Fabulous for Xamarin.Forms



A single page app typically returns a ContentPage. For example:

let view model dispatch =
        title = "Pocket Piggy Bank",
        content = View.Label(text = sprintf "Hello world!")

For other kinds of pages, see Multi-page Applications and Navigation

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Xamarin.Forms has several layouts and features for organizing content on screen. For a comprehensive guide see the Xamarin Guide to Layouts

Xamarin.Forms Layouts


StackLayout organizes views in a one-dimensional line (“stack”), either horizontally or vertically. Views in a StackLayout can be sized based on the space in the layout using layout options. Positioning is determined by the order views were added to the layout and the layout options of the views.

    children = [
        View.Label(text = sprintf "Welcome to the bank!")
        View.Label(text = sprintf "Balance: %s%.2f" model.CurrencySymbol model.Balance)

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AbsoluteLayout positions and sizes child elements proportional to its own size and position or by absolute values. Child views may be positioned and sized using proportional values or static values, and proportional and static values can be mixed.

    backgroundColor = Color.Blue.WithLuminosity(0.9),
    children = [
       View.Label(text = "Top Left", textColor = Color.Black)
           .LayoutBounds(Rectangle(0.0, 0.0, AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize))
       View.Label(text = "Centered", textColor = Color.Black)
           .LayoutBounds(Rectangle(0.5, 0.5, AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize))
       View.Label(text = "Bottom Right", textColor = Color.Black)
           .LayoutBounds(Rectangle(1.0, 1.0, AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize))

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RelativeLayout is used to position and size views relative to properties of the layout or sibling views. Unlike AbsoluteLayout, RelativeLayout does not have the concept of the moving anchor and does not have facilities for positioning elements relative to the bottom or right edges of the layout. RelativeLayout does support positioning elements outside of its own bounds.

An example RelativeLayout is as follows:

    children =
        [ View.Label(text = "RelativeLayout Example", textColor = Color.Red)
              .XConstraint(Constraint.RelativeToParent(fun parent -> 0.0))
          View.Label(text = "Positioned relative to my parent", textColor = Color.Red)
              .XConstraint(Constraint.RelativeToParent(fun parent -> parent.Width / 3.0))
              .YConstraint(Constraint.RelativeToParent(fun parent -> parent.Height / 2.0))

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FlexLayout is similar to the Xamarin.Forms StackLayout in that it can arrange its children horizontally and vertically in a stack. However, the FlexLayout is also capable of wrapping its children if there are too many to fit in a single row or column, and also has many options for orientation, alignment, and adapting to various screen sizes.

    children = [
        View.Label(text = "Seated Monkey", fontSize="Large", textColor=Color.Blue)
        View.Label(text = "This monkey is laid back and relaxed.")
        View.Label(text = "  - Often smiles mysteriously")
        View.Label(text = "  - Sleeps sitting up")

        View.Image(widthRequest = 160.0, heightRequest = 240.0,
            source = "images/160px-Vervet_monkey_Krugersdorp_game_reserve_%285657678441%29.jpg"

        View.Label(margin = Thickness(0.0, 4.0)).FlexGrow(1.0)
        View.Button(text = "Learn More", fontSize = "Large", cornerRadius = 20)

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Grid supports arranging views into rows and columns. Rows and columns can be set to have proportional sizes or absolute sizes. The Grid layout should not be confused with traditional tables and is not intended to present tabular data. Grid does not have the concept of row, column or cell formatting. Unlike HTML tables, Grid is purely intended for laying out content.

An example Grid is as follows:

    rowdefs = [for i in 1 .. 6 -> box "auto"],
    coldefs = [for i in 1 .. 6 -> box "auto"],
    children = [
        for i in 1 .. 6 do
            for j in 1 .. 6 ->
                let color = Color((1.0/float i), (1.0/float j), (1.0/float (i+j)), 1.0)


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ScrollView contains layouts and enables them to scroll offscreen. ScrollView is also used to allow views to automatically move to the visible portion of the screen when the keyboard is showing.

View.ScrollView(View.StackLayout(padding=20.0, children= ...) )

The scroll position can be setted programmatically through the attribute scrollTo. This attribute needs the X and Y coordinates to scroll to and an indication whether it should be animated or not. (Animated/NotAnimated)

Note: Fabulous will try to scroll to these coordinates every time it needs to refresh the UI. Making use of the optional argument is recommended.

You can also subscribe to the event Scrolled to be notified when the scrolling is over.

    ?scrollTo=(if model.ShouldScroll then Some (500.0, 0.0, Animated) else None),
    scrolled=(fun args -> dispatch Scrolled))

For more complex scenarios, you can directly use the method from Xamarin.Forms ScrollView.ScrollToAsync(x, y, animated) This method offers the advantage of being awaitable until the end of the scrolling. To do this, a reference to the underlying ScrollView is needed.

let scrollViewRef = ViewRef<ScrollView>()

View.ScrollView(ref=scrollViewRef, content=(...))

// Some time later (usually in a Cmd)
let scrollToCoordinates x y animated =
    async {
        match scrollViewRef.TryValue with
        | None ->
            return None
        | Some scrollView ->
            do! scrollView.ScrollToAsync(x, y, animated) |> Async.AwaitTask
            return (Some Scrolled)
    } |> Cmd.ofAsyncMsgOption

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CarouselView, Shell

CarouselView and Shell are available in Xamarin.Forms 4.0.


View.Shell(title = "TitleShell",
           items = [
                   items = [
                       View.ShellSection(items = [
                           View.ShellContent(title = "Section 1", content = View.ContentPage(content = View.Button(text = "Button")))         

View.CarouselView(items = [

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