Reactive Properties

The [Reactive] attribute is the foundation of Termina's reactive state management. It uses source generation to create observable properties from private fields.

Basic Usage

public partial class MyViewModel : ReactiveViewModel
{
    [Reactive] private int _count;
    [Reactive] private string _status = "Ready";
}

WARNING

Your class must be partial for the source generator to work.

What Gets Generated

For each [Reactive] field, the source generator creates:

// You write:
[Reactive] private int _count;

// Generator creates:
private readonly BehaviorSubject<int> _countSubject = new(default);

public int Count
{
    get => _countSubject.Value;
    set => _countSubject.OnNext(value);
}

public IObservable<int> CountChanged => _countSubject.AsObservable();

Naming Convention

The field name determines the generated property name:

Field	Property	Observable
_count	Count	CountChanged
_userName	UserName	UserNameChanged
_isVisible	IsVisible	IsVisibleChanged

The _ prefix is removed and the first letter is capitalized.

Default Values

Set default values on the field:

[Reactive] private int _count = 10;
[Reactive] private string _status = "Ready";
[Reactive] private List<string> _items = new();

Using in Pages

Subscribe to the generated *Changed observable in your page:

public class MyPage : ReactivePage<MyViewModel>
{
    protected override ILayoutNode BuildLayout()
    {
        return ViewModel.CountChanged
            .Select(count => new TextNode($"Count: {count}"))
            .AsLayout();
    }
}

Using in ViewModel Logic

Access the current value via the property:

private void HandleIncrement()
{
    Count++;  // Read current value, increment, set new value
}

private void HandleReset()
{
    if (Count != 0)  // Read current value
    {
        Count = 0;   // Set new value
    }
}

Collection Properties

For collections, replace the entire collection to trigger updates:

[Reactive] private List<string> _messages = new();

private void AddMessage(string msg)
{
    // Create new list with added item
    Messages = new List<string>(Messages) { msg };

    // Or using LINQ
    Messages = Messages.Append(msg).ToList();
}

TIP

Mutating a collection in-place (e.g., Messages.Add(msg)) won't trigger an update because the reference hasn't changed. Always assign a new collection.

BehaviorSubject vs Other Observables

[Reactive] properties use BehaviorSubject<T> which:

• Holds current value - Subscribers immediately receive the latest value
• Hot observable - Emits to all subscribers
• Replays last value - New subscribers get the current value immediately

This is ideal for UI state because:

1. The page gets the current value when it subscribes
2. All reactive bindings stay in sync
3. You can read the current value synchronously

Source Generator Details

See Source Generators for implementation details.