F#: ViewModel (Dependency Injection)

 

Intro

In this post, I attempt to demonstrate how cool I am at writing proven code for a user’s registration form. Please note that I use the term proven and testable interchangeably.

 

The viewmodel is the root of the testable codebase for business logic.
As a result, I injected the function (aka: implementation) that handles the submit request for a registration form inside my viewmodel.

Here’s code that shows a workflow accepting the implementation of a submit request inside my viewmodel:

type ViewModel(implementation:Try.SubmitFn) as x =
...
    let submit() =
        let publishEvents events =
            events |> List.iter(fun event -> eventOccurred.Trigger(event))

        validatedForm |> function
                         | Some form ->
                                form |> Command.Execute
                                     |> In.SubmitRegistration.workflow implementation
                                     |> publishEvents
                         | None -> ()

Here’s the entire viewmodel:

namespace Nikeza.Mobile.UILogic.Registration

open System.Windows.Input
open Nikeza.Mobile.UILogic
open Nikeza.Mobile.Profile.Commands.Registration
open Nikeza.Mobile.Profile.Events
open Adapter
open In

type Form = Registration.Types.Form

module Updates =
    let statusOf formValidated events = 
        events |> List.exists formValidated

type ViewModel(implementation:Try.SubmitFn) as x =

    let mutable validatedForm = None

    let eventOccurred = new Event<_>()

    let validate() =
        let isValidated = function
            | FormValidated form -> validatedForm <- Some form; true
            | _ -> false

        { Form.Email=    x.Email
          Form.Password= x.Password
          Form.Confirm=  x.Confirm
        } 
          |> ofUnvalidated
          |> Validate.Execute 
          |> In.ValidateRegistration.workflow
          |> Updates.statusOf isValidated
               
    let submit() =
        let publishEvents events =
            events |> List.iter(fun event -> eventOccurred.Trigger(event))

        validatedForm |> function 
                         | Some form -> 
                                form |> Command.Execute 
                                     |> In.SubmitRegistration.workflow implementation
                                     |> publishEvents
                         | None -> ()

    let validateCommand = DelegateCommand( (fun _ -> x.IsValidated <- validate()) , fun _ -> true)

    let submitCommand =   DelegateCommand( (fun _ -> submit() |> ignore), 
                                            fun _ -> x.IsValidated <- validate(); x.IsValidated )

    let mutable email =    ""
    let mutable password = ""
    let mutable confirm =  ""
    let mutable isValidated = false

    member x.Validate = validateCommand :> ICommand
    member x.Submit =   submitCommand   :> ICommand

    [<CLIEvent>]
    member x.EventOccured = eventOccurred.Publish

    member x.Email
             with get() =      email 
             and  set(value) = email <- value

    member x.Password
             with get() =      password
             and  set(value) = password <- value

    member x.Confirm
        with get() =      confirm
        and  set(value) = confirm <- value

    member x.IsValidated
             with get() =      isValidated
             and  set(value) = isValidated <- value

Test

An automated test targeting business logic should provide its own implementation for submitting a registration request.

My test for submitting a registration form is as follows:

[<Test>]
let ``Registration submitted after being validated`` () =
    
    // Setup
    let mutable registrationSucceeded = false

    let registration = ViewModel(mockSubmit)

    registration.FillIn()
                .EventOccured
                .Add(fun event -> 
                         event |> function
                                  | RegistrationSucceeded _ -> registrationSucceeded <- true
                                  | RegistrationFailed    _ -> registrationSucceeded <- false)

    registration.Validate.Execute()

    // Test
    registration.Submit.Execute()

    // Verify
    registrationSucceeded |> should equal true

Test API

As I stated earlier, I try to be cool when I write code. Thus, to validate my coolness, I used a type extension to initialize the registration viewmodel.

Here’s my test API:

module TestAPI

open Nikeza.Mobile.UILogic.Registration
open Try

let someEmail =    "scott@abc.com"
let somePassword = "some_password"

let mockSubmit : SubmitFn =
    fun _ -> Ok { Id =        ""
                  FirstName = ""
                  LastName =  ""
                  Email =     ""
                  Bio =       ""
                  ImageUrl =  ""
                  Sources =   []
                }

type ViewModel with

    member x.FillIn () =
           x.Email    <- someEmail
           x.Password <- somePassword
           x.Confirm  <- somePassword
           x

Usage

The workflow below relies on a SubmitFn which was originally injected into my viewmodel.

The workflow that receives the implementation of this dependency is as follows:

module SubmitRegistration =
    type private SubmitWorkflow = SubmitFn -> Command -> RegistrationSubmissionEvent list

    let workflow : SubmitWorkflow =
        fun submitFn command -> command |> function
            Command.Execute form ->
                            form |> submitFn
                                 |> ResultOf.Submit.Executed
                                 |> Are.Registration.Submission.events