Simple test showing how to use ExclusiveEntityTransaction in Unity ECS

EntityTransactionTest

using NUnit.Framework;
using Unity.Collections;
using Unity.Entities;
using Unity.Jobs;

[TestFixture]
public class EntityTransactionTests
{
    struct SomeComponent : IComponentData
    { }

    struct TransactionJob : IJob
    {
        public ExclusiveEntityTransaction Transaction;
        public EntityArchetype Archetype;

        public void Execute()
        {
            var arr = new NativeArray<Entity>(10000, Allocator.Temp);
            Transaction.CreateEntity(Archetype, arr);
        }
    }

    // A Test behaves as an ordinary method
    [Test]
    public void SimpleExample()
    {
        var world1 = new World("World 1");
        var world2 = new World("World 2");

        var em1 = world1.EntityManager;
        var em2 = world2.EntityManager;

        var q1 = em1.CreateEntityQuery(typeof(SomeComponent));
        var q2 = em2.CreateEntityQuery(typeof(SomeComponent));

        // Note if you create the archetype from em1, it would
        // cause the transaction to happen in World1, even though
        // you call BeginTransaction/EndTransaction from em2.
        var archetype = em2.CreateArchetype(typeof(SomeComponent));

        // Prevents access to em2 until EndTransaction is called
        var transaction = em2.BeginExclusiveEntityTransaction();
        var job = new TransactionJob
        {
            Archetype = archetype,
            Transaction = transaction
        }.Schedule();

        Assert.AreEqual(0, q1.CalculateEntityCount());
        Assert.AreEqual(0, q2.CalculateEntityCount());

        job.Complete();
        em2.EndExclusiveEntityTransaction();

        Assert.AreEqual(0, q1.CalculateEntityCount());
        Assert.AreEqual(10000, q2.CalculateEntityCount());

        em1.MoveEntitiesFrom(em2);

        Assert.AreEqual(10000, q1.CalculateEntityCount());
        Assert.AreEqual(0, q2.CalculateEntityCount());
    }
}