How to mvvmCross with F# that will increase your productivity

prerequisites

  • Understanding of C#, Xamarin, Mvvm/MvvmCross

F# supports Object-Orientated programming. MvvmCross is a framework for building apps with MvvM design pattern. This blog post walks through combing them using a PCL. A new project will be created. A new F# core PCL will then be added. The C# core will then be translated to F#. A small problem awaits….

Getting setup

Follow the IDE template and get an MvvmCross project setup with a C# PCl. We will change the core once this has been added. If you haven’t added a template you can add it to your IDE from Xamarin Studio or Visual Studio. Once the project is created and all the packages have been installed, run your app to double check everything installed correctly. I’ve named my app MvvmCrossViewModelDemo

Adding F# core

Next we will add an F# core to replace the C# Core. On the solution right click and add a Forms PCL, make sure to choose F# for the language drop down. For the name you can choose anything, this post will use ‘central’. Once the F# PCL has been added, do the following:
– remove windows 8.1 support from the pcl, ie select ‘.NET Portable Subset (.NET Framework 4.5, Windows 8)’
— This is showing as profile7 on my machine
– add the required packages via Nuget (or Paket if you know how)
— MvvmCross
— MvvmCross.Binding
— MvvmCross.Core
— MvvmCross.Platform
— Xamarin.Forms

Porting MvxApplication

Rename the file MyPage.fs to App.fs

The App.cs is the first block of code to port. It appears as follows:

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public class App : MvvmCross.Core.ViewModels.MvxApplication
{
  public override void Initialize()
  {
    CreatableTypes()
      .EndingWith("Service")
      .AsInterfaces()
      .RegisterAsLazySingleton();

    RegisterAppStart<ViewModels.FirstViewModel>();
  }
}

This is a direct port so the F# looks as follows:

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// F# port of MvvmCross MvxApplication
type App() = 
    inherit MvvmCross.Core.ViewModels.MvxApplication() 
    
    override this.Initialize() = 
        this.CreatableTypes()
            .EndingWith("Service")
            .AsInterfaces()
            .RegisterAsLazySingleton()
        this.RegisterAppStart<ViewModels.FirstViewModel>()

Update the App.fs file to be as follows:

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namespace MvvmCrossViewModelDemo.Central
open MvvmCross.Platform.IoC

type App() = 
    inherit MvvmCross.Core.ViewModels.MvxApplication() 

    override this.Initialize() = 
    
        this.CreatableTypes()
            .EndingWith("Service")
            .AsInterfaces()
            .RegisterAsLazySingleton()

        this.RegisterAppStart<ViewModels.FirstViewModel>()

Adding views

Simply add the views to the F# PCL project as you would with a C# project. Be sure to select Forms ContentPage Xaml
When added, copy the xaml from below (or the C# project) and paste into the xaml for the F# project. Don’t forget to update the two class references (x:Class) for each of the files at the top of the xaml. I’ve included just the FirstPage.xaml:

FirstPage.xaml

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<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:forms="using:Xamarin.Forms" x:Class="MvvmCrossViewModelDemo.Central.FirstPage" Title="First Page">
    <!-- Jul 22 2015 Xamarin have not yet provided Device.OnPlatform property for W81. The below syntax works by putting "5, 0, 5, 95" into the default-->
    <ContentPage.Padding Thickness="5, 0, 5, 95">
        <OnPlatform x:TypeArguments="Thickness" iOS="5, 20, 5, 0" Android="5, 0, 5, 0" WinPhone="5, 0, 5, 0" />
    </ContentPage.Padding>
    <ContentPage.ToolbarItems>
        <ToolbarItem Name="Menu1" Text="About" ClassId="About" Order="Primary" Command="{Binding ShowAboutPageCommand}">
            <ToolbarItem.Icon>
                <OnPlatform x:TypeArguments="FileImageSource" WinPhone="Toolkit.Content/ApplicationBar.Add.png" />
            </ToolbarItem.Icon>
        </ToolbarItem>
    </ContentPage.ToolbarItems>
    <StackLayout Spacing="10" Orientation="Vertical">
        <Label FontSize="24" Text="Enter your nickname in the box below" />
        <Entry Placeholder="Who are you?" TextColor="Red" Text="{Binding YourNickname}" />
        <Label FontSize="24" Text="{Binding Hello}" />
    </StackLayout>
</ContentPage>

Porting the view models

A full version of the App.fs with the view models is at the bottom of this blog if you want to skip to the end.
Let’s start with the AboutViewModel, paste the following code below in App.fs below open MvvmCross.Platform.IoC:

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open MvvmCross.Core.ViewModels

module ViewModels = 

    type AboutViewModel() =
        inherit MvxViewModel()

We need the import MvvmCross.Core.ViewModels in order to subclass MvxViewModel, just as we do in C#. (Note the project won’t compile until everything is complete)

Porting the FirstViewModel

The FirstViewModel requires a bit more work to translate, so is covered bit by bit. FirstViewModel should be placed below AboutViewModel in the ViewModels modules. F# is indentation sensitive so this means it should be indented two tabs.

