Styling in Xamarin.Forms with CSS

Less than a week ago, CSS support was merged into the master branch of Xamarin.Forms. I've seen some people wonder what the point of CSS support is and how it may be useful. Personally I'm excited for CSS support. I want to tell you why.

I've never really been a fan of XAML styling. Not in Xamarin.Forms, nor in WPF or UWP. I've always thought it's too limited. A style can only be specified for a single type of control at a time for example.

Want to specify a default margin for all child items of a StackLayout? You'll have to create a style for each type of control inside that StackLayout.

With CSS you'll be able to define a property on a container element and have that style "trickle down" to its children. This means you'll be able to set the text color on a Grid and all it's children will use that text color: Labels, Buttons, Editors, etc. Best of all, it can be reused on a StackLayout later for the same effect.

It's possible to make selections based on the parent of a control too. Say you want the children of all Frames to have a default margin of 10. You can easily specify this too with the following selector: Frame > *. In other words, "any kind of control that's a direct child of a Frame.

If you want all the children of the children, and their children, and their children, all the way down (all descendants in other words), you can specify that too: Frame *. "All controls that are inside a Frame.

Xamarin.Forms will also support a special kind selector which is not present in regular CSS. The ^ selector. This selector will match a class and all it's subclasses. ^ContentPage will match any ContentPage and any subclass of ContentPage you may have created. This means you'll be able to style of the pages in your app in one place.

This makes building themes a lot easier. Imagine defining a style that will change the all the included controls and pages, and your custom controls and pages. If you want to support multiple themes in your app, you'll be able to swap out the loaded CSS file and completely change the way the app looks.

I think CSS will help Xamarin.Forms look better. It will be easier to create a custom look for your app, which I think is something Xamarin.Forms really needs. As theming has been a bit difficult in the past, a lot of Xamarin.Forms apps look alike. While it's not a bad thing that they look like the native apps on ther respective platfomrs, it's often desirable to have a color scheme in the app that complements a brand or have a unique color scheme for the app.

I know a lot of people don't like CSS, but I personally love it. It's a simple syntax to learn, but it has a lot of possibilities when it comes to more advanced scenarios. The initial support for CSS won't support some of the more advanced selectors, but I think they're not really needed for most scenarios anyways.

If you want all the technical details you can read up on the Github issue, or Adam Pedley's write up.

Happy coding!

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