Intro
This post is the third post within my series of posts regarding the construction of a trading application using Xamarin.Forms and F#. In this post, I share my source code for selling shares of a stock.
User Interface
The following reflects the user-experience for selling a stock:
The following XAML reflects the above UI:
<?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:manageTrades="clr-namespace:ManageTrades.ViewModels;assembly=ManageTrades" x:Class="FnTrade.SellPage">
<Grid Margin="10,5">
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
<RowDefinition Height="auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Label Grid.Row="0" Grid.Column="0" Text="{Binding Symbol, StringFormat='Selling ({0:F0})'}}" FontSize="Large" />
<Label Grid.Row="2" Grid.Column="0" Text="{Binding Shares, StringFormat='{0:F0} shares available'}}" FontSize="Medium" HorizontalTextAlignment="Start" HorizontalOptions="Start" />
<Label Grid.Row="3" Grid.Column="0" Text="{Binding StockPrice, StringFormat='{}{0:c} per share'}}" FontSize="Medium" HorizontalTextAlignment="Start" HorizontalOptions="Start" />
<Label Grid.Row="4" Grid.Column="1" Text="{Binding Total, StringFormat='{}{0:c} Invested'}}" FontSize="Large" HorizontalOptions="Start" />
<Entry Grid.Row="6" Grid.Column="0" Grid.ColumnSpan="2" Text="{Binding SellQty, Mode=TwoWay}" Placeholder="Enter number of shares to sell" HorizontalTextAlignment="Center" Keyboard="Numeric" />
<Label Grid.Row="7" Grid.Column="1" Text="{Binding SellValue, StringFormat='{}{0:c}'}" HorizontalTextAlignment="End" />
<Button Grid.Row="9" Grid.Column="0" Grid.ColumnSpan="2" Text="Confirm" Command="{Binding Confirm}" IsEnabled="{Binding CanSell}" />
</Grid>
</ContentPage>
Here’s some more UI in regards to the user story of selling shares:
ViewModels
The HomeViewmodel was implemented as follows:
namespace Home.ViewModels
open System.Windows.Input
open Core.IntegrationLogic
open Core.Entities
open Integration.Factories
open Services
open Search
open System.Collections.ObjectModel
type HomeViewModel() as this =
inherit ViewModelBase()
let dispatcher = getDispatcher()
let accountId = getAccountId()
let broker = getBroker() :> IBroker
let searchCommand =
DelegateCommand( (fun _ -> this.StockInfo <- getQuote broker this.Symbol) , fun _ -> true) :> ICommand
let sellCommand =
DelegateCommand( (fun o -> dispatcher.Sell (o :?> SharesInfo) ) ,
fun _ -> true) :> ICommand
let buyCommand =
DelegateCommand( (fun o -> dispatcher.Buy { Owner.AccountId=accountId ; Symbol=o :?> string} ) ,
fun _ -> true) :> ICommand
let mutable symbol = ""
let mutable stockInfo = None
let mutable investments = ObservableCollection<SharesInfo>()
member this.Load() =
let result = broker.InvestmentsOf accountId
this.Investments <- ObservableCollection<SharesInfo>(result)
member this.Symbol
with get() = symbol
and set(value) = symbol <- value
base.NotifyPropertyChanged(<@ this.Symbol @>)
member this.StockInfo
with get() = stockInfo
and set(value) = stockInfo <- value
base.NotifyPropertyChanged(<@ this.StockInfo @>)
member this.Investments
with get() = investments
and set(value) = investments <- value
base.NotifyPropertyChanged(<@ this.Investments @>)
member this.Search = searchCommand
member this.Sell = sellCommand
member this.Buy = buyCommand
The SellViewModel was implemented as follows:
namespace ManageTrades.ViewModels
open System
open System.Windows.Input
open Core.IntegrationLogic
open Core.Entities
open Integration.Factories
open Services
type SellViewModel(info:SharesInfo) as this =
inherit ViewModelBase()
let dispatcher = getDispatcher()
let accountId = getAccountId()
let broker = getBroker() :> IBroker
let mutable sellQty = ""
let mutable canSell = false
let mutable sellValue = 0m
let confirm = DelegateCommand( (fun _ -> this.ConfirmSell()) ,
fun _ -> true ) :> ICommand
member this.Symbol with get() = info.Shares.Symbol
member this.Shares with get() = info.Shares.Qty
member this.StockPrice with get() = info.PricePerShare
member this.Total with get() = ((decimal)info.Shares.Qty * info.PricePerShare)
