T/ckeditor5/1636: MultiCommand

[jodator]

Suggested merge commit message (convention)

Feature: Introduce MultiCommand that allows to group many commands as one.

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Additional information

This is part of IndentBlock PR.

The changes in core:

1. Created Indent feature plugins:

   • Indent - typical glue plugin.
   • IndentEditing - introduces the "multi" 'indent' and 'outdent' commands.
   • IndentUI - introduces the 'indent' and 'outdent' toolbar buttons.

   We talked about defining Indent feature (buttons) with @Reinmar but I'm divided on where to put this feature. From one side this feature might not be needed by some - thus it might be better to put it in... 🥁 ...another repository as a separate ckeditor5-indent feature. But OTOH we will probably include this in most of the builds to use 'indent' and 'outdent' buttons.

2. The MultiCommand that allows to register many commands under one alias.

   I've created the MultiCommand as a way to programmatically register other indentation commands under one command so the UI buttons would attach to one command only event thought there are voices against (by @pjasiun dunno @Reinmar voice on this).

   The command allows other features to register a child command (if it is defined in command collection). The MultiCommand changes its isEnabled state to true if any of registered commands is enabled. It also executes first enabled command (if any).

   I've created this command as we want to expose 'indent' and 'outdent' buttons that will enable to indent list and indent blocks. We already have 'indentList' (I'll skip the outdent as it is similar) command and a newly introduced 'indentBlock' command. We want to have one button for both (as in CKEditor 4). Now, I don't like the idea of overriding, ie indentBlock command by indentList command if the command is in list and not in a block. This isn't clean and might introduce some WTFs to others. Now the MultiCommand might be just InputCommand with the same interface as we have only one usage for such construct now and in forseable future. But I'm strongly against any ifs or other hacks in one commands that would tightly couple both features. (early stage POC with buttons)

   The MultiCommand allows to have any of indent feature to be present and does not require loading of other.

[coveralls]

Coverage remained the same at 100.0% when pulling 0b23e78 on t/ckeditor5/1636 into 21b3fd3 on master.

[jodator]

I've invited you @pjasiun to review this alongisde @oleq just because the previous comments on this issue.

[Reinmar]

Why do you wrap it with an object?

[Reinmar]

Because we use Collection? Then why using Collection?

[jodator]

I think that I've started with collection to use some of its delegation powers but it turned out I use as an array. I'll change that.

[Reinmar]

Just addChildCommand()?

[Reinmar]

Although, "register" reads quite well in the docs. So I don't have a strong opinion on that.

[jodator]

The "register" implies something more than just adding it (like adding to a collection). So if it also reads well in the docs I'll leave it as is.

[Reinmar]

Good point. We actually start watching those commands.

[Reinmar]

I became unsure about this. I'll report a followup, though.