一、官网的Physics文档

https://docs.unity.cn/cn/2019.4/ScriptReference/Physics.html

1、Physics下带NonAlloc都是无GC的，项目中必须使用NonAlloc相关的函数来进行物理检测；

Physics.OverlapBox有GC，Physics.OverlapBoxNonAlloc无GC

GC对比.png

二、Physics.NonAlloc的几个函数有时候生效有时候不生效问题

1、先看一下下面的测试代码

public class ColliderTest : MonoBehaviour
{
    private Collider[] m_Colliders;

    void Update()
    {
        m_Colliders = Physics.OverlapBox(Vector3.zero, new Vector3(2, 2, 2));
        Debug.Log("OverlapBox：" + m_Colliders.Length);

        var count = Physics.OverlapBoxNonAlloc(Vector3.zero, new Vector3(2, 2, 2), m_Colliders);
        Debug.Log("OverlapBoxNonAlloc：" + count);
    }
}

结果都可以

成功.png

2、现在我们切换到项目中的无GC写法的代码中去

public class ColliderTest : MonoBehaviour
{
    private Collider[] m_Colliders;

   void Update()
    {
        var count = Physics.OverlapBoxNonAlloc(Vector3.zero, new Vector3(2, 2, 2), m_Colliders);
        Debug.Log("OverlapBoxNonAlloc：" + count);
    }
}

结果没有任何输出，失败了，为什么？

失败.png

三、问题探究解决

1、我们看下两次代码都有哪些变化

（1）成功：测试代码的Physics.OverlapBox接收的Collider数组用的是m_Colliders

（2）成功：测试代码的Physics.OverlapBoxNonAlloc参数也是用的m_Colliders

（3）失败：测试代码的Physics.OverlapBoxNonAlloc参数是用的m_Colliders

2、发现问题所在

（1）成功：m_Colliders数组默认没有初始化，Physics.OverlapBox接收的数组赋值给了m_Colliders，此时m_Colliders就被初始化了，当Physics.OverlapBoxNonAlloc传递进去的数组就已经是初始化后的了，所以成功了；

（2）失败：m_Colliders数组没有被初始化，直接传递给了Physics.OverlapBoxNonAlloc的参数，Physics.OverlapBoxNonAlloc内部并不会对一个没有初始化的Collider[]进行初始化，因为Collider[]初始化的长度由外部使用者决定的，所以使用Physics.NonAlloc相关的函数外部传递进去的缓存数组必须先初始化

private Collider[] m_Colliders = new Collider[1];

2、问题解决

public class ColliderTest : MonoBehaviour
{
    private Collider[] m_Colliders = new Collider[1];

    void Update()
    {
        var count = Physics.OverlapBoxNonAlloc(Vector3.zero, new Vector3(2, 2, 2), m_Colliders);
        Debug.Log("OverlapBoxNonAlloc：" + count);
    }
}