Version: 2022.2
Language : English
Copying NativeContainer structures
Create and run a job

Custom NativeContainer example

The following is a complete example of a custom NativeContainer as an append-only list. It demonstrates basic protection of read and write operations, as well as creating and invalidating aliasing views. For another example, see the NativeContainerAttribute API documentation.

using System;
using System.Runtime.InteropServices;
using Unity.Collections.LowLevel.Unsafe;
using Unity.Collections;

// Marks the struct as a NativeContainer. This tells the job system that it contains an AtomicSafetyHandle.
[NativeContainer]
public unsafe struct NativeAppendOnlyList<T> : IDisposable where T : unmanaged
{
    // Raw pointers aren't usually allowed inside structures that are passed to jobs, but because it's protected
    // with the safety system, you can disable that restriction for it
    [NativeDisableUnsafePtrRestriction]
    internal void* m_Buffer;
    internal int m_Length;
    internal Allocator m_AllocatorLabel;

    // You should only declare and use safety system members with the ENABLE_UNITY_COLLECTIONS_CHECKS define.
    // In final builds of projects, the safety system is disabled for performance reasons, so these APIs aren't
    // available in those builds.
#if ENABLE_UNITY_COLLECTIONS_CHECKS
    
    // The AtomicSafetyHandle field must be named exactly 'm_Safety'.
    internal AtomicSafetyHandle m_Safety;
    
    // Statically register this type with the safety system, using a name derived from the type itself
    internal static readonly int s_staticSafetyId = AtomicSafetyHandle.NewStaticSafetyId<NativeAppendOnlyList<T>>();
#endif

    public NativeAppendOnlyList(Allocator allocator, params T[] initialItems)
    {
        m_Length = initialItems.Length;
        m_AllocatorLabel = allocator;

        // Calculate the size of the initial buffer in bytes, and allocate it
        int totalSize = UnsafeUtility.SizeOf<T>() * m_Length;
        m_Buffer = UnsafeUtility.MallocTracked(totalSize, UnsafeUtility.AlignOf<T>(), m_AllocatorLabel, 1);

        // Copy the data from the array into the buffer
        var handle = GCHandle.Alloc(initialItems, GCHandleType.Pinned);
        try
        {
            UnsafeUtility.MemCpy(m_Buffer, handle.AddrOfPinnedObject().ToPointer(), totalSize);
        }
        finally
        {
            handle.Free();
        }

#if ENABLE_UNITY_COLLECTIONS_CHECKS
        // Create the AtomicSafetyHandle and DisposeSentinel
        m_Safety = AtomicSafetyHandle.Create();

        // Set the safety ID on the AtomicSafetyHandle so that error messages describe this container type properly.
        AtomicSafetyHandle.SetStaticSafetyId(ref m_Safety, s_staticSafetyId);
        
        // Automatically bump the secondary version any time this container is scheduled for writing in a job
        AtomicSafetyHandle.SetBumpSecondaryVersionOnScheduleWrite(m_Safety, true);

        // Check if this is a nested container, and if so, set the nested container flag
        if (UnsafeUtility.IsNativeContainerType<T>()) 
            AtomicSafetyHandle.SetNestedContainer(m_Safety, true);
#endif
    }

    public int Length
    {
        get
        {
#if ENABLE_UNITY_COLLECTIONS_CHECKS
            // Check that you are allowed to read information about the container 
            // This throws InvalidOperationException if you aren't allowed to read from the native container,
            // or if the native container has been disposed
            AtomicSafetyHandle.CheckReadAndThrow(m_Safety);
#endif
            return m_Length;
        }
    }

    public T this[int index]
    {
        get
        {
#if ENABLE_UNITY_COLLECTIONS_CHECKS
            // Check that you can read from the native container right now.
            AtomicSafetyHandle.CheckReadAndThrow(m_Safety);
#endif

            // Read from the buffer and return the value
            return UnsafeUtility.ReadArrayElement<T>(m_Buffer, index);
        }

        set
        {
#if ENABLE_UNITY_COLLECTIONS_CHECKS
            // Check that you can write to the native container right now.
            AtomicSafetyHandle.CheckWriteAndThrow(m_Safety);
#endif
            // Write the value into the buffer
            UnsafeUtility.WriteArrayElement(m_Buffer, index, value);
        }
    }

    public void Add(T value)
    {
#if ENABLE_UNITY_COLLECTIONS_CHECKS
        // Check that you can modify (write to) the native container right now, and if so, bump the secondary version so that
        // any views are invalidated, because you are going to change the size and pointer to the buffer
        AtomicSafetyHandle.CheckWriteAndBumpSecondaryVersion(m_Safety);
#endif

        // Replace the current buffer with a new one that has space for an extra element
        int newTotalSize = (m_Length + 1) * UnsafeUtility.SizeOf<T>();
        void* newBuffer = UnsafeUtility.MallocTracked(newTotalSize, UnsafeUtility.AlignOf<T>(), m_AllocatorLabel, 1);
        UnsafeUtility.MemCpy(newBuffer, m_Buffer, m_Length * UnsafeUtility.SizeOf<T>());
        UnsafeUtility.FreeTracked(m_Buffer, m_AllocatorLabel);
        m_Buffer = newBuffer;
        
        // Put the new element at the end of the buffer and increase the length
        UnsafeUtility.WriteArrayElement(m_Buffer, m_Length++, value);
    }

    public NativeArray<T> AsArray()
    {
#if ENABLE_UNITY_COLLECTIONS_CHECKS
        // Check that it's safe for you to use the buffer pointer to construct a view right now.
        AtomicSafetyHandle.CheckGetSecondaryDataPointerAndThrow(m_Safety);
        
        // Make a copy of the AtomicSafetyHandle, and mark the copy to use the secondary version instead of the primary
        AtomicSafetyHandle handleForArray = m_Safety;
        AtomicSafetyHandle.UseSecondaryVersion(ref handleForArray);
#endif

        // Create a new NativeArray which aliases the buffer, using the current size. This doesn't allocate or copy
        // any data, it just sets up a NativeArray<T> which points at the m_Buffer.
        var array = NativeArrayUnsafeUtility.ConvertExistingDataToNativeArray<T>(m_Buffer, m_Length, Allocator.None);
        
#if ENABLE_UNITY_COLLECTIONS_CHECKS
        // Set the AtomicSafetyHandle on the newly created NativeArray to be the one that you copied from your handle
        // and made to use the secondary version.
        NativeArrayUnsafeUtility.SetAtomicSafetyHandle(ref array, handleForArray);
#endif
        
        return array;
    }

    public void Dispose()
    {
#if ENABLE_UNITY_COLLECTIONS_CHECKS
        AtomicSafetyHandle.CheckDeallocateAndThrow(m_Safety);
        AtomicSafetyHandle.Release(m_Safety);
#endif

        // Free the buffer
        UnsafeUtility.FreeTracked(m_Buffer, m_AllocatorLabel);
        m_Buffer = null;
        m_Length = 0;
    }
}

Additional resources

Copying NativeContainer structures
Create and run a job
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