Workaround to force Xamarin.Forms WebView to use a dark mode CSS for local content on Android

Workaround to force Xamarin.Forms WebView to use a dark mode CSS for local content on Android

Recently I updated my blog reader app to support the dark mode newer iOS and Android version support. While everything went smooth on iOS and the update is already live in the App Store, I had some more work to do on Android. One of the bigger problems: the WebView I use to view posts does not automatically switch to dark mode with Xamarin.Forms.

What’s causing this problem?

On part of the problem is that the WebView does not support the CSS query “prefers-color-scheme“. This works as intended on iOS however and is a problem specific to Android. You can refer to this issue on the Xamarin.Forms repository on Github.


I am not sure if this problem will ever get solved by the Xamarin.Forms team. I tried to play around with some Javascript solutions that are floating around the web to keep just one CSS file. In the end however, I went with a Xamarin.Forms only approach following the KISS principle.

Xamarin.Forms has a working theme detection mechanism. Based on the return value of the Application.Current.RequestedTheme property, I am loading either the dark mode CSS file or the light mode CSS file (which is default in my case).

Shipping the CSS files is easy, we just need to add them to the Assets folder and set the Build action to AndroidAsset. This results in the following structure within the Android project:

All files that are shipped that way are accessible via the android_asset file uri:

<link rel="stylesheet" href="file:///android_asset/dummycss_light.css">

Now that everything is set up in the Android project, let’s head over to the Xamarin.Forms project. In the sample for this post, I am loading a local html file, while I generate the html dynamically in my blog reader app. The idea works the same way in both cases:

private async Task SetThemeAndLoadSource()
    _html = await LoadHtmlFromFileAsync();

    _html = Application.Current.RequestedTheme == OSAppTheme.Dark ?
            _html.Replace("light", "dark") :
            _html.Replace("dark", "light");

    this.TestWebView.Source = new HtmlWebViewSource() { Html = _html };

As you can see, I just override the light and dark part of the CSS file name I load into the HTML. That’s all the “magic” that needs to happen here. Just one little thing left to add – what if the user changes the theme while the app is running? Xamarin.Forms has the solution built in as well – just handle the RequestedThemeChanged event and override the file name again, followed by setting the HtmlWebViewSource again:

private async void Current_RequestedThemeChanged(object sender, AppThemeChangedEventArgs e)
    await SetThemeAndLoadSource();


As most of us are already used to, we sometimes need to find some workarounds when dealing with Xamarin.Forms. While this problem could have been solved with a bunch of Javascript and a custom CSS in a WebViewRenderer as well (I tried that, but didn’t like the complexity), you can achieve reliable results with the workaround above just in Xamarin.Forms.

You can find a working sample here on Github. As always, I hope this post will be helpful for some of you.

Until the next post, happy coding, everyone!
Posted by msicc in Android, Dev Stories, Xamarin, 1 comment

Dev Story Series (Part 5 of many): Styling a WebView or WebBrowser element

This post is about styling our WebView or WebBrowser in our app. Until now, we only got the HTML string that we are displaying in our WebView or WebBrowser. It looks like this:


The content we receive from our WordPress post content includes already all kind of HTML tags like paragraphs, lists, links, images. That is the advantage for this solution: no parsing is needed, the string can be displayed as is. Both the WebView and the WebBrowser framework element (no, they are not controls) are able to read and render CSS code. And this is how we can match the whole element for our app.

HTML Pages can be styled by using a so called cascading style sheet (CSS), which is similar to XAML code. With a little bit of searching on the web you will be able to style “translate” your XAML properties into CSS.

Here is a sample CSS String:

<STYLE type="text/css">
body{background:#034786; width:450px; }
p{font-family:'Segoe UI';color: white;font-size:medium;}
h1{font-family:'Segoe UI';color: white;}
h2{font-family:'Segoe UI';color: white;}
h3{font-family:'Segoe UI';color: white;}
h4{font-family:'Segoe UI';color: white;}
pre{background-color: #C0C0C0; width:100%;}
blockquote{font-family:'Segoe UI';font-style:italic;}
a:link{font-family: 'Segoe UI';color: #C0C0C0; font-size: medium; text-decoration:underline}
li{font-family: 'Segoe UI';color: white;font-size: medium;list-style-type: square;}
img {text-align:center; width:100%: height:100%;}

Every CSS string has to be surrounded with “<STYLE type=”text/css”> </STYLE> “. Between those two Style tags, you can set different properties for each kind of HMTL tag:

  • body = the whole page is embedded in the body. this is where we set the background of our content as well as the width and the height
  • p = paragraphs. paragraphs can contain text as well as images or other multimedia content. Mainly used for text like in our blog post, we style how the user is able to read our blog post.
  • h1 – h4 = different kinds of headers. you can define four styles of headers
  • pre = is for lines of code
  • blockquote = if we quote people or other sites, we use quotes to clarify this aren’t our words. should be styled a bit differently than the rest of our blog (e.g. Italic)
  • a:link = how hyperlinks will be styled
  • li = this is how our list will be styled in this view
  • img = how we want to see our pictures in our post

Hint for using CSS in code behind:

If you just C&P the CSS string from above, it will result in some errors from Visual Studio. Visual Studio does not like the new lines in strings, so you have to add it as one line. Also is it not possible to use ‘”‘ within a string declaration. It has to be “escaped”, which we are doing with a simple before it: ‘”‘. I mentioned it because I learned it the very hard way by trying to solve it for 2 hours.

