[Twig] add computed properties system

[kbond]

Q	A
Bug fix?	no
New feature?	yes
Tickets	n/a
License	MIT

This adds a computed component template variable to make "computed methods" easier. In your component's template, to call a method on your component and cache the return (to use again later), instead of calling this.methodName, call computed.methodName:

{# templates/components/featured_products.html.twig #}

<div>
    <h3>Featured Products</h3>

    {% for product in computed.products %}
        ...
    {% endfor %}

    ...
    {% for product in computed.products %} {# use cache, does not result in a second query #}
        ...
    {% endfor %}
</div>

[kbond]

Should we make the public properties available via computed (computed.prop1)? Right now only public methods are available.

[weaverryan]

Should we make the public properties available via computed (computed.prop1)? Right now only public methods are available.

Ideally yes... just so that this and component work identically. But more so, could we re-use the Twig logic for calling the method / fetching the property? For example, we know that Twig checks several things when we say this.foo to get that property / call that method. Can we reuse that inside the proxy class?

[kbond]

Can we reuse that inside the proxy class?

I believe so, yes. I only check get/no-get method prefix now (so computed.getMethod and computed.method both work and refer to the same cache). I'll take a look at twig's internals and try to match.

[kbond]

Ok, I think this is ready. I ensure computed.x works with public properties, components implementing \ArrayAccess, getter's, isser's and hasser's.

[weaverryan]

This looks great! i just wanted to triple-check about reusing the Twig logic

[weaverryan]

So I'm guessing there wasn't a nice way to "steal" and re-use the Twig native functionality?

[kbond]

There was not...

[kbond]

I looked at using twig_get_attribute but it requires injecting the twig environment and Source.

[weaverryan]

Wow, Source, that's a new class to me :)

[kbond]

Rebased and conflicts resolved.

[weaverryan]

Thank you Kevin!