ReactiveUI

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# What is ReactiveUI?

[ReactiveUI](https://reactiveui.net/) is a composable, cross-platform model-view-viewmodel framework for all .NET
platforms that is inspired by functional reactive programming, which is a paradigm that allows you
to [abstract mutable state away from your user interfaces and express the idea around a feature in one readable place](https://www.youtube.com/watch?v=3HwEytvngXk)
and improve the testability of your application.

[🔨 Get Started](https://reactiveui.net/documentation/getting-started/) [🛍 Install Packages](https://reactiveui.net/documentation/getting-started/installation/) [🎞 Watch Videos](https://reactiveui.net/documentation/resources/videos) [🎓 View Samples](https://reactiveui.net/documentation/resources/samples/) [🎤 Discuss ReactiveUI](https://reactiveui.net/slack)

## Your first view model

A view shows data that changes, so it needs to know when a value changes. Written by hand, each property needs a
field and a setter that raises a change notification. Each command needs a property that wraps a method. ReactiveUI
writes that code for you while your project builds.

A **source generator** is a compiler add-on that writes C# code during the build. The ReactiveUI packages bring two
of them.
[ReactiveUI.SourceGenerators](https://www.reactiveui.net/documentation/source-generators/) writes reactive properties
and commands. [ReactiveUI.Binding](https://www.reactiveui.net/documentation/binding/) writes the code behind
`WhenAnyValue`, `ToProperty` and the view bindings. You install neither yourself.

**1. Install the ReactiveUI package for your UI framework.** A class library that holds only view models installs
`ReactiveUI`.

```bash
dotnet add package ReactiveUI.WPF
```

**2. Declare the view model as a `partial` class and mark its members.**

```csharp
using ReactiveUI;
using ReactiveUI.SourceGenerators;

public partial class LoginViewModel : ReactiveObject
{
    private readonly IObservable<bool> _canLogIn;

    public LoginViewModel()
    {
        _canLogIn = this.WhenAnyValue(
            static x => x.UserName,
            static x => x.Password,
            static (userName, password) => userName.Length > 0 && password.Length > 0);

        _isValidHelper = _canLogIn.ToProperty(this, static x => x.IsValid);
    }

    [Reactive]