Legacy Documentation: Version 5.0
AssetBundles FAQ
Enhanced Asset Bundle Workflow

BuildingAssetBundles in 5.x

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Authoring AssetBundles

In Unity 4.x AssetBundles were created using editor scripts. To simplify this process Unity 5.x Editor includes AssetBundle authoring tools. Now, if you select an Asset in the editor a drop down will appear at the bottom of the Inspector window inviting you to specify which AssetBundle (if any) the asset should be packaged into. By default the AssetBundle option is set to None, meaning the asset will not be written into an AssetBundle. You can create new AssetBundle, give them names, and then use these new AssetBundle names as the destination for the asset.

AssetBundle creation
AssetBundle creation

In the picture the StateMachine asset has been added to an AssetBundle called scene1/animdata. This AssetBundle may contain other assets which have previously been added.

Empty AssetBundles can be created by using the “New…” menu option. After you have created a new, empty AssetBundle it will appear in the list allowing you to add assets to it. AssetBundle names are always lower-case. If you use upper-case characters in the name they will be converted to lower-case. Using a forward slash in the name of the AssetBundle effectively creates folders, and the menu will have sub-menus, as the picture above shows.

If you create AssetBundles which have no assets assigned to them, then the “Remove Unused Names” option can be used. This will delete the empty AssetBundle.

The meta file belonging to an Asset will have the chosen AssetBundle name written into it.

Exporting AssetBundles

AssetBundles are exported from the editor using script code. (This is similar to the 4.x approach.) The following script exports AssetBundles:

using UnityEditor;

public class CreateAssetBundles
    [MenuItem ("Assets/Build AssetBundles")]
    static void BuildAllAssetBundles ()
        BuildPipeline.BuildAssetBundles ("AssetBundles");

This script creates a menu item at the bottom of the Assets menu. When that menu item is selected the function will be called and the AssetBundles will be built. This will start a build dialog with a progress bar. The BuildPipeline.BuildAssetBundles function will create the AssetBundles into an output folder called “AssetBundles”. This folder must have been created in the project folder. Within that folder the AssetBundles that have been labelled will be created.

Each AssetBundle that is exported will have the name created in the AssetBundle menu. Additionally, each AssetBundle will have an associated file with a .manifest extension. This manifest file is a text file that you can open with any text editor. It provides information such as the file CRC and asset dependencies. The AssetBundle in the example above has a manifest file that looks like this:

ManifestFileVersion: 0
CRC: 2422268106
    serializedVersion: 2
    Hash: 8b6db55a2344f068cf8a9be0a662ba15
    serializedVersion: 2
    Hash: 37ad974993dbaa77485dd2a0c38f347a
HashAppended: 0
- Class: 91
  Script: {instanceID: 0}
  Asset_0: Assets/Mecanim/StateMachine.controller
Dependencies: {}

In addition to these, there are another two files created: another AssetBundle and another manifest file. These two are always created whenever AssetBundles are created. They are created for each folder that AssetBundles are created in, thus if you always create AssetBundles in the same place, you will only get two extra files. The additional manifest file - in this example AssetBundles.manifest - can be used in much the same way as other manifest files but will show information on how AssetBundles relate and depend on each other. In this case, since we only have a single AssetBundle, it has no other dependencies.

ManifestFileVersion: 0
      Name: scene1assetbundle
      Dependencies: {}

AssetBundle Editor Tools

Getting names of AssetBundles

The following editor script can display the names of the AssetBundles which the build process can create.

using UnityEditor;
using UnityEngine;

public class GetAssetBundleNames
    [MenuItem ("Assets/Get AssetBundle names")]
    static void GetNames ()
        var names = AssetDatabase.GetAllAssetBundleNames();
        foreach (var name in names)
            Debug.Log ("AssetBundle: " + name);

Getting told when an asset changes AssetBundle

You can use the OnPostprocessAssetbundleNameChanged method from the AssetPostprocessor class, to get a callback when the AssetBundle an asset is associated with changes.

using UnityEngine;
using UnityEditor;

public class MyPostprocessor : AssetPostprocessor {

    void OnPostprocessAssetbundleNameChanged ( string path,
            string previous, string next) {
        Debug.Log("AB: " + path + " old: " + previous + " new: " + next);

AssetBundle Variants

AssetBundle Variants is a new 5.x feature. This can be used to achieve a result similar to virtual assets. For example, you can set AssetBundle a variants like “MyAssets.hd” and “MyAssets.sd”. Make sure the assets exactly match. The objects in these two variant AssetBundles will have the exactly same internal IDs which is ensured by the Unity build pipeline. So these two variant AssetBundles can be switched out arbitrarily with AssetBundles of different variant extension at runtime.

How to set assetbundle variants: 1. From the editor use the one extra variant name, to the right of the asset labels GUI. 2. In code use AssetImporter.assetBundleVariant option.

AssetBundle variants
AssetBundle variants

The full AssetBundle name will be the combination of the AssetBundle name and the variant name. For example, if you want to add “MyAssets.hd” as a variant AssetBundle, you should set the AssetBundle name to “MyAssets” and AssetBundle variant to “hd”. And the final AssetBundle is “MyAssets.hd”.

But if you only set the AssetBundle name like “MyAssets.hd”, then it’s just a normal AssetBundle which is not variant AssetBundle. “MyAssets”+“hd” and “MyAssets.hd”+”” cannot coexist as they lead to the same full AssetBundle name.

Scripting Advice

API to mark the asset into AssetBundle

Simple APIs

Simple APIs are available to build AssetBundles, BuildPipeline.BuildAssetBundles(). These APIs are pretty simple, you only need to provide:

  • Output path for all the AssetBundles.
  • BuildAssetBundleOptions which will be described later.
  • BuildTarget which is same as before.
  • Also have one overloaded version to provide an array of AssetBundleBuild which contains one map from assets to AsssetBundles. This provides flexibility to you, you can set your mapping information by script and build from it. And this mapping information won’t break/replace the existed one in the asset database.

APIs to manipulate AssetBundle names in the asset database


Manifest file

A manifest file is created for every AssetBundle which contains the following information:

  • The manifest file is next to the AssetBundle.
  • CRC
  • Asset file hash. A single hash for all the assets included in this AssetBundle, only used for incremental build check.
  • Type tree hash. A single hash for all the types included in this AssetBundle, only used for incremental build check.
  • Class types. All the class types included in this AssetBundle. These are used to get the new single hash when doing the type tree incremental build check.
  • Asset names. All the assets explicitly included in this AssetBundle.
  • Dependent AssetBundle names. All the AssetBundles which this AssetBundle depends on.
  • This manifest file is only used for incremental build, not necessary for runtime.

Single manifest file

We generate a single manifest file which includes:

  • All the AssetBundles.
  • All the AssetBundle dependencies.

Single manifest AssetBundle

It only contains an AssetBundleManifest object which has following APIs:

AssetBundle loading APIs changed

Now we have:


A typetree is written to the AssetBundle by default. The only exception is Metro as it has different serialization solution.

AssetBundles FAQ
Enhanced Asset Bundle Workflow