New addon API

[Tyriar]

I added back the old addons so we can ship the new API in v3, I think we should move ahead with this so that we can start converting the other addons over to the new API incrementally.

Fixes #1128
Related #2076

The API

The final API ended up being extremely simple:

export class Terminal {
    /**
     * (EXPERIMENTAL) Loads an addon into this instance of xterm.js.
     * @param addon The addon to load.
     */
    loadAddon(addon: ITerminalAddon): void;
}

export interface ITerminalAddon extends IDisposable {
    /**
     * (EXPERIMENTAL) This is called when the addon is activated within xterm.js.
     */
    activate(terminal: Terminal): void;
}

In fact the only interesting part about the new addon API is how the addon's dispose method gets wrapped once loaded so that it can unload itself from the Terminal when called.

Terminal events

The idea of exposing a more rich API with events that get fired on the addon itself came up, but I'm leaning towards having addons handle that themselves, at least for now. For example if the addon only works after open it could check Terminal.element:

const addon = {
  activate: (term: Terminal) => {
    if (term.element) {
      // it's already open, init now
    } else {
      // otherwise handle it when it happens
      term.onOpen(() => ...);
    }
  },
  dispose: () => { }
}

Example usage

The usage from the demo is close to this:

term.loadAddon(new WebLinksAddon());
attachAddon = new AttachAddon();
term.loadAddon(attachAddon);

Notice we keep a reference to attachAddon as we might want to dispose it later (detach).

Addon implementations

• https://github.com/xtermjs/xterm-addon-attach
• https://github.com/xtermjs/xterm-addon-web-links

[jerch]

LGTM 👍 - just a few questions from my side.

[jerch]

Reversed disposing - should do the trick for most linear dependencies 👍
(Guess we will never see cycling deps, they are a nightmare on its own)

[Tyriar]

Yeah this is something we can look into supporting as it comes up, not supported for now

[jerch]

Guess Ive overlooked it - where are the addon disposals hooked into the terminal dispose chain?
Also does the API allow to do nasty things like terminal.dispose() within addon.dispose?

[Tyriar]

Guess Ive overlooked it - where are the addon disposals hooked into the terminal dispose chain?

It just moved to dispose in ./public/Terminal as the addon manager now lives there which is much nicer (as they build on the API, not TerminalCore).

Also does the API allow to do nasty things like terminal.dispose() within addon.dispose?

Well you could as Terminal.dispose is part of the API, no way around that really nor should we try imo. I see a possible infinite loop because of that, I think just moving isDisposed = true above the addon.dispose call should fix that.

[Tyriar]

The fullscreen addon will be removed when we transition over as it does so little, for fit it can be converted after the renderer API is done in #2005