问题
I have an Android App that uses webview, and lately I'm trying to figure out how to add a dark theme by using the new @media (prefers-color-scheme: dark) CSS syntax. I have the correct CSS written on my page, and if I open it in Chrome with the dark mode of Chrome turning on, it works. I also have my AppTheme inheriting Theme.AppCompat.DayNight, and my app shows dark loading dialog etc. when I turn on dark mode for the entire OS on my device. Even the auto-complete options for the <input> elements become dark. But still, the webpage loaded with my webview doesn't turn dark. According to this page, webviews should support this feature, but I just can't get it to work. What am I missing here?
I also just found out that in API 29 there's this WebSettings.setForceDark() method; could it be the thing I'm looking for? I hope to find a solution that works with lower API level though.
By the way, my current workaround is to inject a JavaScript interface like this:
webView.addJavascriptInterface(new JSInterface(), "jsInterface");
...
public class JSInterface {
@JavascriptInterface
public boolean isNightMode() {
int nightModeFlags = getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK;
return nightModeFlags == Configuration.UI_MODE_NIGHT_YES;
}
}
And then in my webpage, call the jsInterface.isNightMode() method and dynamically load different CSS file based on the result. It certainly works and responses to the global dark mode setting as desired, but I still wonder if I can make prefers-color-scheme work.
回答1:
Android Webview handles day/night mode a bit differently from the rest of the views. Setting your theme to dark will change the WebView components (scrollbar, zoom buttons etc.) to a dark mode version, but will not change the content it loaded.
To change the content you need to use the setForceDark method of the webview settings to make it change its contents as well. A compatibility version of this method can be found in the AndroidX webkit package.
Add the following dependency to your gradle build:
implementation 'androidx.webkit:webkit:1.3.0-rc01'
Note: The required version of the package (1.3.0) is still in pre-release at the moment of writing this answer. So keep this in mind if you want to use these features in production
And add the following lines of code to your webview intitialization:
if(WebViewFeature.isFeatureSupported(WebViewFeature.FORCE_DARK)) {
WebSettingsCompat.setForceDark(myWebView.getSettings(), WebSettingsCompat.FORCE_DARK_ON);
}
The isFeatureSupported check is there to make sure the Android System WebView version the user has installed on their device supports dark mode (since this can be updated or downgraded independently from the Android version through Google Play).
Note: The setForceDark feature requires Android System WebView v76 or up to be installed on the running device.
The force dark feature for webview content has two so-called strategies:
User agent darkening: The webview will set its content to dark mode by automatically inverting or darkening colors of its content.
Theme based darkening: The webview will change to dark mode according to the theme of the content (which includes the
@media (prefers-color-scheme: dark)query).
To set which strategy the webview should use to apply force dark you can use the following code:
if(WebViewFeature.isFeatureSupported(WebViewFeature.FORCE_DARK_STRATEGY)) {
WebSettingsCompat.setForceDarkStrategy(myWebView.getSettings(), WebSettingsCompat.DARK_STRATEGY_WEB_THEME_DARKENING_ONLY);
}
Note: Strategy selection requires Android System WebView v83 or up to be installed on the running device. WebView versions that support setForceDark but do not support strategy selections (v76 to v81) will use user agent darkening
The supported strategy options are:
- DARK_STRATEGY_USER_AGENT_DARKENING_ONLY: Only use user agent darkening and ignore any themes in the content (default)
- DARK_STRATEGY_WEB_THEME_DARKENING_ONLY: Only use the dark theme of the content itself to set the page to dark mode. If the content doesn't have a dark theme, the webview won't apply any darkening and show it "as is".
- DARK_STRATEGY_PREFER_WEB_THEME_OVER_USER_AGENT_DARKENING: Use the dark theme of the content itself to set the page to dark mode. If content doesn't have a dark theme, use user agent darkening.
How do Javascript checks work for darkened webviews?
The JavaScript call window.matchMedia('(prefers-color-scheme: dark)') will match in both the user agent darkening and web theme darkening strategy.
This darkens my webviews even if I am running the app/device in light mode. How do I only enable this when running the app in dark mode?
It depends on how your app implements dark mode.
Force Dark
If your app is using the new auto-dark Android 10 Force Dark feature for views (a feature similarly named, but not fully related to webview forcedark), then you can simply set the force dark mode of the webview to FORCE_DARK_AUTO to handle this for you.
DayNight or Dark theme
If you are using a DayNight or Dark theme to handle dark mode, a night theme check around the webview forcedark code is the only way to handle this automatically:
int nightModeFlags = getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK;
if (nightModeFlags == Configuration.UI_MODE_NIGHT_YES) {
//Code to enable force dark and select force dark strategy
}
回答2:
What I do, based on your question code, is to force based on current status:
int nightModeFlags = getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK;
if (nightModeFlags == Configuration.UI_MODE_NIGHT_YES) {
webSettings.setForceDark(WebSettings.FORCE_DARK_ON);
}
This way @media (prefers-color-scheme: dark) works, but @media (prefers-color-scheme: light) still doesn't work (tried using FORCE_DARK_OFF and FORCE_DARK_AUTO in an else)
回答3:
There are two relevant pieces of documentation here:
https://developer.android.com/guide/topics/ui/look-and-feel/darktheme#force_dark
https://developer.android.com/reference/android/view/View.html#setForceDarkAllowed(boolean)
The key points are
- The WebView must allow force dark, and all its parents must too.
- The theme must be marked as light.
This means that getting FORCE_DARK_AUTO behaviour to work in WebView is a little complex. Adding the following to an AppCompat.DayNight theme does work, but is maybe not the best solution as it may apply Android force dark to views other than the WebView.
<item name="android:forceDarkAllowed">true</item>
<item name="android:isLightTheme">true</item>
The other option is to handle this manually, by setting FORCE_DARK_ON/OFF based upon the relevant configuration change.
Currentlly WebView does not support prefers-color-scheme partly because force dark was implemented before prefers-color-scheme standardisation, and partly because there's no way to integrate that sensibly (without style flashes) with force dark. The intent is to provide the ability to choose either force dark or prefers-color-scheme through androidx.webkit.
回答4:
I don't think WebView honours the prefers-color-scheme CSS media query yet.
The new API for setForceDark has three states: on, off or auto.
ON - your content will always be rendered darker everytime.
OFF - your content will always be rendered light everytime.
AUTO - the content will be rendered darker if your app's theme is darker OR the device OS is in dark mode because the user toggles the OS level switch or battery saver mode turned on.
I believe support for older versions of Android and also control over whether to use prefers-color-scheme instead of WebView's force dark is coming soon via AndroidX. Due date is unknown at present.
For now I would recommend setting the WebView to setForceDark Auto. This will work on Android Q and above.
I would keep an eye on AndroidX release notes for the rest of the support you require for devices on Android P and below.
来源：https://stackoverflow.com/questions/57449900/letting-webview-on-android-work-with-prefers-color-scheme-dark