Port the media layout algorithm from the Android app

[grishka]

As discussed, I tried to port the media layout algorithm from the Android app. Android sources: the layout algorithm itself and the custom ViewGroup that actually puts the views on the screen. This new layout algorithm works with up to 10 media attachments.

I'm not familiar with Swift and UIKit, and definitely not with ReactiveSwift that permeates the entire codebase of the app. I might have done something stupid.

This PR isn't quite ready for merging. There's something inside MediaView that doesn't respect its frame and won't crop vertically, resulting in overlapping views:

There's also a bug I don't know how to fix because I don't know where to hook into either the app or UIKit to re-run the layout with new maxSize when the view size changes. Android would automatically measure and then lay out the view subtree when the dimensions of its parent ViewGroup change, but iOS apparently doesn't do that? To reproduce, open an attachment, rotate the device to landscape orientation, then close the attachment.

[CLAassistant]

All committers have signed the CLA.

[grishka]

Also, teeny-tiny gifs. I'm definitely missing some crucial piece of understanding of how UIKit views work.

[j-f1]

I think you need to call super.layoutSubviews() in your layoutSubviews otherwise the subviews won’t layout their own subviews.

[grishka]

Nope, it made no difference =/

[j-f1]

Does this mean 4 attachments will never be rendered as a 2x2 grid?

[grishka]

Yes, it's always 1 bigger one and 3 smaller ones

[grishka]

I limited the aspect ratio to 9:20 to avoid weird skinny layouts with images of extreme aspect ratios. I chose 9:20 specifically so that vertical screenshots from modern phones don't end up getting cropped. Modern "bezel-less" phones have 9:18-ish screens.

[grishka]

Another interesting observation — if a post has a broken layout, but I then tap it to open separately, that screen has a correct layout.

Might it be that images loading asynchronously affect the layout inside MediaView somehow?

[grishka]

I decided to take another look at it and remembered that the layout inspector exists. So something is adding these constraints to the UIImageView, making the layout break when I set frames manually. Now it's only a matter of finding what it is.