – which is the process of removing and passive/default listener and adding an active listener.There are three states for event listeners and their
passiveflag. They can betrue,falseorvoid. If they aretrue, then the listener is passive. If they arefalsethen the listener is active and finally, if they arevoidthen we use the browser default and we don't specify an option value forpassivewhen creating the listener viaaddEventListener. To wire all this up, we have to unify the value ofElementListenerMap, so that it's an object telling us of the last previously usedpassivevalue. We represent this as{ passive: void | boolean, listener: any => void }. The upgrade order should go frompassive (true) -> default (void) -> active (false).Now we have this refactor in place, it makes it possible for the modern event system and the work going into
useEventto live together nicely and have a nice optimial system where they can share event listeners together. For Flare, we change the pattern so it uses the new format, but it really doesn't affect the others as Flare will soon be removed, so we keep theElementListenerMapkeys different (they have a_active/_passivesuffixes to not clash with the legacy/modern event system listeners).