Only thing we now need to do is pass the CSS String together with our HTML string from our JSON to our WebView or WebBrowser element:

WebBrowser.NavigateToString(CSSString + ContentString);

After navigating to this both strings, our content is now displayed like native:


One last tip:

I recommend to set the Visibility of your WebBrowser or WebView to “Collapsed” until the whole rendering has done. Once the “navigation” has finished, set it via code to visible. This way the user does not recognize that we are rendering the post content for him. Just display a loading animation until that is done. Both elements have a “LoadCompleted” event. Once the rendering (= the navigation to our string) is done, the content of our blog post is shown as it would be natively in our app.

As always, I hope this is helpful for some of you and feel free to leave a comment below.

Posted by msicc in Archive, 0 comments

Dev Story Series (Part 4 of many): How to open links from a WebBrowser/WebView in Internet Explorer


Today I will share my solution of how to open links from a WebBrowser or WebView on Windows Phone and Windows 8 (as I did in my app for msicc.net).

Generally links are opened within the same WebBrowser or WebView element. On Windows Phone you can solve this problem pretty easy with only one simple method:

public void WebBrowser_Navigating(object sender, NavigatingEventArgs e)
            e.Cancel = true;
            WebBrowserTask wbt = new WebBrowserTask();
            wbt.Uri = e.Uri;

In Windows 8 this gets a bit more complex. There is no Navigating method, and this is why we have to combine different languages together. We will use the ScriptNotify event to pass the link via a small java script and launch IE with the new link.

First, you need to add the function to your HTML string.  I used a RegEx method to add all the pattern that is needed to all links. As I know that a lot of us (especially junior devs like me) have their problems with RegEx, here is my method to add them:

public static string AddScriptToLink(string text)
            const string hrefScript = " onclick="return OnLinkClick('{0}')" ";
            const string pattern = @"href=""(.*?)""";

            var result = text;

            var matches = Regex.Matches(text, pattern);
            var sortedMatches = matches.Cast<Match>().OrderByDescending(x => x.Index);
            foreach (var match in sortedMatches)
                var replacement = string.Format(hrefScript, match.Groups[1].Value);
                result = result.Insert(match.Index, replacement);

            return result;

After you did that, you will be able to use the following script:

<script type='text/javascript'>
	function OnLinkClick(a) 
		 return false;    

I pass this script together with the HTML string (HTML methods need to be first!) to the WebView. I recommend to save the script as a static string, so you have do insert only the name of the string.

If you add the ScriptNotify Event to your WebView, you will be able to use LaunchUriAsync with the value of e, which is the link.

private async void WebView_ScriptNotify(object sender, NotifyEventArgs e)
            await Windows.System.Launcher.LaunchUriAsync(new Uri(e.Value));

I am not sure if there is a better way to do this, but that is my way. It works like it should and does not hurt the experience of my app.

As always I hope this is helpful for someone out there. If you have any way to improve that, feel free to leave a comment below.

Posted by msicc in Archive, 0 comments

Dev Story Series (Part 3 of many): Why I use a WebBrowser/WebView to display WordPress post content

When it comes to display the post content on a blog reader app, it starts to become a bit challenging. The post content is formatted to look great on your website. But when we pull our posts into an app, there is only the naked, HTML formatted string.

As developer, you have to think about several things now:

  • What part of the content do I want to be displayed?
  • How do I get the images there?
  • What if there is a video in the post?
  • Where do I put the Links in?
  • How can I handle enumerations?
  • and so on…

There is the HTMLAgilityPack out there, but I never got a satisfying result out of it. The next method would be to write a custom parser. This is what I have done before, in the old version of the app for my WordPress blog. It did work, but I had to invest a real big amount of time in it before I got a result that I was able to live with. I was also not too experienced with RegEx (and I am still not) that I could set up a perfect parser.

When I was creating the Windows 8 version of my app, I wanted to achieve a good reading experience. On the other side I  wanted the code to be as reliable as possible, because there are often changes on WordPress that can have impact on my app.

As I mentioned above, the post content is already formatted. It is formatted in HTML.  I decided to render the content string instead of parsing it.

It is pretty easy to do that. Just pass the content string to your desired details page, and use a WebBrowser on Windows Phone or a WebView on Windows 8. Without any parsing, just by “navigating” to the passed string, we will get a result like this:


So we have already a readable result, and if my app has only white background, I could leave it like it is and go on.  Without any additional line of code.

Using the WebBrowser/WebView brings also additional advantages:

  • Pinch-to-Zoom support
  • Orientation support
  • automatic image downloading without any additional control
  • WebView on Windows 8 embeds videos automatically

I don’t want to hide that there are a few points that we need to handle, which will be subject of additional posts:

  • styling of content to match our app colors
  • Navigation to links (including a solution for video links on Windows Phone)
  • Scroll direction in Windows 8 WebView

I know it might be not the best practice for displaying web content, but I am really satisfied what I achieved by using the WebBrowser and WebView element in my apps.

I hope the upcoming blog posts will be helpful for some of you to create also a good user experience by using these elements. Of course these posts will contain some code. Before starting the posts about it I just wanted to share why I used these elements.


Posted by msicc in Archive, 0 comments