Add a picture_tag helper

[jpbalarini]

Motivation / Background

We wanted to use the HTML <picture> tag in our website for responsive images (specifically, for art direction) and found that Rails did not have built-in support for it. By introducing the picture_tag helper, we can simplify the usage of the <picture> element in Rails applications, enabling developers to incorporate responsive images in their projects easily.

The need for a picture_tag in Rails led to the creation of this gem and this RailsConf talk. After presenting the talk, a lot of people told me I should add this feature to Rails, so I'm porting some of the gem's functionality here.

The picture tag is a more "robust" version of the img tag, because it supports resolution switching, art direction, and image support fallback, so I think we should have it in Rails (the img tag only supports resolution switching). More information here:

• Picture tag
• Responsive images & picture element
• Responsive images & Art direction

Detail

This PR adds support for the HTML picture tag. It supports passing one element (String), multiple (an Array), or a Block if you need full control of what it's being generated. All properties that are passed to the helper will apply to the picture tag. If you need to pass properties to the img tag, you can do it inside the :image key.

Since the picture tag requires an img tag (contrary to the audio_tag or video_tag, which only require sources), the last element you provide will be used as the source for the img tag. For complete control over the picture tag, a block can be passed, which will populate the contents of the tag accordingly.

It can be used like this for a single source:

<%= picture_tag("picture.webp") %>

which will generate the following:

<picture>
    <img src="/images/picture.webp" />
</picture>

For multiple sources:

<%= picture_tag("picture.webp", "picture.png", :class => "mt-2",  :image => { alt: "Image", class: "responsive-img" }) %>

will generate:

<picture class="mt-2">
    <source srcset="/images/picture.webp" />
    <source srcset="/images/picture.png" />
    <img alt="Image" class="responsive-img" src="/images/picture.png" />
</picture>

Full control via a block:

<%= picture_tag(:class => "my-class") do %>
    <%= tag(:source, :srcset => image_path("picture.webp")) %>
    <%= tag(:source, :srcset => image_path("picture.png")) %>
    <%= image_tag("picture.png", :alt => "Image") %>
<% end %>

will generate:

<picture class="my-class">
    <source srcset="/images/picture.webp" />
    <source srcset="/images/picture.png" />
    <img alt="Image" src="/images/picture.png" />
</picture>

Additional information

Another option would be to be more explicit on the image and sources that are generated (could be more cumbersome).
Something like this:

picture_tag("image.jpg", sources: [{ srcset: "image_small.jpg", media: "(min-width: 400px)" }, { srcset: "image_big.jpg" }], alt: "An example image", class: "responsive-image")

The issue with this is that we would need a way to specify which properties belong to the img tag and which to the picture tag itself. I did not go with this approach since I tried to more or less replicate how the audio_tag, video_tag, and image_tag helpers work.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[ihoka]

Just wondering why it is necessary to flatten! here.