TipCalc Android Project

Edit

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’ .NET Standard project which contained:

Our ‘business logic’ - ICalculationService
Our ViewModel - TipViewModel
Our App - which contains some bootstrapping code.

We’re now ready to add out first User Interface!

So… let’s start with Android.

Same as we did with the Core project, we will use a standard template to create the Android project.

Create a new Android project

Add a new project to your solution - a ‘.Net Android Application’ application with name TipCalc.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 that MainActivity :)

Also, make sure you delete activity_main.xml in the /resources/Layout folder.

Install MvvmCross

Open the Nuget Package Manager and search for the package MvvmCross.

If you don’t really enjoy the NuGet UI experience, then you can alternatively open the Package Manager Console, and type:

Install-Package MvvmCross

Add a reference to TipCalc.Core project

Add a reference to your TipCalc.Core project - the project we created in the first step.

Creating Setup class

Every MvvmCross UI project requires a Setup class. The Setup class is responsible for performing the initialization of the MvvmCross framework, including:

The IoC Container and DI engine
The Data-Binding engine
The ViewModel / View lookups
The whole navigation system
Plugins

The Setup class is also responsible for initializing your App class.

Finally, let’s add Setup class:

using Microsoft.Extensions.Logging;
using MvvmCross.Platforms.Android.Core;
using TipCalc.Core;

namespace TipCalc.Droid;

public class Setup : MvxAndroidSetup<App>
{
    protected override ILoggerFactory? CreateLogFactory() => default!;

    protected override ILoggerProvider? CreateLogProvider() => default!;
}

Add an Android Application class

The Android Application class will allow us to specify the MvvmCross framework some key classes to be used for initialization:

using Android.Runtime;
using MvvmCross.Platforms.Android.Views;
using TipCalc.Core;

namespace TipCalc.Droid;

[Application]
public class MainApplication : MvxAndroidApplication<Setup, App>
{
    public MainApplication(IntPtr javaReference, JniHandleOwnership transfer)
        : base(javaReference, transfer)
    {
    }
}

We won’t go deeper into what role does MainApplication have on the Android platform, but let’s talk a bit about the MvvmCross bits:

MvxAndroidApplication provides some behavior for initializing the framework in runtime - although it isn’t really the only way to configure the Android project.
App here is a reference to our TipCalc.Core.App class.

Add your View

Add the Android Layout XML (AXML)

This tutorial doesn’t attempt to give an introduction to Android XML layout, but any knowledge is actually really necessary at this point. If you are very new to Android, you can read more about Android XML on the official documentation.

To achieve the basic layout that we need:

We will add a new .xml file - called TipView.xml into the /Resources/Layout folder.
We will edit this file using the XML editor - the designer gives us a visual display, while the VS editor sometimes gives us XML Intellisense. Open the file, go to the “Source” tab and replace the file content with the following code:

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

We will now add a local app namespace - http://schemas.android.com/apk/res-auto - which 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-auto"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

Notice that the LinearLayout has by default a horizontal orientation - for XAMLites, this layout is just like a StackPanel except that it is very important to specify the vertical orientation
Within this layout we will add some TextViews to provide some static text labels:

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

We will also add a short, wide View with a yellow background to provide a small amount of chrome after the last TextView:

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

Now it’s time to add some Views for data display and data entry, which we will also databind to properties in our TipViewModel:
- Add an EditText for text data entry of the SubTotal:

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

Add a SeekBar for touch/slide entry of the Generosity:

<SeekBar
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:max="100"
    local:MvxBind="Progress Generosity" />

Now for a last step, add a TextView to display the final Tip result:

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

Putting everything together, your .axml file should look like this:

<?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-auto"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:text="SubTotal" />
    <EditText
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          local:MvxBind="Text SubTotal" />
    <TextView
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:text="Generosity" />
    <SeekBar
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:max="100"
          local:MvxBind="Progress Generosity" />
    <View
          android:layout_width="match_parent"
          android:layout_height="1dp"
          android:background="#ffff00" />
    <TextView
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          android:text="Tip to leave" />
    <TextView
          android:layout_width="match_parent"
          android:layout_height="wrap_content"
          local:MvxBind="Text Tip" />
</LinearLayout>

About the data-binding syntax

You may have noticed that each of the data-binding blocks within our first sample look very similar, for example:

local:MvxBind="Text SubTotal"

What this line means is:

Data-Binding:

The property Text of the TextView
To the property SubTotal on the DataContext - which in this case will be a TipViewModel.

This means:

Whenever the TipViewModel calls RaisePropertyChanged on SubTotal then the View should update its content
And whenever the user enters text into the View, 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.

Add the View class

With our .axml layout complete, we can now move back to C# and add an Activity, which is used to display the content. These Activity classes are very special objects on Android, which provide a context to your app and a placeholder to display widgets on the UI.

This Activity will act as our MVVM View. Please follow these steps:

Create a Views folder within your TipCalc.Droid project.
Within this folder create a new C# class called TipView.
This class should inherit from MvxActivity<TipViewModel>:

public class TipView : MvxActivity<TipViewModel>

Add an Activity attribute over the class and set the MainLauncher property to true. This attribute lets Xamarin.Android add it automatically to your AndroidManifest file:

[Activity(Label = "Tip Calculator", MainLauncher = true, Theme = "@style/Theme.AppCompat")]

Override the method OnCreate and call SetContentView() right after the call to base:

protected override void OnCreate(Bundle bundle)
{
    base.OnCreate(bundle);
    SetContentView(Resource.Layout.TipView);
}

As a result this completed class is very simple:

using MvvmCross.Platforms.Android.Views;
using TipCalc.Core.ViewModels;

namespace TipCalc.Droid;

[Activity(Label = "Tip Calculator", MainLauncher = true, Theme = "@style/Theme.AppCompat")]
public class TipView : MvxActivity<TipViewModel>
{
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        SetContentView(Resource.Layout.TipView);
    }
}

The Android project is complete!

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

When it starts… you should see something like this:

Android TipCalc

If you then want to make it ‘more beautiful’, then try adding a few attributes to some of your .xml - things like:

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

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!

Next!