Getting started


You will need to install a few different tools to work with Fabulous:

  • .NET 6.0 SDK or newer (link)
  • A compatible IDE:
    • Visual Studio 2021 or newer (link)
    • Visual Studio 2019 for Mac or newer (link)
    • JetBrains Rider (link - available for Windows and macOS)
  • During installation, you will be asked to choose the workloads you want. Be sure to select the mobile development workload and F#.

Note: if you choose JetBrains Rider, you will also need to install either Visual Studio or Visual Studio Mac since Rider won’t install those workloads by itself.

Creating a new Fabulous project

1. Download the latest templates

First, we need to install the latest templates for Fabulous. This is done via command line.
Open a terminal in the folder where you want to store the new solution, and type the following command:

dotnet new -i Fabulous.XamarinForms.Templates

This will add Fabulous.XamarinForms to the templates available for dotnet new.

dotnet new --list

Result of dotnet new –list

2. Create the project

Now, we can create the solution using the newly installed template:

dotnet new fabulous-xf -n FabHelloWorld

Note: if you plan to use Visual Studio on Windows, use fabulous-xf-vswin instead. Visual Studio doesn’t support the default template.

3. Build and run

In your IDE, you will be able to choose the configuration profile, the project to start and the device or simulator to use.

Debugging on Android device or emulator

For debugging Android, select Android project and the profile Debug | Any CPU, then select any device or emulator you want.

Selecting Android for debugging

Debugging on a iPhone device

For debugging iOS on a real iPhone device, select the iOS project and the profile Debug | iPhone, then select any connected device you want.

Selecting iPhone for debugging

Debugging on an iPhone simulator

For debugging iOS on a simulator, select the iOS project and the profile Debug | iPhoneSimulator, then select any simulator you want.

Selecting iPhoneSimulator for debugging

You can start debugging like usual and the app will launch.

Interacting with the Hello World application

Understanding the project structure

Fabulous implements the MVU design pattern. MVU stands for Model-View-Update.

Diagram explaining the MVU cycle
Credits: Beginnnig Elm -

MVU is a simple state machine for making reliable UI applications.

We have a Model storing the state of the application.
This state is immutable, it can only be mutated during the evaluation of the update function.
This is done to help prevent side effects from making your app unstable.

To trigger an update, we have Msg (messages) that are dispatched from the user interface or from internal subscriptions (like a timer).
Upon dispatch, Fabulous will call the update function to know how the state will change.

Everytime the state changes, Fabulous will also call the view function.
This function returns a virtual view (much like Virtual Dom if you’re used to React) that Fabulous will compare with the current UI to determine which changes to make. Thus we can understand the UI as a function of the state, like shown in this image:

UI equals function of state
Credits: Flutter docs - Start thinking declaratively

In the blank template, you will find everything inside the App.fs file of the FabHelloWorld shared project.

The Model type defines the application state. You can store any value you need for the lifetime of the application inside it.

type Model =
    { Count: int }

The Msg discriminated union lets you define all events that will change the application state.

type Msg =
    | Increment
    | Decrement

init is a function called at the start of the application to get the default application state.

let init () =
    { Count = 0 }

update is a function called whenever a Msg is received. Inside you can update the current Model to change the application state.

let update msg model =
    match msg with
    | Increment -> { model with Count = model.Count + 1 }
    | Decrement -> { model with Count = model.Count - 1 }

view is a function returning a view description for the current Model.
You can notice that controls that can be interacted with can dispatch a Msg, e.g. Button("Increment", Increment).

let view model =
            VStack() {
                Label("Hello from Fabulous v2!")
                    .font(namedSize = NamedSize.Title)

                (VStack() {
                    Label($"Count is {model.Count}")

                    Button("Increment", Increment)
                    Button("Decrement", Decrement)
                    .centerVertical(expand = true)

Program.statefulApplication bootstraps your Fabulous application by linking together the 3 functions init, update and view.

let program =
    Program.statefulApplication init update view