NativeContainerAttribute

描述

允许您创建自己的自定义原生容器。

Native containers let you create new container types, which do not allocate any GC memory and give explicit control over allocations. They data contained must be blittable. Native Containers can also be used on jobs, the job system understand NativeContainers and the job debugger ensures that all access to the containers is safe and throws exceptions if any usage code contains race conditions or contains non-deterministic behaviour.

原生容器必须嵌入一个 AtomicSafetyHandle，以确保作业调试系统能够检测到所有可能的竞争条件。DisposeSentinel 用于立即检测任何泄漏。

Note that creating your own custom container has to be done by carefully following the code example below. It is strongly recommended to add test coverage for all scenarios when creating a custom container, particularly for the integration into jobs, ensuring that all race conditions are prevented. When implemented incorrectly, a custom container can easily crash Unity without throwing any useful exception.

using System.Diagnostics;
using System;
using Unity.Collections.LowLevel.Unsafe;
using Unity.Collections;

// Marks our struct as a NativeContainer.
// If ENABLE_UNITY_COLLECTIONS_CHECKS is enabled,
// it is required that m_Safety with exactly this name.
[NativeContainer]
// The [NativeContainerSupportsMinMaxWriteRestriction] enables
// a common jobification pattern where an IJobParallelFor is split into ranges
// And the job is only allowed to access the index range being Executed by that worker thread.
// Effectively limiting access of the array to the specific index passed into the Execute(int index) method
// This attribute requires m_MinIndex & m_MaxIndex to exist.
// and the container is expected to perform out of bounds checks against it.
// m_MinIndex & m_MaxIndex will be set by the job scheduler before Execute is called on the worker thread.
[NativeContainerSupportsMinMaxWriteRestriction]
// It is recommended to always implement a Debugger proxy
// to visualize the contents of the array in VisualStudio and other tools.
[DebuggerDisplay("Length = {Length}")]
[DebuggerTypeProxy(typeof(NativeCustomArrayDebugView<>))]
public unsafe struct NativeCustomArray<T> : IDisposable where T : unmanaged
{
    internal void* m_Buffer;
    internal int m_Length;

#if ENABLE_UNITY_COLLECTIONS_CHECKS
    internal int m_MinIndex;
    internal int m_MaxIndex;
    internal AtomicSafetyHandle m_Safety;
    internal static readonly SharedStatic<int> s_staticSafetyId = SharedStatic<int>.GetOrCreate<NativeCustomArray<T>>();
#endif

    internal Allocator m_AllocatorLabel;

    public NativeCustomArray(int length, Allocator allocator)
    {
        long totalSize = UnsafeUtility.SizeOf<T>() * length;

#if ENABLE_UNITY_COLLECTIONS_CHECKS
        // Native allocation is only valid for Temp, Job and Persistent
        if (allocator <= Allocator.None)
            throw new ArgumentException("Allocator must be Temp, TempJob or Persistent", "allocator");
        if (length < 0)
            throw new ArgumentOutOfRangeException("length", "Length must be >= 0");
        if (!UnsafeUtility.IsBlittable<T>())
            throw new ArgumentException(string.Format("{0} used in NativeCustomArray<{0}> must be blittable", typeof(T)));
#endif

        m_Buffer = UnsafeUtility.Malloc(totalSize, UnsafeUtility.AlignOf<T>(), allocator);
        UnsafeUtility.MemClear(m_Buffer, totalSize);

        m_Length = length;
        m_AllocatorLabel = allocator;

#if ENABLE_UNITY_COLLECTIONS_CHECKS
        m_MinIndex = 0;
        m_MaxIndex = length - 1;
        m_Safety = CollectionHelper.CreateSafetyHandle(allocator);
        CollectionHelper.SetStaticSafetyId<NativeCustomArray<T>>(ref m_Safety, ref s_staticSafetyId.Data);
#endif
    }

    public int Length { get { return m_Length; } }

    public unsafe T this[int index]
    {
        get
        {
#if ENABLE_UNITY_COLLECTIONS_CHECKS
            // If the container is currently not allowed to read from the buffer
            // then this will throw an exception.
            // This handles all cases, from already disposed containers
            // to safe multithreaded access.
            AtomicSafetyHandle.CheckReadAndThrow(m_Safety);

            // Perform out of range checks based on
            // the NativeContainerSupportsMinMaxWriteRestriction policy
            if (index < m_MinIndex || index > m_MaxIndex)
                FailOutOfRangeError(index);
#endif
            // Read the element from the allocated native memory
            return UnsafeUtility.ReadArrayElement<T>(m_Buffer, index);
        }

        set
        {
#if ENABLE_UNITY_COLLECTIONS_CHECKS
            // If the container is currently not allowed to write to the buffer
            // then this will throw an exception.
            // This handles all cases, from already disposed containers
            // to safe multithreaded access.
            AtomicSafetyHandle.CheckWriteAndThrow(m_Safety);

            // Perform out of range checks based on
            // the NativeContainerSupportsMinMaxWriteRestriction policy
            if (index < m_MinIndex || index > m_MaxIndex)
                FailOutOfRangeError(index);
#endif
            // Writes value to the allocated native memory
            UnsafeUtility.WriteArrayElement(m_Buffer, index, value);
        }
    }

    public T[] ToArray()
    {
#if ENABLE_UNITY_COLLECTIONS_CHECKS
        AtomicSafetyHandle.CheckReadAndThrow(m_Safety);
#endif

        var array = new T[Length];
        for (var i = 0; i < Length; i++)
            array[i] = UnsafeUtility.ReadArrayElement<T>(m_Buffer, i);
        return array;
    }

    public bool IsCreated
    {
        get { return m_Buffer != null; }
    }

    public void Dispose()
    {
#if ENABLE_UNITY_COLLECTIONS_CHECKS
        CollectionHelper.DisposeSafetyHandle(ref m_Safety);
#endif

        UnsafeUtility.Free(m_Buffer, m_AllocatorLabel);
        m_Buffer = null;
        m_Length = 0;
    }

#if ENABLE_UNITY_COLLECTIONS_CHECKS
    private void FailOutOfRangeError(int index)
    {
        if (index < Length &amp;&amp; (m_MinIndex != 0 || m_MaxIndex != Length - 1))
            throw new IndexOutOfRangeException(string.Format(
                "Index {0} is out of restricted IJobParallelFor range [{1}...{2}] in ReadWriteBuffer.\n" +
                "ReadWriteBuffers are restricted to only read &amp; write the element at the job index. " +
                "You can use double buffering strategies to avoid race conditions due to " +
                "reading &amp; writing in parallel to the same elements from a job.",
                index, m_MinIndex, m_MaxIndex));

        throw new IndexOutOfRangeException(string.Format("Index {0} is out of range of '{1}' Length.", index, Length));
    }

#endif
}

// Visualizes the custom array in the C# debugger
internal sealed class NativeCustomArrayDebugView<T> where T : unmanaged
{
    private NativeCustomArray<T> m_Array;

    public NativeCustomArrayDebugView(NativeCustomArray<T> array)
    {
        m_Array = array;
    }

    public T[] Items
    {
        get { return m_Array.ToArray(); }
    }
}