Feat: support of multiple selection in media upload

[dratwas]

In this PR I added support for multiple selections of media in MediaUpload component. It is a breaking change and apps will crash without changes from listed PRs.

Associated issue - #1298

Related gutenberg PR - WordPress/gutenberg#17397
Related Wordpres-iOS PR - wordpress-mobile/WordPress-iOS#12404
Related Wordpress-Android PR - wordpress-mobile/WordPress-Android#10556
To test:

• set multiple prop to true in image/video block
• run Wordpress-iOS or Wordpress-Android
• add image/video block - multiple select should be enabled for selecting media from the device or WP Media
• the array of selected media should be passed to the callback function

NOTE: Multiple selections of media from the device on iOS doesn't work since this functionality is implemented by the client app. Our example app from ios directory doesn't support multiple selections from the device because it uses build-in UIImagePickerController. In order to make it works in our example app, I would need to add an external library that supports multiple selections.

[pinarol]

This is looking pretty good overall, I just wanted to point out some things early on.

So what are some pending work here?

[dratwas]

Prop multiple works on iOS with these changes so basically I need to check how the cancelation mechanism works and make it works with multiple as well and add tests.

And Android implementation

[pinarol]

thanks @dratwas ! did you have chance to try multiple uploads also?

[pinarol]

and add tests.

really appreciate that!

[dratwas]

thanks @dratwas ! did you have chance to try multiple uploads also?

AFAIK this is a responsibility of the client app. I implemented it in Wordpress-iOS here https://github.com/wordpress-mobile/WordPress-iOS/pull/12404/files#diff-0992f0530230d5beb61d0569d50f4459R40
I tried it and native side uploads media and sends all "mediaUpload" events related to it but it is not handled on JS side because at this moment JS implementation of image/edit.native.js assumes that we can get only one image. So it is prepared to support multiple media upload but needs to be handled in gallery block.

[pinarol]

I tried it and native side uploads media and sends all "mediaUpload" events related to it

That's actually enough for this particular task thanks! Sorry If I confused you, the rest will be handled in gallery block. I just wanted to make sure we are sending the "mediaUpload" events generated by simultaneous uploads. 👍

[pinarol]

Could you be able to update the branch from develop ?

[pinarol]

Could you also add the associated issue to PR description?

[pinarol]

Could you be able to update the branch resolve merge conflicts one more time? Sorry for being late for reviewing, I'll start after this. Thanks!

[pinarol]

iOS changes look good to me 🎉

[mkevins]

I tested this on Android, and it is working nicely. 🎉