feature: add channel registry

[johnhooks]

What?

Add a Channel_Registry class to register notification channels.

Why?

To attempt registering channels in code.

Implementing the data model of PR #168 has the challenge of when and how to look up plugin registered channels. If channel data is stored in a database table and an integer id column is used to link a channel to subscription and messages, how does a plugin know the id of the channel it registered? This data would have to be looked up on message emit, and also there wouldn't be a good mechanism to prevent plugins from registering similarly named channels.

Using a registry similar block types might work well. The channel is registered in code and the channel name/key/slug (as in core/post-edit) would link a message source to its channel. Subscriptions could also use the same key to link users to channels. This would also allow checking for conflicts at channel registration.

Keeping in mind that if/when a plugin is uninstalled, a message has to contain all the data required to render, it is necessary to store channel key and title in the message.

Both systems have pros and cons. This strategy removes the need for a channel table, and simplifies the logic of emitting a message. Though it will result in duplication of data in the database.

How?

• Channel_Registry class, heavy based on core Block_Type_Registry
• Channel class
• register_channel function

Further considerations

• When/if a plugin is uninstalled its channel subscriptions would remain in the database subscriptions table. Though this is a very small table compared with the queue table. And remembering user preferences is probably normal behavior, so there is a seamless experience if a plugin is reinstalled.
• The channel icon could be stored in the message's icon column if/when a message doesn't specify it's own.

[johnhooks]

The register_channel function seems awkward. It could look cleaner to pass an array with channel constructor parameters.

Though this achieves creating a channel with translations for title and description.

wp-feature-notifications/wp-feature-notifications.php

Lines 74 to 81 in a33543c

	register_channel(
	new Notifications\Channel(
	'core/updates',
	__( 'WordPress Updates', 'wp-feature-notifications' ),
	'WordPress', // TODO the linter won't leave a lowercased name for Dashicons.
	__( 'WordPress core update events.', 'wp-feature-notifications' ),
	)
	);

[johnhooks]

This looks a lot cleaner to me, though PHP doesn't help with type hinting the array. Named arguments would be most preferred, but that would require a minimum PHP version of 8.

wp-feature-notifications/wp-feature-notifications.php

Lines 77 to 86 in cb4954e

	register_channel(
	new Notifications\Channel(
	'core/updates',
	array(
	'title' => __( 'WordPress Updates', 'wp-feature-notifications' ),
	'icon' => 'wordpress',
	'description' => __( 'WordPress core update events.', 'wp-feature-notifications' ),
	)
	)
	);

[erikyo]

wasn't something like this?
@see \WP\Notifications\register_channel()

[johnhooks]

Blocked by #255

[johnhooks]

Work on this is being continued in #259