member this.SellQty
with get() = sellQty
and set(value) = sellQty <- value
let success , validQty = Int32.TryParse sellQty
if success then this.CanSell <- validQty > 0 &&
validQty <= this.Shares
else this.CanSell <- false
this.UpdateSellValue()
base.NotifyPropertyChanged(<@ this.SellQty @>)
member this.SellValue
with get() = sellValue
and set(value) = sellValue <- value
base.NotifyPropertyChanged(<@ this.SellValue @>)
member this.CanSell with get() = canSell
and set(value) = canSell <- value
base.NotifyPropertyChanged(<@ this.CanSell @>)
member this.Confirm = confirm
member private this.ConfirmSell() =
if this.CanSell then
dispatcher.ConfirmSell { AccountId = accountId
Symbol = this.Symbol
Quantity = Int32.Parse this.SellQty }
member private this.UpdateSellValue() =
let success , validQty = Int32.TryParse sellQty
if success
then this.SellValue <- (decimal)validQty * info.PricePerShare
else this.SellValue <- 0m
The RequestSellConfirmedViewModel is as follows:
namespace ManageTrades.ViewModels
open Core.IntegrationLogic
open Core.Entities
open Services
open Integration.Factories
type RequestSellConfirmedViewModel(request:RequestInfo) =
inherit ViewModelBase()
let broker = getBroker() :> IBroker
let mutable total = 0m
member this.Symbol with get() = request.Symbol
member this.Quantity with get() = request.Quantity
member this.Total with get() = total
and set(value) = total <- value
base.NotifyPropertyChanged(<@ this.Total @>)
member this.Load() =
match broker.GetInfo request.Symbol with
| Some info -> this.Total <- info.Price * (decimal) request.Quantity
| None -> failwith (sprintf "Failed to retrieve stock information for %s" this.Symbol)
Application Logic
My app subscribes to events so that it can manage page navigation:
public partial class App : Application
{
public App()
{
InitializeComponent();
MainPage = new NavigationPage(new HomePage());
}
protected override void OnStart()
{
getDispatcher().SellRequested +=
async (s, e) => await MainPage.Navigation.PushAsync(
new SellPage(new SellViewModel(e as SharesInfo)));
getDispatcher().ExecuteSellRequested +=
async (s, e) => await MainPage.Navigation.PushAsync(
new RequestSellConfirmedPage(new RequestSellConfirmedViewModel(e as RequestInfo)));
}
protected override void OnSleep() { }
protected override void OnResume() { }
}
}
The following are events that are declared within my Dispatcher class:
namespace Services
open System.Diagnostics
open System
type Dispatcher() =
let sellRequested = new Event<EventHandler<_>,_>()
let buyRequested = new Event<EventHandler<_>,_>()
let confirmSellRequested = new Event<EventHandler<_>,_>()
let executeSellRequested = new Event<EventHandler<_>,_>()
[<CLIEvent>]
member this.SellRequested = sellRequested.Publish
member this.Sell owner = sellRequested.Trigger(this , owner)
[<CLIEvent>]
member this.BuyRequested = buyRequested.Publish
member this.Buy owner = buyRequested.Trigger(this , owner)
[<CLIEvent>]
member this.ConfirmSellRequested = confirmSellRequested.Publish
member this.ConfirmSell info = confirmSellRequested.Trigger(this , info)
[<CLIEvent>]
member this.ExecuteSellRequested = executeSellRequested.Publish
member this.ExecuteSell info = executeSellRequested.Trigger(this , info)
Domain Types
The following are my domain types:
module Core.Entities
type StockInfo = {
Symbol : string
Price : decimal
DayLow : decimal
DayHigh : decimal
}
type Shares = {
AccountId : string
Symbol : string
Qty : int
}
type SharesInfo ={
Shares : Shares
PricePerShare : decimal
Total : decimal
}
type Owner = { AccountId:string ; Symbol:string }
type RequestInfo = {
AccountId : string
Symbol : string
Quantity : int
}
type InsufficientFunds = {
PurchaseAttempt : RequestInfo
Balance : decimal
StockPrice : decimal
}
type PurchaseResult =
| PurchaseRequested of RequestInfo
| UnknownSymbol of RequestInfo
| InvalidQuantity of RequestInfo
| InsufficientFunds of InsufficientFunds
type SellResult =
| SellRequested of RequestInfo
| InsufficientQuantity of RequestInfo
Conclusion
The source code to my implementation can be found on GitHub.
Scott Nimrod's Blog 