[2.3] Reworked gallery.phtml to move generation of gallery json strings to own block functions

[gwharton]

Description

Reworked gallery.phtml to move generation of gallery json strings to own block functions
Introduced unit tests to cover generation of gallery json strings with full type checking.

The resultant code is much easier to diagnose and test. It uses the json_encode and json_decode routines instead of the old manual way of generating json strings that were used in the old gallery.phtml which is prone to error and mistakes.

Fixed Issues (if relevant)

1. none. Rework and unit tests.

Manual testing scenarios

1. View source on any product page.
2. Ensure the gallery section has fully formed json for gallery and gallery fullscreen sections.

Stock Magento should be

"options": {"nav":"thumbs","loop":true,"keyboard":true,"arrows":true,"allowfullscreen":true,"showCaption":false,"width":700,"thumbwidth":88,"thumbheight":110,"height":700,"transitionduration":500,"transition":"slide","navarrows":true,"navtype":"slides","navdir":"horizontal"}

and

"fullscreen": "nav":"thumbs","loop":true,"navdir":"horizontal","navarrows":false,"navtype":"slides","arrows":true,"showCaption":false,"transitionduration":500,"transition":"slide"}

All strings/bools/ints are now typed properly as per the Magento dev docs for the Magento 2 Gallery Widget.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[gwharton]

The 2.2 Backport PR is here #18443

Changes incorporated

[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-3987 has been created to process this Pull Request

[sidolov]

Hi @gwharton , it's a really good changes that helps external developers customize gallery configuration, but unfortunately, we are not allowed to add new public methods to blocks that marked as @api.
As alternative solution you may extract logic to the separate class, inject it to block via di configuration and use in the phtml template. You can find the example in the /app/code/Magento/Sales/view/adminhtml/templates/order/create/form/address.phtml
Thank you!

[gwharton]

@sidolov, Thats neat. I hadn't realised you could do that.

I've updated the 2.3 and 2.2 PR's accordingly. Just waiting on the Travis backlog, but pretty sure they should come in green. If not I will resolve.

[magento-cicd2]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

[gwharton]

Just waiting on a travis pass.

I have reverted all changes to Gallery.php and GalleryTest.php as all active changes were moved to GalleryOptions.php and GalleryOptionsTest.php and the changes that were left in the original Gallery.php and GalleryTest.php were cleanup code unrelated to this PR and a distraction.

This PR feels much cleaner now.

[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-3987 has been created to process this Pull Request

[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-3987 has been created to process this Pull Request

[VasylShvorak]

✔️ QA passed

[m2-assistant]

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