How Uno works on Android

This article explores Android-specific details of Uno's internals, with a focus on information that's useful for contributors to Uno. For an overview of how Uno works on all platforms, see this article.

Some particulars of Android:

Some classes are written in Java

Several base classes and helper classes are written in native Java code. These are located in Uno.UI.BindingHelper.Android.

The Xamarin.Android framework gives complete access to the Android API from managed C#, so why write anything in Java? The reason is performance. The interop between Java and C# can be costly, and instrumented profiling identified that certain virtual methods when overridden in C# and called in heavily-used paths (eg measure and arrange) imposed a measurable performance penalty. Over time, these 'hot' methods have been lowered to Java classes, particularly the UnoViewGroup class.

The Uno.UI.BindingHelper.Android project builds these Java types, and wraps them in a Xamarin binding library, making them available via C#.

UIElement inherits from ViewGroup

UIElement in Uno is a native view on Android, inheriting from the general ViewGroup type. To elaborate, UIElement's base classes are the following: Android.Views.ViewAndroid.Views.ViewGroupUno.UI.UnoViewGroupUno.UI.Controls.BindableViewMicrosoft.UI.Xaml.UIElement

Recall that UIElement implements DependencyObect as an interface in Uno.


Uno's measure and arrange logic is triggered from the native Android layout cycle. For a schematic of the control flow, see Layouting in Android.