Get rid of soft requirements

[Reinmar]

Background: https://github.com/ckeditor/ckeditor5-core/issues/192 and https://github.com/ckeditor/ckeditor5-core/issues/148.

There are two types of plugins which we're not currently listing in requires():

• Optional ones – those which may change the logic of your plugin when they are loaded, but which are not necessary for your plugin to work.

  Example: if Notification is loaded, use its API to display a warning. Do nothing otherwise.

• Soft (dynamic) dependencies – those which are in fact required for your plugin to work correctly, but we didn't want to specify which implementation of this functionality we depend on. Instead, a bit like in "program to an interface" we wanted to depend on an interface. The fact that a plugin called 'Clipboard' will be available, but we don't need to assume which instance it is precisely.

The first mechanism works fine. It was improved in https://github.com/ckeditor/ckeditor5-core/issues/148 (plugins.get() throws, so you get the error ASAP).

The second might be improved (with soft requirements – using strings in requires() as initially discussed in https://github.com/ckeditor/ckeditor5-core/issues/148), but it's actually not used by us. I checked @jodator's research (https://github.com/ckeditor/ckeditor5-core/issues/148#issuecomment-445794316) and did some more and we use this approach in very few cases.

Therefore, let's agree that we don't use soft requirements. If you depending on a clipboard, define it by a class in your requirements. This isn't ideal, but this is how reality works, so I have no intention to deny it that :D.

Also, there's always the option to use this technique with NormalModuleReplacementPlugin to inject a different implementation by tuning webpack's configuration. While less convenient, the option exists, so no one will be locked.

What needs to be done?

We need to use requires() in places where it's missing now – this can be found by scanning for plugins.get( ' and filtering out the "optional dependency" cases.

[Reinmar]

@jodator, I know you want to do this :D

[jodator]

The examples should be swapped - in most cases we do not need Clipboard plugin to work - we only add some functionality to it. So, for me, this is an optional dependency.

The Notification plugin is used as a normal requirement and has proper requires() defined.

Now I'm confused - should we always use hard imports in editor.plugins.get() calls even if its properly required by requires()?

I could only found this one case when plugin used get( 'string' ) that wasn't defined in requires(): https://github.com/ckeditor/ckeditor5-editor-balloon/blob/28ab7bc959f0886c99081c36cc5811b69f2924d2/src/ballooneditorui.js#L51

I found that mostly commands and utility methods use the editor.plugins.get( 'Plugin' ).

[jodator]

Another question: I understand that we must disallow such constructs:

class MyPlugin {
    static get requires() {
        return [ 'Clipboard' ]
    }
}

This construct will work because we map plugin names to constructors and explicitly load them:

https://github.com/ckeditor/ckeditor5-core/blob/5b2507b5494fa26a94d970057fa69aeb9426dbeb/src/plugincollection.js#L259

[jodator]

About that BalloonToolbar plugin use - it is OK. It is not a plugin but a class of and BalloonEditor which always requires BalloonToolbar.