Reactive programming in Unity with UniRx.

2017-11-25

In the past I’ve written about how we use reactive programming in Wunderlist. Reactive programming leverages IObservable and IObserver to create and subscribe to streams of data in your app. Not only typical data like lists and tasks can be streamed but also things like timers, keyboard, controller and mouse inputs. Unity3D has its own Rx implementation called UniRx and here is an example of what you can do with it.

shots

Create a new Unity3D project. Call it RxTest
Save the main scene as ‘main’
Open the Player settings and find the Other settings, Configuration section.
Change the Scripting Runtime to .NET 4.6 (Unity might restart)

Lib in unity

Download UniRx from the asset store in Unity.
Create an empty GameObject and rename it to app
With app selected, click Add Component in the inspector.
Select New Script and call it App
Doubleclick ‘App’ to open in Visual Studio
Replace the generated code with:

using UnityEngine;
using UnityEngine.UI;

public class App : MonoBehaviour
{
public Text TextBlock;
}

Switch back to Unity
In the hierarchy, click create > UI > Text
Select app
Drag Text to the App (script) Text Block placeholder

drag

Switch to Visual Studio

In the next code, a ReactiveProperty of type int is added for the counter. The reactive property has a method to register to changes to update a UI.Text. In our example, we overload SubscribeToText with a Func that transforms an int into a string with some formatting.

using UniRx;
using UnityEngine;
using UnityEngine.UI;

public class App : MonoBehaviour
{
public Text TextBlock;

private ReactiveProperty<int> _counter = new ReactiveProperty<int>();

private void Start()
{
_counter.SubscribeToText(TextBlock, i => $"Shots fired: {i}");
}
}

Filtering an observable stream

Finally it is time to create an Observable that creates a stream of clicks. UniRx has a function UpdateAsObservable that transforms the Update event to an observable. We start by filtering this stream by checking if the mouse button is down. By this time, we throttle the events to emit every 500 milliseconds and finally subscribe to the observable by calling OnClick. Notice how the value of Reactive Property _counter in the OnClick method is incremented and the UI.Text automatically updates.

using System;
using UniRx;
using UniRx.Triggers;
using UnityEngine;
using UnityEngine.UI;

public class App : MonoBehaviour
{
public Text TextBlock;
private ReactiveProperty<int> _counter = new ReactiveProperty<int>();

private void Start()
{
this.UpdateAsObservable()
    .Where(_ => Input.GetMouseButton(0))
    .ThrottleFirst(TimeSpan.FromMilliseconds(500))
    .Subscribe(OnClick);

_counter.SubscribeToText(TextBlock, i => $"Shots fired: {i}");
}

private void OnClick(Unit x)
{
_counter.Value++;
}
}

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