Summary:
@public
This diff implements background colors for the `RCTWKWebView` component by proxying the background color prop to the underlying `WKWebView` and its underlying `UIScrollView`.
There's few differences between `backgroundColor` in `RCTWebView` and `RCTWKWebView` implementations:
1. With `UIWebView,` the background color gets applied after the page loads. With `WKWebView`, this isn't necessarily true. This results in a white flicker on solid backgrounds because sometimes, the background color is set before the page loads. This video illustrates the problem: https://our.intern.facebook.com/intern/px/p/9QBH
1. As far as I can tell, `WKWebView` doesn't handle transparent backgrounds correctly. Either that, or I could be setting the background color incorrectly. I set the background color to `rgba(1, 1, 1, 0.5)` and recorded how both `RCTWebView` and `RCTWKWebView` render. These two videos indicate the differences:
**RCTWebView: Lighter background**
https://pxl.cl/9R13
**RCTWKWebView: Darker background**
https://pxl.cl/9R1b
I tried to replicate this on the web. According to [[ https://our.intern.facebook.com/intern/fiddle/zCHu/ | this fiddle ]], `RCTWebView` is correct. Clearly, RCTWKWebView is rendering transparent backgrounds a bit darker than necessary. This doesn't seem simple to debug, so I've created a task to document this work: T23815343. I'll get to it eventually.
Reviewed By: shergin
Differential Revision: D6398209
fbshipit-source-id: 1812cb68133bc18a3278f6b328d7b085362528b0
215fa14efc