对象池 object pool

对象池适用于：

• 对象的创建很耗时
• 对象频繁地释放再创建

对象池的实现：将释放的对象放进对象池中，在新建对象时，从对象池取对象

public class ObjectPool<T> where T : class, new()
{
    private Stack<T> m_objectStack = new Stack<T>();

    public T New()
    {
        return (m_objectStack.Count == 0) ? new T() : m_objectStack.Pop();
    }

    public void Store(T t)
    {
        m_objectStack.Push(t);
    }
}

改进第一步：对象被重新使用前，reset所有的成员变量。

public class ObjectPool<T> where T : class, new()
{
    private Stack<T> m_objectStack;

    private Action<T> m_resetAction; //indicate whether need reset
    private Action<T> m_onetimeInitAction; //indicate whether need initialize

    public ObjectPool(int initialBufferSize, Action<T>
        ResetAction = null, Action<T> OnetimeInitAction = null)
    {
        m_objectStack = new Stack<T>(initialBufferSize);
        m_resetAction = ResetAction;
        m_onetimeInitAction = OnetimeInitAction;
    }

    public T New()
    {
        if (m_objectStack.Count > 0)
        {
            T t = m_objectStack.Pop();

            if (m_resetAction != null)
                m_resetAction(t);

            return t;
        }
        else
        {
            T t = new T();

            if (m_onetimeInitAction != null)
                m_onetimeInitAction(t);

            return t;
        }
    }

    public void Store(T obj)
    {
        m_objectStack.Push(obj);
    }
}

参数T被指明为”where T:class,new()”，意味着有两个限制。首先，T必须为一个类（毕竟，只有引用类型需要被obejct-pool）；其次，它必须要有一个无参构造函数。

以下是池的使用方法：

class SomeClass : MonoBehaviour
{
    private ObjectPool<List<Vector3>> m_poolOfListOfVector3 =
        //32为假设的最大数量
        new ObjectPool<List<Vector3>>(32,
        (list) => {
            list.Clear();
        },
        (list) => {
            //初始化容量为1024
            list.Capacity = 1024;
        });

    void Update()
    {
        List<Vector3> listVector3 = m_poolOfListOfVector3.New();

        // do stuff

        m_poolOfListOfVector3.Store(listVector3);
    }
}

改进第二步：允许一次性回收所有已经池化的对象

public class ObjectPoolWithCollectiveReset<T> where T : class, new()
{
    private List<T> m_objectList; //Stack<T>被List<T>代替,可以跟踪最近创建或释放的对象在list中索引
    private int m_nextAvailableIndex = 0;

    private Action<T> m_resetAction;
    private Action<T> m_onetimeInitAction;

    public ObjectPoolWithCollectiveReset(int initialBufferSize, Action<T>
        ResetAction = null, Action<T> OnetimeInitAction = null)
    {
        m_objectList = new List<T>(initialBufferSize);
        m_resetAction = ResetAction;
        m_onetimeInitAction = OnetimeInitAction;
    }

    public T New()
    {
        if (m_nextAvailableIndex < m_objectList.Count)
        {
            // an allocated object is already available; just reset it
            T t = m_objectList[m_nextAvailableIndex];
            m_nextAvailableIndex++;

            if (m_resetAction != null)
                m_resetAction(t);

            return t;
        }
        else
        {
            // no allocated object is available
            T t = new T();
            m_objectList.Add(t);
            m_nextAvailableIndex++;

            if (m_onetimeInitAction != null)
                m_onetimeInitAction(t);

            return t;
        }
    }

    public void ResetAll() //原本的store()
    {
       //重置索引
        m_nextAvailableIndex = 0;
    }
}

局限：显然，当我们需要一个其它类型的对象池时就需要重新申明一个。或许我们可以实现一个对用户透明。创建管理所有类型池的ObjectPool。