๐Ÿ“ป Music Store App

A tutorial demonstrating how to build a music store app.

The completed project can be found at https://github.com/AvaloniaUI/Avalonia.MusicStore.

It was created by Dan Walmsley for a webinar hosted by JetBrains.

In this tutorial, you will see just how easy it is to build great looking visual desktop applications using Avalonia.

This guide has instructions for Rider on OSX, however the steps will be almost the same on other operating systems, and reasonably similar on other IDEs such as Visual Studio.

Our livestream assumes some knowledge of XAML, MVVM development, however this guide should fill in the gaps for beginners.

A little background to Avalonia

Avalonia is a "Template" based UI framework. This means that controls have Templates that describe how they look on the screen.

This is a different approach to other UI frameworks where native controls are used, for example a Button will take on the standard look of a button on its respective operating system.

Some apps require or desire a native look and feel. Others require a "pixel perfect" style and design. This pixel perfect approach is becoming more popular and common place. Good examples of this kind of application are "Slack" where the companies branding is clear throughout the UI. It also looks the exact same no matter which OS you run it on.

Avalonia is suited to these "Pixel Perfect" style apps, providing the advantage of native code and speed.

If you are already familiar with MVVM you may wish to skip this next section, if your new to Avalonia read on.

Model View ViewModel

The best architecture to use Avalonia with (not compulsory, just works best) is MVVM (Model View ViewModel).

It sounds complicated, and there are many over complicated guides and tutorials on the internet.

MVVM is simply a way to enforce Separation of concerns. For your quick demo tutorial app, this may seem overkill. Keeping UI and business logic separated in such a way. However the apps that your will soon be building, often start small but quickly grow. Your customers will spring new requirements on you, and you will need to shoe horn them into your software.

Following the MVVM approach will alleviate these difficulties and help keep your UI code scalable.

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How does MVVM work?

Back to separation of concerns, in any GUI application there are at least 3 main concerns:

  • UI - Layout, Style, Content

  • UI Logic - How the controls interact with each other and the user.

  • Business Logic - The actual functionality your application provides, dealing with a database, controlling some hardware you built for IOT, ordering products from your store.

Why is it a good idea to keep these separated?

If your code combines or mixes these 3 aspects together, it will tie the design of your UI directly to your business logic. This will make it incredibly difficult to make large changes to the way your application works. This was common place in the not too distant past.

How does MVVM achieve this?

Model (Business Logic)

All your business logic can be contained in plain classes, they can be designed and implemented however you want. Most project you already may have this. The general term we call these business logic classes is Models.

Your models can simply expose methods (functions), properties and use events to notify other parts of the MVVM architecture when something has changed in the system.

For example if your domain, is a music store. Perhaps your business logic provides a list of the top 10 albums. It may happen that the list changes and this change can be propagated by use of an event.

So now we have the Model part of MVVM, all self contained, the models know nothing about any of the other parts.

ViewModel (UI Logic)

You user interface is essentially the way your users interact with your business logic or models. There needs to be a way for your UI to interact with the business logic. We do this using a ViewModel. A ViewModel knows about the Model that it represents. It does not know anything about the layout or design of the UI or View part.

A ViewModel is essentially special type of Model that represents all the data that will be displayed in the UI. It also represents all the actions that can be done with the UI. For example what happens when a button is clicked.

This keeps things like disabling buttons when the user hasnt input the correct information away from your business logic.

A ViewModel will subscribe to or observe events on a model so that it knows when something in your system has changed, with the intention that it can then update the UI so the user will know about it.

A ViewModel will also call or trigger functionality in response to user input like a Button being clicked.

View (UI)

The view provides the layout and content of the UI. It knows about the ViewModel and the information it provides that can be displayed in the UI.

The view describes how to "present" data and provides controls the user can interact with.

A View can retrieve data to display with the use of Bindings . Bindings can also be used for interactions to be communicated back to the ViewModel .

Summary

A model provides the business logic and talks to the ViewModel.

A ViewModel provides the UI logic and invokes the business logic in response to user input.

A View provides the look, layout and content of the UI.

Lets get started building something!

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