Since the beginning of Xamarin's lifetime as a company, their motto has always been to expose the native APIs on iOS and Android directly to C#. This was a great strategy at the beginning, because applications built with Xamarin.iOS or Xamarin.Android were pretty much indistinguishable from a native Objective-C or Java application. Code sharing was generally limited to non-UI code that left a potential gap to fill in the Xamarin ecosystem: a cross-platform UI abstraction. Xamarin.Forms is the solution to this problem, a cross-platform UI framework that renders native controls on each platform. Xamarin.Forms is a great solution for those who know C# (and XAML), but also might not want to get into the full details of using the native iOS and Android APIs.

In this chapter, we will cover the following:

Creating Hello World in Xamarin.Forms

To understand how a Xamarin.Forms application is put together, let's begin by creating a simple "Hello World" application.

Open Xamarin Studio and perform the following steps:

• Create a new solution.
• Navigate to the C# | Mobile Apps section.
• Create a new Blank App (Xamarin.Forms Portable) solution.
• Name your solution something appropriate such as HelloForms.

Notice the three new projects that were successfully created: HelloForms, HelloForms.Android, and HelloForms.iOS. In Xamarin.Forms applications, the bulk of your code will be shared, and each platform-specific project is just a small amount of code that starts the Xamarin.Forms framework.

Let's examine the minimal parts of a Xamarin.Forms application:

• App.cs in the HelloForms PCL library. This class holds the startup page of the Xamarin.Forms application. A simple static method, GetMainPage(), returns the startup page of the application. In the default project template,
• A ContentPage is created with a single label that will be rendered as a UILabel on iOS and a TextView on Android.
• MainActivity.cs in the HelloForms.Android Android project. This is the main launcher activity of the Android application. The important part for Xamarin.Forms here is the call to Forms.Init(this, bundle) that initializes the Android-specific portion of the Xamarin.Forms framework. Next is a call to SetPage(App.GetMainPage()) that displays the native version of the main Xamarin.Forms page.
• AppDelegate.cs in the HelloForms.iOS iOS project. This is very similar to Android, except iOS applications startup via a UIApplicationDelegate class. Forms.Init() will initialize the iOS-specific parts of Xamarin.Forms, while App.GetMainPage().CreateViewController() will generate a native controller that can be used as the RootViewController of the main window of the application.

Go ahead and run the iOS project; you should see something similar to the following screenshot:

If you run the Android project, you will get a UI very similar to the iOS, but using the native Android controls, as shown in the following screenshot:

Tip

Even though not covered in this book, Xamarin.Forms also supports Windows Phone applications. However, a PC running Windows and Visual Studio is required to develop for Windows Phone. If you can get a Xamarin.Forms application working on iOS and Android, then getting a Windows Phone version working should be a piece of cake.