Tip Calc A Xamarin.Android UI project

We started with the goal of creating an app to help calculate what tip to leave in a restaurant

We had a plan to produce a UI based on this concept:

Sketch

To satisfy this we built a 'Core' Portable Class Library project which contained:

our 'business logic' - ICalculation
our ViewModel - TipViewModel
our App which contains the application wiring, including the start instructions.

We're now ready to add out first User Interface.

So... let's start with Android.

To create an Android MvvmCross UI, you can use the Visual Studio project template wizards, but here we'll instead build up a new project 'from empty', just as we did for the Core project.

Create a new Android UI Project

Add a new project to your solution - a 'Blank App (Android)' application with name TipCalc.UI.Droid

Within this, you'll find the normal Android application constructs:

the Assets folder
the Resources folder
the MainActivity.cs

Delete MainActivity.cs

No-one really needs an MainActivity :)

Also, delete Main.axml in the /resources/Layout folder.

Install MvvmCross

In the Package Manager Console, enter...

Install-Package MvvmCross.HotTuna.MvvmCrossLibraries

Add a reference to TipCalc.Core.csproj

Add a reference to your TipCalc.Core project - the project we created in the last step which included:

your Calculation service,
your TipViewModel
your App wiring.

Add a Setup class

Every MvvmCross UI project requires a Setup class.

This class sits in the root namespace (folder) of our UI project and performs the initialisation of the MvvmCross framework and your application, including:

the Inversion of Control (IoC) system
the MvvmCross data-binding
your App and its collection of ViewModels
your UI project and its collection of Views

Most of this functionality is provided for you automatically. Within your Droid UI project all you have to supply are:

your App - your link to the business logic and ViewModel content

For TipCalc here's all that is needed in Setup.cs:

using Android.Content;
using Cirrious.MvvmCross.Droid.Platform;
using Cirrious.MvvmCross.ViewModels;
using TipCalc.Core;

namespace TipCalc.UI.Droid
{
	public class Setup : MvxAndroidSetup
	{
		public Setup(Context applicationContext)
			: base(applicationContext)
		{
		}

		protected override IMvxApplication CreateApp()
		{
			return new App();
		}
	}
}

Add your View

Add the Android Layout XML (AXML)

This tutorial doesn't attempt to give an introduction to Android XML layout.

Instead all I'll say here is the bare minimum. If you are new to Android, then you can find out more about Android XML from lots of places including the official documentation at: http://developer.android.com/guide/topics/ui/declaring-layout.html. If you are coming from a XAML background - you are a XAMLite - then I'll include some simple XAML-AXML comparisons to help you out.

To achieve the basic layout:

we'll add a new AXML file - View_Tip.axml in the /resources/Layout folder

we'll edit this using either the Xamarin Android designer or the Visual Studio XML editor - the designer gives us a visual display, while the VS editor sometimes gives us XML Intellisense.

  <?xml version="1.0" encoding="utf-8"?>
  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:orientation="vertical"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent">
  </LinearLayout>

we'll add a local app namespace - http://schemas.android.com/apk/res/TipCalc.UI.Droid - this is just like adding a namespace in XAML.

  <?xml version="1.0" encoding="utf-8"?>
  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:local="http://schemas.android.com/apk/res/TipCalc.UI.Droid"
      android:orientation="vertical"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent">
  </LinearLayout>

notice that this 'layout' is already by default a vertical LinearLayout - for XAMLites, this layout is just like a StackPanel except that it is very important to specify the vertical orientation

within this layout we'll add some TextViews to provide some static text labels - for XAMLites, these are like TextBlocks

  <TextView
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      android:text="SubTotal" />
  <TextView
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      android:text="Generosity" />
  <TextView
      android:layout_width="fill_parent"
      android:layout_height="wrap_content"
      android:text="Tip to leave" />

we'll also add a short, wide View with a yellow background to provide a small amount of chrome:

  <View
      android:layout_width="fill_parent"
      android:layout_height="1dp"
      android:background="#ffff00" />

we'll add some Views for data display and entry, and we'll databind these Views to properties in our TipViewModel

an EditText for text data entry of the SubTotal - for XAMLites, this is a TextBox