Create the class with subclass:

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type FirstViewModel() =  
    inherit MvxViewModel()

Add the variable to hold the string with its associated property for YourNickName :

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let yourNickname = ref ""
member this.YourNickname 
    with get() = !yourNickname 
    and set(value) =
        if (this.SetProperty(yourNickname, value)) then 
            this.RaisePropertyChanged("Hello")
        else 
            ()

My first attempt at doing this, (and I’m sure most people learning F# would be the same), was to use let mutable yourNickname = "" to hold the mutable string. However it doesn’t work, (the code compiles but value/property is never updated). This is because of the implementation of SetProperty in MvvmCross.

The implementation of SetProperty is:

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protected bool SetProperty<T> (ref T storage, T value, [CallerMemberName] string propertyName = null)
{
    if (object.Equals (storage, value)) {
        return false;
    }
    storage = value;
    this.RaisePropertyChanged (propertyName);
    return true;
}

Notice that ref that is passed in. In F# if you take a ref of a mutable (ie ref yourNickname) you don’t get the same result as taking a ref in C#. In F# it creates a wrapper that holds a mutable value. This is not what we want as we won’t be able to compare it with the raw string; it wil always return false. For more details see this post Pass by reference. By using a ref value in the view model, this problem is avoided and all we need to remember is to dereference yourNickname when we want the string.

As a result of using the ref we now have this ! in the getter:

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with get() = !yourNickname

As stated earlier, we need to return a string and we’re using a reference type which is a wrapper type. The exclamation mark is a short hand to return the enclosed mutable field.

Next we can add the getter the for the string. Again we use ! to get the string value out:

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member this.Hello = sprintf "Hello %s" !yourNickname

Finally we can add in the command to navigate to the AboutViewModel:

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member private this.ShowAboutViewModel() = 
    this.ShowViewModel<AboutViewModel>()

member this.ShowAboutPageCommand 
    with get() = new MvxCommand(fun () -> this.ShowAboutViewModel() |> ignore)

You’ll notice that we had to create a private method to make the call to base method ShowViewModel. The reason for this is that MvxCommand command takes in a function, and calling ShowViewModel must be called from a subclass ie it’s not public method. The compiler gives an error if a function is passed in directly that calls ShowViewModel as it cannot prove that the call is made from a subclass ie it’s enforcing the method is not public. The private method to FirstViewModel proves that the call is made from a subclass, so everything now compiles.

That’s it for porting the C# to F#. Note that with F# we don’t need lots of files. The view models are declared above the app class, and F# compiler enforces that, making the code easy to navigate. Here is how the entire App.fs should look:

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namespace MvvmCrossViewModelDemo.Central

open MvvmCross.Platform.IoC
open MvvmCross.Core.ViewModels

module ViewModels = 

    type AboutViewModel() =
        inherit MvxViewModel()

    type FirstViewModel() =  
        inherit MvxViewModel()
    
        let yourNickname = ref ""

        member this.YourNickname 
            with get() = !yourNickname 
            and set(value) =
                if (this.SetProperty(yourNickname, value)) then 
                    this.RaisePropertyChanged("Hello")
                else 
                    ()

        member this.Hello = sprintf "Hello %s" !yourNickname

        member private this.ShowAboutViewModel() = 
            this.ShowViewModel<AboutViewModel>()

        member this.ShowAboutPageCommand 
            with get() = new MvxCommand(fun () -> this.ShowAboutViewModel() |> ignore)
        
type App() = 
    inherit MvvmCross.Core.ViewModels.MvxApplication() 

    override this.Initialize() = 
    
        this.CreatableTypes()
            .EndingWith("Service")
            .AsInterfaces()
            .RegisterAsLazySingleton()

        this.RegisterAppStart<ViewModels.FirstViewModel>()

The last wiring effort

For both the iOS and droid project, remove the project reference to the C# core and change it to the F# core (named Central in the post). Alternativly you could delete the C# core if you want to be sure you are using F#. Compile these projects, and you will get a compile error in the Setup.cs files. Change Core to Central in Setup.cs and everything should compile, ie:

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return new Central.App();

Wrapping up

This post has shown how to create an F# core with mvvmCross. Remember the following:
– use ref for your variables if you need to call SetProperty, or any other method that takes in a ref value
– use private methods if need to pass a function that invokes a protected method on a base class

As always, leave a comment if you found this helpful or know a better way!

Multiple items
val ref : value:'T -> 'T ref

Full name: Microsoft.FSharp.Core.Operators.ref

——————–
type 'T ref = Ref<'T>

Full name: Microsoft.FSharp.Core.ref<_>

val set : elements:seq<'T> -> Set<'T> (requires comparison)

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.set

val sprintf : format:Printf.StringFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.sprintf

val ignore : value:'T -> unit

Full name: Microsoft.FSharp.Core.Operators.ignore