How do I get a Unity Job to update external data?

Question

I'm relatively new to Unity, and I wanted to use the Job system for a complex pathfinding algorithm, since it will likely be used alongside multiple instances of NPCs attempting to find a valid route. Just as a test, I ran a single instance:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using Unity.Mathematics;
using Unity.Collections;
using Unity.Jobs;
using Unity.Burst;
using System.Linq;
using Unity.VisualScripting;

public class Pathfinding : MonoBehaviour {



    public List<Vector3> PathList = null;

    public static Pathfinding Instance { get; private set; }

    public Pathfinding() {
        Instance = this;
    }

    void Start(){
        Vector3 startPosition = new Vector3(1, 2);
        Vector3 startPosition2 = new Vector3(2, 3);
        PathList = FindPath(startPosition, startPosition2);
    }
    


     public List<Vector3> FindPath(Vector3 startPosition, Vector3 endPosition){

        Debug.Log("Activated");
        List<Vector3> pathList = null;
        NativeList<int2> paths = new NativeList<int2>(Allocator.Persistent);
        


        // NativeArray<JobHandle> jobHandleArray = new NativeArray<JobHandle>(findPathJobCount, Allocator.TempJob);

        
        FindPathJob findPathJob = new FindPathJob { 
                startPosition = new int2((int) startPosition.x, (int) startPosition.y), 
                endPosition = new int2((int) endPosition.x, (int) endPosition.y),
                paths = paths // output paths
                
            };

            
        JobHandle jobHandle = findPathJob.Schedule();
        jobHandle.Complete();

        if(paths.Length != 0){
            Debug.Log("paths not empty");
            for(int i = paths.Length-1; i >= 0; i--){
                int xcoord = paths[i].x;
                int ycoord = paths[i].y;
                pathList.Add(new Vector3(xcoord, ycoord));
            }
        }
        else{
            Debug.Log("paths are empty");
        }
        
        paths.Dispose();
                
        
        // JobHandle.CompleteAll(jobHandleArray);
        // jobHandle.Dispose();
        return pathList;
     }

    

    [BurstCompile]
    private struct FindPathJob : IJob {

        public int2 startPosition;
        public int2 endPosition;
        public NativeList<int2> paths;

        public void Execute() {
            int2 gridSize = new int2(100, 100);

            NativeArray<PathNode> pathNodeArray = new NativeArray<PathNode>(gridSize.x * gridSize.y, Allocator.Temp);

// pathfinding algorithm code


            if (endNode.cameFromNodeIndex == -1) {
                // Didn't find a path!
                Debug.Log("Didn't find a path!");
            } else {
                // Found a path
                paths = CalculatePath(pathNodeArray, endNode);
                Debug.Log("Found Paths!");
                Debug.Log("Paths size: " + paths.Length);
                
                
                foreach (int2 pathPosition in paths) {
                    Debug.Log(pathPosition);
                }
                
                
            }

            pathNodeArray.Dispose();
            neighbourOffsetArray.Dispose();
            openList.Dispose();
            closedList.Dispose();
        }
        
        


    }

}

The print statement when a path is found in the job prints "Found Job!" and a valid path, but the code calling paths in the FindPath() function instead says "paths are empty". My question is, how do I get paths in FindPath() to update when it is changed in the job?

Note that there is little to no benefit to schedule a job on a separate thread with JobHandle jobHandle = findPathJob.Schedule(); and then pause the main thread until it is finished by calling jobHandle.Complete(); immediately afterwards. A better pattern is to let the job run in the background and periodically check its status via jobHandle.IsCompleted in subsequent Updates or in a coroutine. — Philipp
– Philipp, Commented Jan 15 at 12:48

eero1111 · Accepted Answer · 2025-01-14 15:07:25Z

First of all, don't use constructors in MonoBehaviours like this:

    public Pathfinding() {
        Instance = this;
    }

their use is undocumented and undefined by Unity. Use Awake/Start instead for initializing MonoBehaviours. (also see: In Unity, how do I correctly implement the singleton pattern? for correct singleton implementation)

Second, the reason why your jobs code doesn't work is because you are reassigning paths to a new NativeList (which is using Allocator.Temp) inside of the job. It's not safe to access paths after that from outside the job. From Unity docs:

Temp allocations are only safe to use in the thread and the scope where they were allocated.

source: Allocator Overview

To fix it, you should assign the result of CalculatePath inside of the job to a temp paths variable, and then copy the values from there into paths.

Something like this:

  [BurstCompile]
    private struct FindPathJob : IJob {

        public int2 startPosition;
        public int2 endPosition;
        public NativeList<int2> paths;



        public void Execute() {
            int2 gridSize = new int2(100, 100);

            NativeArray<PathNode> pathNodeArray = new NativeArray<PathNode>(gridSize.x * gridSize.y, Allocator.Temp);

// pathfinding algorithm code


            if (endNode.cameFromNodeIndex == -1) {
                // Didn't find a path!
                Debug.Log("Didn't find a path!");
            } else {
                // Found a path
                NativeList<int2> calculatedPaths = CalculatePath(pathNodeArray, endNode);
                Debug.Log("Found Paths!");
                Debug.Log("Paths size: " + calculatedPaths.Length);
                
                
                foreach (int2 pathPosition in calculatedPaths) {
                    paths.Add(pathPosition);
                    Debug.Log(pathPosition);
                }
                
                
            }

            pathNodeArray.Dispose();
            neighbourOffsetArray.Dispose();
            openList.Dispose();
            closedList.Dispose();
        }

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