ObjectPool 對象池設計模式

Micosoft.Extension.ObjectPool

源碼架構、模式分析：

三大基本對象：

1. ObjectPool抽象類
2. ObjectPoolProvider抽象類
3. IPooledObjectPolicy接口

ObjectPool的默認實現是DefaultPool,對象池的建立由ObjectPoolProvider抽象類的默認實現DefaultObjectPoolProvider類實現。

public class DefaultObjectPoolProvider : ObjectPoolProvider
{
    /// <summary>
    /// The maximum number of objects to retain in the pool.
    /// </summary>
    public int MaximumRetained { get; set; } = Environment.ProcessorCount * 2;

    /// <inheritdoc/>
    public override ObjectPool<T> Create<T>(IPooledObjectPolicy<T> policy)
    {
        if (policy == null)
        {
            throw new ArgumentNullException(nameof(policy));
        }

        if (typeof(IDisposable).IsAssignableFrom(typeof(T)))
        {
            return new DisposableObjectPool<T>(policy, MaximumRetained);
        }

        return new DefaultObjectPool<T>(policy, MaximumRetained);
    }
}

IPooledObjectPolicy接口有對應的抽象類及默認實現，對象池中的對象(也就是池對象)的建立或返回由此策略類來控制。

public override T Get()
{
    var item = _firstItem;
    if (item == null || Interlocked.CompareExchange(ref _firstItem, null, item) != item)
    {
        var items = _items;
        for (var i = 0; i < items.Length; i++)
        {
            item = items[i].Element;
            if (item != null && Interlocked.CompareExchange(ref items[i].Element, null, item) == item)
            {
                return item;
            }
        }

        item = Create();
    }

    return item;
}

// Non-inline to improve its code quality as uncommon path
[MethodImpl(MethodImplOptions.NoInlining)]
private T Create() => _fastPolicy?.Create() ?? _policy.Create();

public override void Return(T obj)
{
    if (_isDefaultPolicy || (_fastPolicy?.Return(obj) ?? _policy.Return(obj)))
    {
        if (_firstItem != null || Interlocked.CompareExchange(ref _firstItem, obj, null) != null)
        {
            var items = _items;
            for (var i = 0; i < items.Length && Interlocked.CompareExchange(ref items[i].Element, obj, null) != null; ++i)
            {
            }
        }
    }
}

==從總體的設計思路來講，提供者Provider用來提供具體的對象池，傳入的參數也是策略類，而策略類則把控池對象的具體處理。dotnet core中不少源碼的開發模式都是這種：經過Policy構建Provider，經過Provider建立最終的類。也就是說，只經過策略類來實現自定義擴展，這種架構能夠借鑑和思考。==

官方實現：

Object reuse with ObjectPool in ASP.NET Core

參考示例：

.NET Core中Object Pool的簡單使用

.Net Core中的ObjectPool

.NET Core 3.0之深刻源碼理解ObjectPool（二）