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Implement 'backgroundColor' style


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:
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**
**RCTWKWebView: Darker background**

I tried to replicate this on the web. According to [[ | 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
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RSNara authored and facebook-github-bot committed Aug 16, 2018
1 parent 1af17f1 commit 215fa14efc2a817c7e038075163491c8d21526fd
Showing with 18 additions and 0 deletions.
  1. +18 −0 React/Views/RCTWKWebView.m
@@ -15,6 +15,7 @@ @interface RCTWKWebView () <WKUIDelegate, WKNavigationDelegate, WKScriptMessageH

@implementation RCTWKWebView
UIColor * _savedBackgroundColor;

- (void)dealloc
@@ -56,6 +57,19 @@ - (void)didMoveToWindow

- (void)setBackgroundColor:(UIColor *)backgroundColor
_savedBackgroundColor = backgroundColor;
if (_webView == nil) {

CGFloat alpha = CGColorGetAlpha(backgroundColor.CGColor);
self.opaque = _webView.opaque = (alpha == 1.0);
_webView.scrollView.backgroundColor = backgroundColor;
_webView.backgroundColor = backgroundColor;

* This method is called whenever JavaScript running within the web view calls:
* - window.webkit.messageHandlers.[MessageHanderName].postMessage
@@ -236,6 +250,8 @@ - (void) webView:(WKWebView *)webView

[self setBackgroundColor: _savedBackgroundColor];

- (void)evaluateJS:(NSString *)js
@@ -292,6 +308,8 @@ - (void) webView:(WKWebView *)webView
} else if (_onLoadingFinish) {
_onLoadingFinish([self baseEvent]);

[self setBackgroundColor: _savedBackgroundColor];

- (void)injectJavaScript:(NSString *)script

3 comments on commit 215fa14


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oferRounds replied Aug 28, 2018

oh, the video links are broken :\ (came here from the blog’s link)


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pascalgagneur replied Oct 17, 2018

All links seem broken to me


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Monte9 replied Oct 24, 2018

Yeah seems like it might only be accessible internally by fb employees. (

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