Unity AssetBundle Dependencies

Unity AssetBundle Dependencies

In the last few weeks I’ve spent quite a lot of time with Unity’s Asset Bundle system. Understanding how dependencies were tracked. What determines GC cleanup of assets and understanding why the editor profiler produces particular results has been a bit of a struggle. I’m lucky enough to work for a group that allows me to have access to the Unity source code however so this has probably been slightly less painful than it has been for others going down this same path.

First, I’d like to acknowledge what appears to be the most useful piece of Unity Answers information that I’ve come across, located here. This seems to explain the general mechanics of how to include dependencies using the push and then pop method in an editor script.

For my purposes, I created a test project that contained several prefabs all containing simple GUI elements. These elements all used the same fonts and referenced the same textures. This was to verify that, in the editor profiler and in instruments, assets were in fact being shared.

I’ve included my example editor script below to demonstrate some of the methods I used for packing the bundles. In my case, I am traversing all assets in my prefab folder and treating elements with the word “Global” in their name as shared. If I were to have many dependency bundles instead of the one that I have in this example I would have to sort the order in which I am packing these bundles.

using UnityEngine;
using UnityEditor;

using System.IO;
using System.Collections.Generic;

public class AssetBundleBuild : MonoBehaviour
{
    [MenuItem("AssetBundle/Build Bundles")]
    private static void BuildBundles()
    {
        DirectoryInfo directory = new DirectoryInfo(Application.dataPath + "/Prefabs");
        FileInfo[] files = directory.GetFiles("*.prefab", SearchOption.AllDirectories);

        bool shouldPush;

        BuildPipeline.PushAssetDependencies();
        foreach (FileInfo f in files)
        {
            shouldPush = !f.Name.Contains("Global");
            if (shouldPush)
            {
                BuildPipeline.PushAssetDependencies();
            }

            // Get each asset path
            string path = Application.dataPath + "/Bundles/" + f.Name.Substring(0, f.Name.LastIndexOf(".")) + ".bundle";
            string assetPath = f.FullName.Substring(f.FullName.IndexOf("Assets", f.FullName.Length - f.FullName.IndexOf("Assets")));

            Object asset = AssetDatabase.LoadMainAssetAtPath(assetPath);
            //Debug.Log("Asset to pack " + asset + " , " + asset.name);

            BuildAssetBundleOptions options = 
                BuildAssetBundleOptions.DisableWriteTypeTree | 
                BuildAssetBundleOptions.CollectDependencies | 
                BuildAssetBundleOptions.CompleteAssets | 
                BuildAssetBundleOptions.DeterministicAssetBundle;

            if (!shouldPush)
            {
                Object[] d = EditorUtility.CollectDependencies(new Object[] { asset });

                List<Object> dSource = new List<Object>();
                List<string> dNames = new List<string>();

                // In this case I'm attempting to manually collect dependencies for tracking purposes
                // however this does not always seem to be necessary unless you have complex prefab heirarchies
                foreach (Object o in d)
                {
                    if (o != null && !dSource.Contains(o))
                    {
                        Debug.Log(" -- d " + o + " , " + o.name + " , " + o.GetType());

                        dSource.Add(o);
                        dNames.Add(o.name);
                    }
                }

                Debug.Log("::BUILDING DEPENDENCY BUNDLE:: " + asset.name + " , " + dSource.Count);
                BuildPipeline.BuildAssetBundleExplicitAssetNames(
                    dSource.ToArray(), 
                    dNames.ToArray(), 
                    path, 
                    options, 
                    EditorUserBuildSettings.activeBuildTarget);
            }
            else
            {
                Debug.Log("::NON DEPENDENCY:: " + asset.name);
                BuildPipeline.BuildAssetBundleExplicitAssetNames(
                    new Object[] { asset }, 
                    new string[] { asset.name }, 
                    path, 
                    options, 
                    EditorUserBuildSettings.activeBuildTarget);

                if (shouldPush)
                {
                    BuildPipeline.PopAssetDependencies();
                }
            }
        }

        BuildPipeline.PopAssetDependencies();

        Debug.Log("[AssetBundleBuild] Complete.");
    }
}

Now, from the standpoint of packing shared dependencies, this seemed to work with most asset types such as textures or prefabs but when I included fonts, while they wouldn’t be duplicated across multiple bundles, I would always see two copies of my fonts. One set would be flagged as being ‘used by scripts and native code’ and the other would only be flagged as ‘used by native code’. After performing numerous tests, I discovered that upon opening the dependency bundle containing the font I was interested in, if there was a copy of the font in the project directory as well, both versions would be loaded into Resources. I haven’t been able to verify whether this happens on device as well but my inclination is that it only occurs in the editor.

The Problem of the Decompression Buffer

Another problem that became visible when working with large quantities of bundles that (based off of conversations with Unity technical representatives and the 4.2.0b3 beta client) may have changed in such a way as to render the problem less dire is that Unity, as of 4.1.5, allocates an 8mb decompression buffer per each asset bundle being opened. If there is an attempt to parallelize these requests, each request will allocate it’s own decompression buffer. This can be rather disconcerting as one can see how the allocation amount (especially on restricted memory devices) can really get a guy down.

Although 4.2.0b3 seems to be reducing the decompression buffer size down to 0.5mb per bundle the problem of parallelization still persists. The only immediate solution for individuals loading any quantity of bundles seems to be amortizing the requests in such a way as to prevent too much overlap. If someone out there has a suggestion to mitigate this problem otherwise please drop me a line mcelroy.jon[at]gmail.com

출처 : http://blog.jonmcelroy.com/post/55526714506/unity-assetbundle-dependencies