Implement {% elif %} in the template language

[Aymeric Augustin]

Following a discussion on IRC today, this feature is deemed useful by some, but others think it add too much logic in the templates.

#3100 may be related.

[Aymeric Augustin]

#3100 is about supporting arguments on intermediate tag tokens, and it's a pre-requisite for this ticket.

Here are the arguments I saw on IRC.

Pros:

• templates should be able to express presentational logic: for instance, depending of an object's state, I want to show different properties
• the most straightforward workaround is messy: {% if a %} ... {% else %}{% if b %} ... {% else %}{% if c %} ... {% endif %}{% endif %}{% endif %}
• a more elaborate workaround involves partial templates, e.g. {% include myobj.state_template %} where state_template returns "partials/state_<STATE>.html", but that's overkill for simple cases

Cons:

• templates shouldn't contain too much logic — that's a well understood design choice
• increasing the amount of logic in the templates doesn't help us fight performance issues

Tentatively marking as accepted, please bump back to DDN if you disagree.

[Russell Keith-Magee]

@aaugustin -- Count me as +1 on this; I can't see any argument where massively nested ifs are preferable to a flat series of {% elif %} statements. I don't think this increases the amount of logic in a template; it just makes the logic that we've already allowed slightly more friendly.

[Aymeric Augustin]

Attached patch implements {% elif %}. It complements 3100-backwards-compatible.patch from #3100.

[Aymeric Augustin]

In [17187]:

Fixed #17348 -- Implemented {% elif %}. Refs #3100.