Note: input elements with type="week" are not supported in Internet Explorer or Firefox.

(not tested though)

Also im wondering if the underlying DateType can be used with a format Y-\WW[-w] (https://en.wikipedia.org/wiki/ISO_week_date)

I wonder if we need to support different input formats for this form type. Right now it only supports a string that is in the same format as the one supported by the HTML5 week input type. However, I could imagine that some developers would like to have a kind of structure like an array with a week and a year element.

@ro0NL DateType doesn't like date format without y M d, I doubt it can be used directly, I thing the WeekType will be independant, more like the TimeType.

@xabbuh I love the idea, it will behave like the TimeType, right?

@dFayet maybe compatibility with DateType is too far fetched, yet i tend to believe year-weekno-weekday is just as valid as year-month-day for a date. Thus we should be able to use DateTime as a model... yet, im not sure what PHP is doing at this point: https://3v4l.org/7BMDB

If we add a form for type=week, I think it would make sense to also add a validator to ensure that a string contains an ISO week. this is especially important given that some browsers will fallback to a text type (which is fine by default; projects could add some JS-based picker as fallback).

If we want to improve the fallback a bit, we could add a pattern validation (\d{4}-W\d{2}) on the field so that the text fallback has client-side validation too. But I'm not sure we should do it in core (it might interact badly with a JS-based picker added by projects).

Anyway, I think adding a field type rendering a <input type=week /> makes sense.

@dFayet Is this still work in progress or are you waiting for a review? I don't want to put pressure on you just ensure that you are not desperately waiting for a review. :)

Your're right to ask @xabbuh :D

The WIP tag might be too much as, beside the tests, and the questions added in the edit of this PR, I think this one is ready for some reviews.

The subject of the DataTransformers is my main hesitation, should I write the tests now, or wait some feedback?

@dFayet Looking at how the updated transformers would look like, it seems to me that having new transformers would be better. We may then also reconsider the idea that the norm data should be DateTime objects. IMO that's not really needed and we could use an array with two elements as norm data instead as well.

@dFayet Thank you for working on this. I have left you a bunch of comments. But they are actually only about a few details. The overall picture looks quite good to me. 👍

no chance to be merged at 4.4 ?

@dFayet I have incorporated the review feedback. This looks ready to me now. Thank you for your work on this. I have kept your commit so you will still get the full credit.

Thank to you @xabbuh but let's not forget that this is at least a 50-50 between us (+1 for you for your quick reaction)

Thank you @dFayet.

when looking at the input week page, would be a nice idea to have min and max for this too? or maybe as Validator?

the step parameter might be a bit more complicated

@Hanmac That's already possible:

$builder->add('week', WeekType::class, [
    'attr' => [
        'min' => '2018-W18',
        'max' => '2018-W26',
    ],
]);