UI Composition
UI composition is the process you use to create the layouts that your apps require. It allows you to build a complex view from an arrangement of components. The advantages are:
- Encapsulation - reduce the complexity of each component by restricting its XAML and code to only what it needs, making your code more understandable and maintainable.
- Reuse - maintain consistent presentation and behaviour of repeated parts of your app.
Avalonia UI makes it easy for you to use UI composition to create the layouts and functions that your apps require.
When you build an app using Avalonia UI, there are several different types of component to choose from:
- Windows
- Built-in Controls
- User Controls
- Custom Controls
- Template Controls
Windows and Built-in Controls
A window in Avalonia UI is a basic unit of layout (for a windowing platform).
Avalonia UI contains a large number of built-in controls that will cover most of your UI requirements.
When you first meet Avalonia UI, you might place a single built-in control in the content zone of a window (above, left). This is the simplest form of UI composition: the window has the title of the app and usually some window state controls (depending on the target platform). The built-in control allows your app to receive some user input, or to present some output with layout and styling.
A slightly more complex app may require one of the built-in layout controls to arrange more than one other built-in control in the content zone of a window (above, right).
To see the full range of Avalonia UI built-in controls, see the reference section here.
Logical and Visual Trees
Whatever arrangement of controls you use, Avalonia UI represents their relationships as a a tree structure, with the 'outermost' control as the root. So for example, the previous UI composition can be represented as the tree shown here:
This is the logical control tree, and it represents the application controls (including the main window) in the hierarchy in which they are defined in the XAML. There are many systems in Avalonia UI that process the logical control tree and its companion the visual control tree.
For more information on the concept of control trees, see here.
User Controls
User controls are the mainstay of UI composition in Avalonia UI.
You can add a user control to the content zone of a main window, to represent a 'page view' (above, left). This allows you to implement a more complex app with multiple pages; where the layout and function of each page is in its own user control (XAML and code) files.
For more information about how to implement a multi-page app using views, see the guide here.
Another use for a user control is as a component control (above, right). You might initially do this to reduce the complexity of a window or page view; but then you might also (perhaps later) reuse the resulting component on another page as well.
Tutorial
For tutorials about DataTemplates see Avalonia.Samples.
Collection Controls
Another variation of UI composition is where you need to present a collection of items.
This scenario will use one of the built-in repeating controls, bound to a collection; together with a data template to represent the items in the collection.
For information about how to TO DO
Custom Controls
In the unlikely scenario that you cannot find an Avalonia UI built-in control to cover your app's UI requirements, then you can 'roll-your-own' custom control from scratch. This allows you to define your own custom properties, events and methods; but it will require you to implement the drawing of the control presentation from scratch as well.
To learn how to implement a custom control, see the guide here.
Templated Controls
A templated control uses the Avalonia UI styling system to substitute a tag in the UI layout with a
For more information about the concepts behind the Avalonia UI styling system, see here.