Fix target content offset edge case

[bryankeller]

Details

This fixes an issue that could cause the target content offset to be computed with incorrect layout metrics (bounds / content size / content insets) when the collection view is first appearing. This can happen if there are some forced layout passes due to navigation bars being sized, for example.

The general issue is that prepareForAnimatedBoundsChange is called with an oldBounds value, but we have no idea what the content inset / content size of the collection view was when that was our current bounds. If we use updated content inset / content size values, but with the previous bounds value, then we'll compute an incorrect target content offset. To make matters worse, there's no exact order of layout lifecycle calls that we can depend on to save our previous inset / bounds. To work around this, we need to store a few bounds + content inset + content size states, which we can later reference in prepareForAnimatedBoundsChange so that we can use the right values.

Previously, we just stored the last values seen in invalidateLayout, rather than the last 3. This isn't good enough, since invalidateLayout will be called many times with differing values, but the values that we actually have access to in prepareForAnimatedBoundsChange might need to be from a few invalidations in the past. What a pain.

Related Issue

N/A

Motivation and Context

Fix internal Airbnb issue

How Has This Been Tested

Example app, Airbnb app

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[brynbodayle]

Ugh, those early layout passes really break a lot of assumptions.