ObserverTrigger: Add wrapper type around ComponentId

[oscar-benderstone]

Objective

Add a newtype to store events, as mentioned in this HackMD.

Solution

• From the Discord, we agreed on creating a EventKey struct that wraps around ComponentId.
• I created EventKey and refactored all previous instances of ComponentId in observer related functions.

Testing

• I tried three examples, whose behavior matches that of main:

  • examples/observer.rs
  • examples/observer_propagation.rs
  • examples/ui/core_widgets.rs

• There is an issue with examples/observer_propagation.rs, but this is also occurs in main: Warning: missing IDs in observer_propagation #19753.

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[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Please review the instructions for writing migration guides, then expand or revise the content in the migration guides directory to reflect your changes.

[alice-i-cecile]

There's an existing observers migration guide; we should add the changes here to that :)

[alice-i-cecile]

I think that these names would be clearer and more consistent as register_event_key / event_key.

[alice-i-cecile]

Suggested change

	/// # Warning
	///
	/// This struct should only be instantiated internally to this crate.
	///
	/// [`EventKey`]s are created in:
	/// - `crate::observer::trigger_dynamic_ref_with_caller`
	/// - `crate::observer::trigger_dynamic_ref_with_caller`
	/// - `crate::observer::observer_multiple_events`
	/// - `crate::observer::observer_dynamic_trigger`
	/// - `crate::observer::runner::hook_on_add`
	/// A unique identifier for an [`Event`], used by [observers].
	///
	/// You can look up the key for your event by calling the [`Event::event_key`] method.
	///
	/// [observers]: crate::observer

We don't need to give a warning about non-public construction: Rust makes that impossible with correct usage of the visibility system. But we should give some docs about what this type is for and how to get it.

[alice-i-cecile]

This list will be really hard to maintain. Better to leave this off, and allow users to use their IDEs go-to-uses instead.

[alice-i-cecile]

Suggested change

	/// Returns id of the underlying [`Event`].
	/// Returns the internal [`ComponentId`].

[alice-i-cecile]

Some small suggestions for names and docs that we should tackle, but this is looking much better. The clarity of this code is much improved!