    <EditText
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        local:MvxBind="Text SubTotal" />

a SeekBar for touch/slide entry of the Generosity - for XAMLites, this is like a ProgressBar

    <SeekBar
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:max="40"
        local:MvxBind="Progress Generosity" />

we'll add a TextView to display the Tip that results from the calculation:

     <TextView
         android:layout_width="fill_parent"
         android:layout_height="wrap_content"
         local:MvxBind="Text Tip" />

Put together, this looks like:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:local="http://schemas.android.com/apk/res/TipCalc.UI.Droid"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="SubTotal" />
    <EditText
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        local:MvxBind="Text SubTotal" />
    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="Generosity" />
    <SeekBar
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:max="40"
        local:MvxBind="Progress Generosity" />
    <View
        android:layout_width="fill_parent"
        android:layout_height="1dp"
        android:background="#ffff00" />
    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="Tip to leave" />
    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        local:MvxBind="Text Tip" />
</LinearLayout>

About the data-binding syntax

Each of the data-binding blocks within our first sample looks similar:

local:MvxBind="Text SubTotal"

What this line means is:

data-bind:
- the property Text on the View
- to the property SubTotal on the DataContext - which in this case will be a TipViewModel
so:
- whenever the TipViewModel calls RaisePropertyChanged on SubTotal then the View should update
- and whenever the user enters text into the View then the SubTotal value should be set on the TipViewModel

Note that this TwoWay binding is different to XAML where generally the default BindingMode is only OneWay.

In later topics, we'll return to show you more options for data-binding, including how to use ValueConverters, but for now all our binding uses this simple ViewProperty ViewModelProperty syntax.

Add the View class

With our AXML layout complete, we can now add the C# Activity which is used to display this content. For developers coming from XAML backgrounds, these Activity classes are roughly equivalent to Page objects in WindowsPhone on WindowsStore applications - they own the 'whole screen' and have a lifecycle which means that only one of them is shown at any one time.

To create our Activity - which will also be our MVVM View:

Create a Views folder within your TipCalc.UI.Droid project
Within this folder create a new C# class - TipView
This class will:

inherit from MvxActivity

     public class TipCalcView : MvxActivity

be marked with the Xamarin.Android Activity attribute, marking it as the MainLauncher for the project
```
     [Activity(Label = "Tip", MainLauncher=true)]
```

provide a new ViewModel Property to specify the type of ViewModel it expects - the TipViewModel

     public new TipViewModel ViewModel
     {
         get { return (TipViewModel)base.ViewModel; }
         set { base.ViewModel = value; }
     }

use OnViewModelSet to inflate its ContentView from the AXML - this will use a resource identifier generated by the Android and Xamarin tools.
```
     protected override void OnViewModelSet()
     {
         SetContentView(Resource.Layout.View_Tip);
     }
```

As a result this completed class is very simple:

using Android.App;
using Cirrious.MvvmCross.Droid.Views;
using TipCalc.Core;

namespace TipCalc.UI.Droid.Views
{
    [Activity(Label = "Tip", MainLauncher = true)]
    public class TipView : MvxActivity
    {
        public new TipViewModel ViewModel
        {
            get { return (TipViewModel) base.ViewModel; }
            set { base.ViewModel = value; }
        }

        protected override void OnViewModelSet()
        {
            base.OnViewModelSet();
            SetContentView(Resource.Layout.View_Tip);
        }
    }
}

The Android UI is complete!

At this point you should be able to run your application.

When it starts... you should see:

If you then want to make it 'more beautiful', then try adding a few attributes to some of your AXML - things like:

    android:background="#00007f"
    android:textColor="#ffffff"
    android:textSize="24dp"
    android:layout_margin="30dp"
    android:padding="20dp"
    android:layout_marginTop="10dp"

Within a very short time, you should hopefully be able to create something 'styled'...

... but actually making it look 'nice' might take some design skills!

Moving on

There's more we could do to make this User Interface nicer and to make the app richer... but for this first application, we will leave it here for now.

Let's move on to Xamarin.iOS and to Windows!

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