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WPF done the Elmish Way

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The good parts of MVVM (the data bindings) with the simplicity and robustness of an MVU architecture for the rest of your app. Never write an overly-complex ViewModel class again!

Elevator pitch

Elmish.WPF is a production-ready library that allows you to write WPF apps with the robust, simple, well-known, and battle-tested MVU architecture, while still allowing you to use all your XAML knowledge and tooling to create UIs.

Some benefits of MVU you’ll get with Elmish.WPF include:

Even with static views, your central model/update code can follow an idiomatic Elmish/MVU architecture. You could, if you wanted, use the same model/update code to implement an app using a dynamic UI library such as Fabulous or Fable.React, by just rewriting the “U” part of MVU.

Static XAML views is a feature, not a limitation. See the FAQ for several unique benefits to this approach!

Elmish.WPF uses Elmish, an F# implementation of the MVU message loop.

Big thanks to @MrMattSim for the wonderful logo!

Sponsor

JetBrains logo

Thanks to JetBrains for sponsoring Elmish.WPF with OSS licenses!

Recommended resources

Getting started with Elmish.WPF

See the SingleCounter sample for a very simple app. The central points are (assuming up-to-date VS2019):

  1. Create an F# Class Library. If targeting .NET 5 or .NET Core, the project file should look like this:

    <Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net5.0-windows</TargetFramework>  <!-- Or another target framework -->
    <UseWpf>true</UseWpf>
  </PropertyGroup>

  <!-- other stuff -->

    If targeting .NET Framework (4.6.1 or later), replace the first line with

    <Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">

  2. Add NuGet reference to package Elmish.WPF.

  3. Define the model that describes your app’s state and a function that initializes it:

    type Model =
  { Count: int
    StepSize: int }

let init () =
  { Count = 0
    StepSize = 1 }

  4. Define the various messages that can change your model:

    type Msg =
  | Increment
  | Decrement
  | SetStepSize of int

  5. Define an update function that takes a message and a model and returns an updated model:

    let update msg m =
  match msg with
  | Increment -> { m with Count = m.Count + m.StepSize }
  | Decrement -> { m with Count = m.Count - m.StepSize }
  | SetStepSize x -> { m with StepSize = x }

  6. Define the “view” function using the Bindings module. This is the central public API of Elmish.WPF.

    Normally in Elm/Elmish this function is called view and would take a model and a dispatch function (to dispatch new messages to the update loop) and return the UI (e.g. a HTML DOM to be rendered), but in Elmish.WPF this function is in general only run once and simply sets up bindings that XAML-defined views can use. Therefore, let’s call it bindings instead of view.

    open Elmish.WPF

let bindings () =
  [
    "CounterValue" |> Binding.oneWay (fun m -> m.Count)
    "Increment" |> Binding.cmd (fun m -> Increment)
    "Decrement" |> Binding.cmd (fun m -> Decrement)
    "StepSize" |> Binding.twoWay(
      (fun m -> float m.StepSize),
      (fun newVal m -> int newVal |> SetStepSize))
  ]

    The strings identify the binding names to be used in the XAML views. The Binding module has many functions to create various types of bindings.

    Alternatively, use statically-typed view models in order to get better IDE support in the XAML.

    open Elmish.WPF

type CounterViewModel(args) =
  inherit ViewModelBase<Model, Msg>(args)

  member _.CounterValue = base.Get() (Binding.OneWayT.id >> Binding.mapModel (fun m -> m.Count))
  member _.Increment = base.Get() (Binding.CmdT.setAlways Counter.Increment)
  member _.Decrement = base.Get() (Binding.CmdT.setAlways Counter.Decrement)
  member _.StepSize
    with get() = base.Get() (Binding.OneWayT.id >> Binding.mapModel (fun m -> m.StepSize))
    and set(v) = base.Set(v) (Binding.OneWayToSourceT.id >> Binding.mapMsg Counter.Msg.SetStepSize)

  7. Create a function that accepts the app’s main window (to be created) and configures and starts the Elmish loop for the window with your init, update and bindings:

    open Elmish.WPF

let main window =
  Program.mkSimple init update bindings
  |> Program.runElmishLoop window

    Alternatively, use a statically-typed view model at the top level.

    open Elmish.WPF

let main window =
  Program.mkSimpleT init update CounterViewModel
  |> Program.runElmishLoop window

    In the code above, Program.runElmishLoop will set the window’s DataContext to the specified bindings and start the Elmish dispatch loop for the window.

  8. Create a WPF app project (using the Visual Studio template called WPF App (.NET)). This will be your entry point and contain the XAML views. Add a reference to the F# project, and make the following changes in the csproj file:

    • Currently, the core Elmish logs are only output to the console. If you want a console window for displaying Elmish logs, change <OutputType>WinExe</OutputType> to <OutputType>Exe</OutputType> and add <DisableWinExeOutputInference>true</DisableWinExeOutputInference>.
    • If the project file starts with the now legacy <Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">, change it to <Project Sdk="Microsoft.NET.Sdk">
    • Change the target framework to match the one used in the F# project (e.g. net5.0-windows).

