这部分内容手册中没有，本文参考的是官方示例代码与官方论坛

https://github.com/Unity-Technologies/EntityComponentSystemSamples

在 OOP 的物理引擎中，碰撞发生后引擎可以通过调用用户提供的回调函数将碰撞事件传递给用户程序。而在 Unity 的 ECS 框架中我们需要使用数据驱动的思路来处理碰撞
但是引擎已经把事件处理封装好了，因此我们实现的时候其实看起来跟回调差不多

简介

Unity Physics 中，刚体可以通过 Trigger 或者 Collision 触发事件，需要勾选 Physics Shape 中的 Is Trigger 或 Raises Collision Events
它们的区别在于 Trigger 不会引起物理上的碰撞，只产生事件

对于产生的 Trigger 事件或 Collision 事件，我们可以通过 ITriggerEventsJob 或 ICollisionEventsJob 来获取并处理

下面的例子可以实现这样的效果
开始时所有的刚体不受重力影响，但是一个刚体被 Trigger 碰到后它就会重新受到重力并下坠

准备 Trigger

首先准备一个 Entity 作为 Trigger，并且让他受到键盘控制运动。
新建一个 Cube 的 GameObject，移除 Box Collider，添加 Convert To Entity，Physics Body，Physics Shape。在 Physics Body 中把重力系数设为 0，在 Physics Shape 中勾选 Is Trigger
然后使用 Authoring 为它添加 PlayerInput 组件。

保存键盘输入的组件
PlayerInput.cs

[Serializable]
public struct PlayerInput : IComponentData {
    public float2 moveDirection;
}

获取键盘输入的系统
FetchInputSystem.cs

public class FetchInputSystem : ComponentSystem {
    protected override void OnUpdate () {
        float2 dir = float2.zero;
        if (UnityEngine.Input.GetKey(UnityEngine.KeyCode.A))
            dir += new float2 (-1, 0);
        if (UnityEngine.Input.GetKey(UnityEngine.KeyCode.D))
            dir += new float2 (1, 0);
        if (UnityEngine.Input.GetKey(UnityEngine.KeyCode.W))
            dir += new float2 (0, 1);
        if (UnityEngine.Input.GetKey(UnityEngine.KeyCode.S))
            dir += new float2 (0, -1);
        dir = math.normalizesafe(dir);
        Entities.ForEach((ref PlayerInput input) => {
            input.moveDirection = dir;
        });
    }
}

根据得到的键盘输入控制 Trigger 移动的系统
MoveSystem.cs

public class MoveSystem : JobComponentSystem {
    [BurstCompile]
    struct MoveJob : IJobForEach<PlayerInput, PhysicsVelocity> {
        public void Execute ([ReadOnly] ref PlayerInput input, ref PhysicsVelocity velocity) {
            var v = velocity.Linear;
            v = new float3 (input.moveDirection, 0) * 1.8f;
            velocity = new PhysicsVelocity { Linear = v };
        }
    }
    protected override JobHandle OnUpdate (JobHandle inputDeps) {
        return new MoveJob ().Schedule (this, inputDeps);
    }
}

准备用于被触发的 Entity

然后准备另一个用于被触发的 Entity，它至少需要有 Physics Shape、Physics Body
当然要在 Physics Body 中将重力系数设为 0

处理 Trigger 事件

写一个简单的 JobComponentSystem 就行了
只是其中的 Job 需要使用 ITriggerEventsJob 接口，个人感觉是物理引擎 Simulate 之后会产生 Trigger 事件的数组，然后通过 ITriggerEventsJob 来遍历它处理每个 Trigger 事件

DetectTriggerSystem.cs

[UpdateAfter (typeof (EndFramePhysicsSystem))]
public class DetectTriggerSystem : JobComponentSystem {
    [BurstCompile]
    struct DetectTriggerJob : ITriggerEventsJob {
        [ReadOnly] public ComponentDataFromEntity<PlayerInput> playerInputGroup;
        public ComponentDataFromEntity<PhysicsGravityFactor> physicsGravityFactorGroup;
        public void Execute (TriggerEvent triggerEvent) {
            var triggeredEntity = triggerEvent.Entities.EntityA;
            if (playerInputGroup.Exists (triggeredEntity))
                triggeredEntity = triggerEvent.Entities.EntityB;
            var cmpt = physicsGravityFactorGroup[triggeredEntity];
            cmpt.Value = 1;
            physicsGravityFactorGroup[triggeredEntity] = cmpt;
        }
    }
    BuildPhysicsWorld buildPhysicsWorld;
    StepPhysicsWorld stepPhysicsWorld;
    protected override void OnCreate () {
        buildPhysicsWorld = World.GetOrCreateSystem<BuildPhysicsWorld> ();
        stepPhysicsWorld = World.GetOrCreateSystem<StepPhysicsWorld> ();
    }
    protected override JobHandle OnUpdate (JobHandle inputDeps) {
        return new DetectTriggerJob {
            playerInputGroup = GetComponentDataFromEntity<PlayerInput> (true),
                physicsGravityFactorGroup = GetComponentDataFromEntity<PhysicsGravityFactor> ()
        }.Schedule (stepPhysicsWorld.Simulation, ref buildPhysicsWorld.PhysicsWorld, inputDeps);
    }
}

处理 Collision 事件

跟上文 Trigger 事件类似，也是使用 JobComponentSystem，Job 需要换成 ICollisionEventsJob 接口，然后调整 Update 的顺序并使用 EndFramePhysicsSystem.HandlesToWaitFor 处理同步，官方给的例子是有问题的，本文根据论坛作了一些修改

贴一下代码
DetectCollisionSystem.cs

[UpdateAfter (typeof (StepPhysicsWorld))]
[UpdateBefore (typeof (EndFramePhysicsSystem))]
public class DetectCollisionSystem : JobComponentSystem {
    [BurstCompile]
    struct DetectCollisionJob : ICollisionEventsJob {
        [ReadOnly] public ComponentDataFromEntity<PlayerInput> playerInputGroup;
        public ComponentDataFromEntity<PhysicsGravityFactor> physicsGravityFactorGroup;
        public void Execute (CollisionEvent collisionEvent) {
            var triggeredEntity = collisionEvent.Entities.EntityA;
            if (playerInputGroup.Exists (triggeredEntity))
                triggeredEntity = collisionEvent.Entities.EntityB;
            var cmpt = physicsGravityFactorGroup[triggeredEntity];
            cmpt.Value = 1;
            physicsGravityFactorGroup[triggeredEntity] = cmpt;
        }
    }
    BuildPhysicsWorld buildPhysicsWorld;
    StepPhysicsWorld stepPhysicsWorld;
    EndFramePhysicsSystem endFramePhysicsSystem;
    protected override void OnCreate () {
        buildPhysicsWorld = World.GetOrCreateSystem<BuildPhysicsWorld> ();
        stepPhysicsWorld = World.GetOrCreateSystem<StepPhysicsWorld> ();
        endFramePhysicsSystem = World.GetOrCreateSystem<EndFramePhysicsSystem> ();
    }
    protected override JobHandle OnUpdate (JobHandle inputDeps) {
        var jobHandle = new DetectCollisionJob {
            playerInputGroup = GetComponentDataFromEntity<PlayerInput> (true), physicsGravityFactorGroup = GetComponentDataFromEntity<PhysicsGravityFactor> ()
        }.Schedule (stepPhysicsWorld.Simulation, ref buildPhysicsWorld.PhysicsWorld, inputDeps);
        endFramePhysicsSystem.HandlesToWaitFor.Add (jobHandle);
        return jobHandle;
    }
}