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Static UWP views for elmish programs running with the Uno Platform

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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 a ViewModel class again!

Elevator pitch

Elmish.Uno 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.Uno is:

  • Simple-to-understand, unidirectional data flow
  • Single source of truth for all the state in your app
  • Simple async/IO
  • Immutable data
  • Pure functions
  • Great testability
  • Simple optimization
  • 78% more rockets 🚀

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

Big thanks to @MrMattSim for the wonderful logo!

Recommended resources

Getting started with Elmish.Uno

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.Uno.

  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.Uno.

    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.Uno
    
    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.

  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.mkSimpleWpf init update bindings
      |> 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:

  • The Elmish.Uno tutorial provides information on general MVU/Elmish concepts and how they apply to Elmish.Uno, as well as the various Elmish.Uno bindings.
  • The samples are complete, working mini-apps demonstrating selected aspects of Elmish.Uno.
  • If you'd like to contribute, please read and follow the Contributor guidelines.

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.Uno’s static-view-based solution has several unique benefits:

  • You can use your existing XAML and MVVM knowledge (that is, the best part of MVVM – the UI bindings – without having to deal with NavigationServices, ViewModelLocators, state synchronization, INotifyPropertyChanged, etc.)
  • Huge mindshare – there are tons of relevant XAML and MVVM resources on the net which can help with the UI and data binding part if you get stuck
  • Automatic support for all 3rd party WPF UI libraries like MaterialDesignInXamlToolkit, since it just uses XAML and bindings (support for 3rd party libraries is commonly a major pain point for dynamic UI solutions)
  • You can use the XAML designer (including design-time data binding)
  • Automatically puts all the power of WPF at your fingertips, whereas dynamic UI solutions have inherent limitations that are not easy to work around

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:

  • A core library containing the model definitions and update functions.
    • This library can include a reference to Elmish (e.g. for the Cmd module helpers), but not to Elmish.WPF, which depends on WPF and has a UI-centred API (specifying bindings). This will ensure your core logic (such as the update function) is free from any UI concerns, and allow you to re-use the core library should you want to port your app to another Elmish-based solution (e.g. Fable.React).
  • An Elmish.WPF project that contains the bindings (or view) function and the call to Program.runElmishLoop.
    • This project would reference the core library and Elmish.WPF.

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:

  • Create a CmdMsg union type with cases for each command you want to execute in the app.
  • Make init and update return model * CmdMsg list instead of model * Cmd<Msg>. Since init and update now return data, they are much easier to test.
  • Create a trivial/too-boring-to-test cmdMsgToCmd function that transforms a CmdMsg to the corresponding Cmd.
  • Finally, create “normal” versions of init and update that you can use when creating Program. Elmish.WPF provides Program.mkProgramWpfWithCmdMsg that does this for you (but there’s no magic going on – it’s really easy to do yourself).

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.

.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.

How can I use Save File / Open File dialogs?

There’s a few things to remember regarding opening on the UI thread and not blocking the Elmish dispatch loop. Check out the FileDialogs sample. In short, write a function like below, and call it using Cmd.OfAsync.

let save text =
  Application.Current.Dispatcher.Invoke(fun () ->
    let guiCtx = SynchronizationContext.Current
    async {
      do! Async.SwitchToContext guiCtx
      let dlg = Microsoft.Win32.SaveFileDialog ()
      // configure dialog (extensions etc.), show it, handle result
    }
  )

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.Uwp.Managed.

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