AsyncTextureCreator.cs

using UnityEngine;

public class AsyncTextureCreator : MonoBehaviour
{
    public int width = 128;
    public int height = 32;
}

AsyncTextureCreatorEditor.cs

using UnityEngine;
using UnityEditor;
using Unity.Burst;
using Unity.Jobs;
using Unity.Collections;
using Unity.Mathematics;

[BurstCompile]
internal struct CreateTextureJob : IJob
{
    [WriteOnly] public NativeArray<Color32> texColors;
    [ReadOnly] [DeallocateOnJobCompletion] public NativeArray<float> array;

    Color ToRGB(float hue)
    {
        hue = (1f - math.cos(math.PI * hue)) * 0.5f;
        hue = 1f - hue;
        hue *= 5f;
        var x = 1 - math.abs(hue % 2f - 1f);
        return
            hue < 1f ? new Color(1f, x, 0f) :
            hue < 2f ? new Color(x, 1f, 0f) :
            hue < 3f ? new Color(0f, 1f, x) :
            hue < 4f ? new Color(0f, x, 1f) :
            new Color(x * 0.5f, 0f, 0.5f);
    }

    public void Execute()
    {
        for (int i = 0; i < array.Length; ++i)
        {
            texColors[i] = ToRGB(array[i]);
        }
    }
}

[CustomEditor(typeof(AsyncTextureCreator))]
public class AsyncTextureCreatorEditor : Editor
{
    AsyncTextureCreator creator => target as AsyncTextureCreator;
    Texture2D _texture;
    JobHandle _jobHandle;
    bool _isTextureUpdateRequested = false;

    bool isTextureUpdateRequired => 
        !_texture || 
        _texture.width != creator.width || 
        _texture.height != creator.height;
    
    void UpdateTexture()
    {
        if (_isTextureUpdateRequested)
        {
            _jobHandle.Complete();
            _texture.Apply();
            _isTextureUpdateRequested = false;
        }

        if (!isTextureUpdateRequired) return;
        
        int width = creator.width;
        int height = creator.height;
        _texture = new Texture2D(width, height);
            
        var texColors = _texture.GetPixelData<Color32>(0);
        var array = new NativeArray<float>(width * height, Allocator.TempJob);

        for (int y = 0; y < height; ++y)
        {
            for (int x = 0; x < width; ++x)
            {
                int i = y * width + x;
                array[i] = i / 256f;
            }
        }

        var job = new CreateTextureJob()
        {
            texColors = texColors,
            array = array,
        };
        
        _jobHandle = job.Schedule();
        _isTextureUpdateRequested = true;
        
        Repaint();
    }
    
    public override void OnInspectorGUI()
    {
        DrawDefaultInspector();
        
        EditorGUILayout.Space();

        UpdateTexture();
        var area = EditorGUILayout.GetControlRect(false, creator.height);
        GUI.DrawTexture(area, _texture, ScaleMode.StretchToFill);
    }
}