Building AssetBundles

There are three class methods you can use to build AssetBundles:

BuildPipeline.BuildAssetBundle allows you to build AssetBundles of any type of asset.
BuildPipeline.BuildStreamedSceneAssetBundle is used when you want to include only scenes to be streamed and loaded as the data becomes available.
BuildPipeline.BuildAssetBundleExplicitAssetNames is the same as BuildPipeline.BuildAssetBundle but has an extra parameter to specify a custom string identifier (name) for each object.

An example of how to build an AssetBundle

Building asset bundles is done through editor scripting. There is basic example of this in the scripting documentation for BuildPipeline.BuildAssetBundle.

For the sake of this example, copy and paste the script from the link above into a new C# script called ExportAssetBundles. This script should be placed in a folder named Editor, so that it works inside the Unity Editor.

Now in the Assets menu, you should see two new menu options.

Build AssetBundle From Selection - Track dependencies. This will build the current object into an asset bundle and include all of its dependencies. For example if you have a prefab that consists of several hierarchical layers then it will recursively add all the child objects and components to the asset bundle.
Build AssetBundle From Selection - No dependency tracking. This is the opposite of the previous method and will only include the single asset you have selected.

For this example, you should create a new prefab. First create a new Cube by going to GameObject -> Create Other -> Cube, which will create a new cube in the Hierarchy View. Then drag the Cube from the Hierarchy View into the Project View, which will create a prefab of that object.

You should then right click the Cube prefab in the project window and select Build AssetBundle From Selection - Track dependencies. At this point you will be presented with a window to save the “bundled” asset. If you created a new folder called “AssetBundles” and saved the cube as Cube.unity3d, your project window will now look something like this.

At this point you can move the AssetBundle Cube.unity3d elsewhere on your local storage, or upload it to a server of your choice.

An example of how to change the properties of the assets when building an Asset Bundle

You can use AssetDatabase.ImportAsset to force reimporting the asset right before calling BuildPipeline.BuildAssetBundle, and then use AssetPostprocessor.OnPreprocessTexture to set the required properties. The following example will show you how to set different texture compressions when building the Asset Bundle.

C

// Builds an asset bundle from the selected objects in the project view,
// and changes the texture format using an AssetPostprocessor.

using UnityEngine;
using UnityEditor;

public class ExportAssetBundles {
    
    // Store current texture format for the TextureProcessor.
    public static TextureImporterFormat textureFormat;
    
    [MenuItem("Assets/Build AssetBundle From Selection - PVRTC_RGB2")]
    static void ExportResourceRGB2 () {
        textureFormat = TextureImporterFormat.PVRTC_RGB2;
        ExportResource();       
    }   
    
    [MenuItem("Assets/Build AssetBundle From Selection - PVRTC_RGB4")]
    static void ExportResourceRGB4 () {
        textureFormat = TextureImporterFormat.PVRTC_RGB4;
        ExportResource();
    }
    
    static void ExportResource () {
        // Bring up save panel.
        string path = EditorUtility.SaveFilePanel ("Save Resource", "", "New Resource", "unity3d");
        
        if (path.Length != 0) {
            // Build the resource file from the active selection.
            Object[] selection = Selection.GetFiltered(typeof(Object), SelectionMode.DeepAssets);
            
            foreach (object asset in selection) {
                string assetPath = AssetDatabase.GetAssetPath((UnityEngine.Object) asset);
                if (asset is Texture2D) {
                    // Force reimport thru TextureProcessor.
                    AssetDatabase.ImportAsset(assetPath);
                }
            }
            
            BuildPipeline.BuildAssetBundle(Selection.activeObject, selection, path, BuildAssetBundleOptions.CollectDependencies | BuildAssetBundleOptions.CompleteAssets);
            Selection.objects = selection;
        }
    }
}

C

// Changes the texture format when building the Asset Bundle.

using UnityEngine;
using UnityEditor;

public class TextureProcessor : AssetPostprocessor
{ 
    void OnPreprocessTexture() {
        TextureImporter importer = assetImporter as TextureImporter;
        importer.textureFormat = ExportAssetBundles.textureFormat;
    }
}

You can also control how the asset is imported using the AssetDatabase.ImportAssetOptions.

In a test environment, you sometimes need to test a change that require AssetBundles to be rebuilt. In these cases, it is advisable to use the option BuildAssetBundleOptions.UncompressedAssetBundle when you build the AssetBundles. This makes it faster to build and load the AssetBundles but they will also be bigger and therefore take longer to download.