Run your behaviours as Udon

A behaviour a test adds with AddComponent runs as plain C#. To test what Udon does with your code, such as halting a behaviour that throws, run it as its compiled Udon program with UdonHost, in a Play Mode test.

The behaviour needs a program asset, which a script made with Assets > Create > U# Script has. Its assembly has to be in player builds and referenced by your Play Mode test assembly, as the assembly definitions of your world and the UdonDoubles assembly the kit creates are.

Steps

1. Extend UdonTestBase in your Play Mode test class:

   public class DoorTests : UdonTestBase
   {
   }

2. In a test, create the behaviour with Udon.Create. It returns the UdonBehaviour that runs the program:

   UdonBehaviour door = Udon.Create<Door>("Door");

3. Arrange and drive it by the names of its fields and methods, then assert on its fields:

   door.SetProgramVariable(nameof(Door.IsLocked), false);
   
   door.SendCustomEvent(nameof(Door.Open));
   
   Assert.That(door.GetProgramVariable(nameof(Door.IsOpen)), Is.EqualTo(true));

UdonTestBase destroys the GameObjects Udon created after every test.

Check that your code throws

When code that runs as Udon throws, Udon halts the behaviour and logs an error, which fails the test unless the test expects it. Expect it with UdonHost.ExpectUdonException, from a [UnityTest]:

[UnityTest]
public IEnumerator Unlock_NoKeyAssigned_HaltsTheDoor()
{
    UdonBehaviour door = Udon.Create<Door>("Door");

    yield return UdonHost.ExpectUdonException(() => door.SendCustomEvent(nameof(Door.Unlock)));
}

The test fails if Unlock doesn't throw.

Wait for delayed events

When your code delays an event with SendCustomEventDelayedFrames or SendCustomEventDelayedSeconds, Udon runs it on a later frame, once the delay passed, and never during the call that delays it. Wait for it in a [UnityTest], longer than the delay:

[UnityTest]
public IEnumerator OpenInASecond_AfterTheSecond_OpensTheDoor()
{
    UdonBehaviour door = Udon.Create<Door>("Door");

    door.SendCustomEvent(nameof(Door.OpenInASecond));
    yield return new WaitForSeconds(1.5f);

    Assert.That(door.GetProgramVariable(nameof(Door.IsOpen)), Is.EqualTo(true));
}

Behaviours that need players

UdonTestBase extends ClientSimTestBase, so start the session first, as in any ClientSim test:

yield return Session.Start();
UdonBehaviour door = Udon.Create<Door>("Door");

To run these tests in a scene of their own, pass a TestScene to the UdonTestBase constructor.