使用Assetbundle时可能遇到的坑

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AssetBundles (Pro only)

AssetBundles are files which you can export from Unity to contain assets of your choice. These files use a proprietary compressed format and can be loaded on demand by your application. This allows you to stream in content, such as models, textures, audio clips, or even entire scenes separately from the scene in which they will be used. AssetBundles have been designed to simplify downloading content to your application. AssetBundles can contain any kind of asset type recognized by Unity, as determined by the filename extension. If you want to include files with custom binary data, they should have the extension ".bytes". Unity will import these files as TextAssets.

When working with AssetBundles, here‘s the typical workflow:

During development, the developer prepares AssetBundles and uploads them to a server.

                 

1. Building AssetBundles. Asset bundles are created in the editor from assets in your scene. The Asset Bundle building process is described in more detail in the section for Building AssetBundles
    
2. Uploading AssetBundles to external storage. This step does not include the Unity Editor or any other Unity channels, but we include it for completeness. You can use an FTP client to upload your Asset Bundles to the server of your choice.

At runtime, on the user‘s machine, the application will load AssetBundles on demand and operate individual assets within each AssetBundle as needed.

                       Downloading AssetBundles and loading assets from them

1. Downloading AssetBundles at runtime from your application. This is done from script within a Unity scene, and Asset Bundles are loaded from the server on demand. More on that in Downloading Asset Bundles.
    
2. Loading objects from AssetBundles. Once the AssetBundle is downloaded, you might want to access its individual Assets from the Bundle. More on that in Loading Resources from AssetBundles

Please read this section of the documentation thoroughly to familiarize yourself with the workflow for using AssetBundles, discover the different features they provide and learn best practices that can save you time and effort during development.

See also:

• Frequently Asked Questions
• Building AssetBundles
• Downloading Asset Bundles
• Loading Asset Bundles
• Keeping track of loaded AssetBundles
• Storing and loading binary data
• Protecting content
• Managing Asset Dependencies
• Including scripts in AssetBundles

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.

 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.

1. 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. 
2. 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.

Building AssetBundles in a production enviroment

When first using AssetBundles it may seem enough to manually build them as seen in the previous example. But as a project grows in size and the number of assets increases doing this process by hand is not efficient. A better approach is to write a function that builds all of the AssetBundles for a project. You can, for example, use a text file that maps Asset files to AssetBundle files.

back to AssetBundles Intro

Streaming Assets

Application.dataPath

Application.streamingAssetsPath

	// print the path to the streaming assets folder	var filePath = System.IO.Path.Combine(Application.streamingAssetsPath, "MyFile");	var result = "";	if (filePath.Contains("://"))	{		var www = new WWW (filePath);		yield www;		result = www.text;	}	else		result = System.IO.File.ReadAllText(filePath);

You can retrieve the folder using theApplication.streamingAssetsPath property. For reference, the location of this folder varies per platform:

On a desktop computer (Mac OS or Windows) the location of the files can be obtained with the following code:

  path = Application.dataPath + "/StreamingAssets";

On iOS, you should use:

  path = Application.dataPath + "/Raw";

...while on Android, you should use:

  path = "jar:file://" + Application.dataPath + "!/assets/";

Note that on Android, the files are contained within a compressed .jar file (which is essentially the same format as standard zip-compressed files). This means that if you do not use Unity‘s WWW class to retrieve the file then you will need to use additional software to see inside the .jar archive and obtain the file.

Unity 3D : 製作與載入 AssetBundle