    Make the following changes to App.xaml.cs to initialize Elmish when the app starts:

    public partial class App : Application
{
  public App()
  {
    this.Activated += StartElmish;
  }

  private void StartElmish(object sender, EventArgs e)
  {
    this.Activated -= StartElmish;
    Program.main(MainWindow);
  }

}

  9. Define your views and bindings in XAML:

    <Window
    x:Class="MyNamespace.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <StackPanel Orientation="Horizontal">
    <TextBlock Text="{Binding CounterValue}" />
    <Button Command="{Binding Decrement}" Content="-" />
    <Button Command="{Binding Increment}" Content="+" />
    <TextBlock Text="{Binding StepSize}" />
    <Slider Value="{Binding StepSize}" TickFrequency="1" Minimum="1" Maximum="10" />
  </StackPanel>
</Window>

  10. Profit! :)

Further resources:

FAQ

Static views in MVU? Isn’t that just a half-baked solution that only exists due to a lack of better alternatives?

Not at all! 🙂

It’s true that static views aren’t as composable as dynamic views. It’s also true that at the time of writing, there are no solid, production-ready dynamic UI libraries for WPF (though there are no lack of half-finished attempts or proof-of-concepts: Elmish.WPF.Dynamic, Fabulous.WPF, Skylight, Uil). Heck, it’s even true that Elmish.WPF was originally created with static views due to the difficulty of creating a dynamic UI library, as described in issue #1.

However, Elmish.WPF’s static-view-based solution has several unique benefits:

In short, for WPF apps, a solution based on static XAML views is currently the way to go.

Do I have to use the project structure outlined above?

Not at all. The above example, as well as the samples, keep all non-UI code in a single project for simplicity, and all the XAML in a C# project for better tooling.

An alternative with a clearer separation of UI and core logic can be implemented by splitting the F# project into two projects:

Another alternative is to turn the sample code on its head and have the F# project be a console app containing your entry point (with a call to Program.runWindow) and referencing the C#/XAML project (instead of the other way around, as demonstrated above).

In general, you have a large amount of freedom in how you structure your solution and what kind of entry point you use.

How can I test commands? What is the CmdMsg pattern?

Since the commands (Cmd<Msg>) returned by init and update are lists of functions, they are not particularly testable. A general pattern to get around this is to replace the commands with pure data that are transformed to the actual commands elsewhere:

The FileDialogsCmdMsg sample demonstrates this approach. For more information, see the Fabulous documentation. For reference, here is the discussion that led to this pattern.

Can I use design-time view models?

Yes. Assuming you have a C# XAML and entry point project referencing the F# project, simply use ViewModel.designInstance (e.g. in the F# project) to create a view model instance that your XAML can use at design-time:

module MyAssembly.DesignViewModels
let myVm = ViewModel.designInstance myModel myBindings

Then use the following attributes wherever you need a design-time VM:

<Window
    ...
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:vm="clr-namespace:MyAssembly;assembly=MyAssembly"
    mc:Ignorable="d"
    d:DataContext="{x:Static vm:DesignViewModels.myVm}">

When targeting legacy .NET Framework, “Project code” must be enabled in the XAML designer for this to work.

If you are using static view models, make sure that the View Model type is in a namespace and add a default constructor that passes a model into ViewModelArgs.simple:

namespace ViewModels

type [<AllowNullLiteral>] AppViewModel (args) =
  inherit ViewModelBase<AppModel, AppMsg>(args)
  
  new() = AppViewModel(App.init () |> ViewModelArgs.simple)

Then use the following attributes just like you would in a normal C# MVVM project:

<Window
    ...
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:vm="clr-namespace:ViewModels.SubModelStatic;assembly=MyAssembly"
    mc:Ignorable="d"
    d:DataContext="{d:DesignInstance Type=vm:AppViewModel, IsDesignTimeCreatable=True}">
.NET Core 3 workaround

When targeting .NET Core 3, a bug in the XAML designer causes design-time data to not be displayed through DataContext bindings. See this issue for details. One workaround is to add a d:DataContext binding alongside your normal DataContext binding. Another workaround is to change

<local:MyControl DataContext="{Binding Child}" />

to

<local:MyControl
  DataContext="{Binding Child}"
  d:DataContext="{Binding DataContext.Child,
                          RelativeSource={RelativeSource AncestorType=T}}" />

where T is the type of the parent object that contains local:MyControl (or a more distant ancestor, though there are issues with using Window as the type).

Can I open new windows/dialogs?

Sure! Just use Binding.subModelWin. It works like Binding.subModel, but has a WindowState wrapper around the returned model to control whether the window is closed, hidden, or visible. You can use both modal and non-modal windows/dialogs, and everything is a part of the Elmish core loop. Check out the NewWindow sample.

Note that if you use App.xaml startup, you may want to set ShutdownMode="OnMainWindowClose" in App.xaml if that’s the desired behavior.

Can I bind to events and use behaviors?

Sure! Check out the EventBindingsAndBehaviors sample. Note that you have to install the NuGet package Microsoft.Xaml.Behaviors.Wpf.

How can I control logging?

Elmish.WPF uses Microsoft.Extensions.Logging. To see Elmish.WPF output in your favorite logging framework, use WpfProgram.withLogger to pass an ILoggerFactory:

WpfProgram.mkSimple init update bindings
|> WpfProgram.withLogger yourLoggerFactory
|> WpfProgram.runWindow window

For example, in Serilog, you need to install Serilog.Extensions.Logging and instantiate SerilogLoggerFactory. The samples demonstrate this.

Elmish.WPF logs to these categories:

The specific method of controlling what Elmish.WPF logs depends on your logging framework. For Serilog you can use .MinimumLevel.Override(...) to specify the minimum log level per category, like this:

myLoggerConfiguration
  .MinimumLevel.Override("Elmish.WPF.Bindings", LogEventLevel.Verbose)
  ...