Tip Calc The Core Project

MvvmCross application's are normally structured with:

one shared 'core' Portable Class Library (PCL) project
- containing as much code as possible: models, view models, services, converters, etc
one UI project per platform
- each containing the bootstrap and view-specific code for that platform

Normally, you start development from the core project - and that's exactly what we'll do here.

To create the core, you can use the Visual Studio project template wizards, but here we'll instead build up a new project 'from empty'.

Create the new Portable Class Library

Using Visual Studio, create your new PCL using the File|New Project wizard.

Call it something like TipCalc.Core.csproj

When asked to choose platforms, select .NET Framework 4.5, Windows 8, Windows Phone Silverlight 8, Windows Phone 8.1, Xamarin.Android and Xamarin.iOS - this will ensure that the PCL is in Profile259. If Visual Studio stops you selecting these targets with the error 'The selection does not match any portable APIs' then use the workaround described here: http://danrigby.com/2014/04/10/windowsphone81-pcl-xamarin-fix/

Profile259 defines a small subset of .Net including:

Microsoft.CSharp
mscorelib
System.Collections
System.ComponentModel
System.Core
System.Diagnostics
System
System.Globalization
System.IO
System.Linq
System.Net
System.ObjectModel
System.Reflection
System.Resources.ResourceManager
System.Runtime
System.Security.Principal
System.ServiceModel.Web
System.Text.Encoding
System.Text.RegularExpressions
System.Threading
System.Windows
System.Xml

To see the full list of assemblies, look in C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETPortable\v4.5\Profile\Profile259

Importantly for us this Profile259 includes everything we need to build our Mvvm applications.

Delete Class1.cs

No-one really needs a Class1 :)

Install MvvmCross

In the Package Manager Console, enter...

Install-Package MvvmCross.HotTuna.MvvmCrossLibraries

Add the Tip Calculation service

Create a folder called 'Services'

Within this folder create a new Interface which will be used for calculating tips:

namespace TipCalc.Core.Services
{
	public interface ICalculation
	{
		double TipAmount(double subTotal, int generosity);
	}
}

Within this folder create an implementation of this interface:

namespace TipCalc.Core.Services
{
	public class Calculation : ICalculation
	{
		public double TipAmount(double subTotal, int generosity)
		{
			return subTotal * ((double)generosity)/100.0;
		}
	}
}

This provides us with some simple business logic for our app

Add the ViewModel

At a sketch level, we want a user interface that:

uses:
- our calculation service to calculate the tip
has inputs of:
- the current bill (the subTotal)
- a feeling for how much tip we'd like to leave (the generosity)
has output displays of:
- the calculated tip to leave

To represent this user interface we need to build a 'model' for the user interface - which is, of course, a 'ViewModel'

Within MvvmCross, all ViewModels should inherit from MvxViewModel.

So now create a ViewModels folder in our project, and in this folder add a new TipViewModel class like:

using Cirrious.MvvmCross.ViewModels;
using TipCalc.Core.Services;

namespace TipCalc.Core.ViewModels
{
    public class TipViewModel : MvxViewModel
    {
        private readonly ICalculation _calculation;
        public TipViewModel(ICalculation calculation)
        {
            _calculation = calculation;
        }

        public override void Start()
        {
            _subTotal = 100;
            _generosity = 10;
            Recalculate();
            base.Start();
        }

        private double _subTotal;

        public double SubTotal
        {
            get { return _subTotal; }
            set { _subTotal = value; RaisePropertyChanged(() => SubTotal); Recalculate(); }
        }

        private int _generosity;

        public int Generosity
        {
            get { return _generosity; }
            set { _generosity = value; RaisePropertyChanged(() => Generosity); Recalculate(); }
        }

        private double _tip;

        public double Tip
        {
            get { return _tip; }
            private set { _tip = value; RaisePropertyChanged(() => Tip);}
        }

        private void Recalculate()
        {
            Tip = _calculation.TipAmount(SubTotal, Generosity);
        }
    }
}

For many of you, this TipViewModel will already make sense to you. If it does then skip ahead to 'Add the App(lication)'. If not, then here are some simple explanations:

the TipViewModel is constructed with an ICalculation service

  private readonly ICalculation _calculation;
 
  public TipViewModel(ICalculation calculation)
  {
      _calculation = calculation;
  }

after construction, the TipViewModel will be started - during this it sets some initial values.

  public override void Start()
  {
      // set some start values
      SubTotal = 100.0;
      Generosity = 10;
      Recalculate();
  	base.Start();
  }

the view data held within the TipViewModel is exposed through properties.

Each of these properties is backed by a private member variable
Each of these properties has a get and a set
The set accessor for Tip is marked private
All of the set accessors call RaisePropertyChanged to tell the base MvxViewModel that the data has changed

The SubTotal and Generosity set accessors also call Recalculate()

    private double _subTotal;
    public double SubTotal
    {
        get { return _subTotal; }
        set {  _subTotal = value; RaisePropertyChanged(() => SubTotal); Recalculate(); }
    }

    private int _generosity;
    public int Generosity
    {
        get { return _generosity; }
        set {  _generosity = value; RaisePropertyChanged(() => Generosity); Recalculate(); }
    }

    private double _tip;
    public double Tip
    {
        get { return _tip; }
        private set {  _tip = value; RaisePropertyChanged(() => Tip); }
    }

The Recalculate method uses the _calculation service to update Tip from the current values in SubTotal and Generosity
```
  private void Recalculate()
  {
      Tip = _calculation.TipAmount(SubTotal, Generosity);
  }
```

Add the App(lication)

With our Calculation service and TipViewModel defined, we now just need to add the main App code.

This code:

will sit in a single class within the root folder of our PCL core project.
this class will inherits from the MvxApplication class
this class is normally just called App
this class is responsible for providing:
- registration of which interfaces and implementations the app uses
- registration of which ViewModel the App will show when it starts
- control of how ViewModels are located - although most applications normally just use the default implementation of this supplied by the base MvxApplication class.

'Registration' here means creating an 'Inversion of Control' - IoC - record for an interface. This IoC record tells the MvvmCross framework what to do when anything asks for an instance of that interface.

For our Tip Calculation app:

we register the Calculation class to implement the ICalculation service
```
       Mvx.RegisterType<ICalculation, Calculation>();
```
this line tells the MvvmCross framework that whenever any code requests an ICalculation reference, then the framework should create a new instance of Calculation. Note the single static class Mvx which acts as a single place for both registering and resolving interfaces and their implementations.
we want the app to start with the TipViewModel
```
       var appStart = new MvxAppStart<TipViewModel>();
       Mvx.RegisterSingleton<IMvxAppStart>(appStart);
```
this line tells the MvvmCross framework that whenever any code requests an IMvxAppStart reference, then the framework should return that same appStart instance.

So here's what App.cs looks like:

using Cirrious.CrossCore;
using Cirrious.CrossCore.IoC;
using Cirrious.MvvmCross.ViewModels;
using TipCalc.Core.Services;
using TipCalc.Core.Viewmodels;

namespace TipCalc.Core
{
    public class App : MvxApplication
    {
        public App()
        {
            Mvx.RegisterType<ICalculation,Calculation>();
            Mvx.RegisterSingleton<IMvxAppStart>(new MvxAppStart<TipViewModel>());
        }
    }
}

Note: What is 'Inversion of Control'?

We won't go into depth here about what IoC - Inversion of Control - is.

Instead, we will just say that:

within each MvvmCross application, there is a single special object - a singleton
This singleton lives within the Mvx static class.
The application startup code can use the Mvx.Register methods in order to specify what will implement interfaces during the lifetime of the app.
After this has been done, then later in the life when any code needs an interface implementation, then it can request one using the Mvx.Resolve methods.

One common pattern that is seen is 'constructor injection':

Our TipViewModel uses this pattern.
It presents a constructor like: public TipViewModel(ICalculation calculation).
When the app is running a part of the MvvmCross framework called the ViewModelLocator is used to find and create ViewModels
when a TipViewModel is needed, the ViewModelLocator uses a call to Mvx.IocConstruct to create one.
This Mvx.IocConstruct call creates the TipViewModel using the ICalculation implementation that it finds using Mvx.Resolve

This is obviously only a very brief introduction.

If you would like to know more, please see look up some of the excellent tutorials out there on the Internet - like http://joelabrahamsson.com/inversion-of-control-an-introduction-with-examples-in-net/

The Core project is complete :)

Just to recap the steps we've followed:

We created a new PCL project using Profile259
We added the MvvmCross libraries
We added a ICalculation interface and implementation pair
We added a TipViewModel which:

inherited from MvxViewModel
used ICalculation
presented a number of public properties each of which called RaisePropertyChanged

We added an App which:

inherited from MvxApplication
registered the ICalculation/Calculation pair
registered a special start object for IMvxAppStart

These are the same steps that you need to go through for every new MvvmCross application.

Moving on

Next we'll start looking at how to add a first UI to this MvvmCross application.

Tip Calc The Core Project

Create the new Portable Class Library

Delete Class1.cs

Install MvvmCross

Add the Tip Calculation service

Add the ViewModel

Add the App(lication)

Note: What is 'Inversion of Control'?

The Core project is complete :)

Moving on

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