feat: assetbundle使用介绍

Design/UsingAssetBundle.md

+# 使用AssetBundle
+
+*首先，相比AssetBundle，更推荐使用[Addressable Assets System](UsingAddressable.md)*
+
+本文对AssetBundle使用做简要介绍，同时分析四种下载方式的内存占用情况
+
+## 一、AssetBundle使用
+注意：小游戏环境不支持assetbundle本地加载
+### 1.1 组织资源
+自行组织资源如何分组，以及各个bundle之间依赖关系，包名相同的资源会打包到一起
+<image src="../image/assetbundle/set-bundlename.png" width="600" />
+### 1.2 AssetBundle打包
+```c#
+public static void Build()
+{
+    string dst = Application.streamingAssetsPath + "/AssetBundles";
+    if (!Directory.Exists(dst))
+    {
+        Directory.CreateDirectory(dst);
+    }
+    BuildPipeline.BuildAssetBundles(dst, BuildAssetBundleOptions.AppendHashToAssetBundleName | BuildAssetBundleOptions.ChunkBasedCompression | BuildAssetBundleOptions.None, BuildTarget.WebGL);
+}
+```
+打包bundle时，请使用如下参数
+- 【重要】BuildAssetBundleOptions.AppendHashToAssetBundleName：bundle带上hash。在小游戏底层对bundle做缓存及缓存淘汰时，hash是重要依据。
+- BuildAssetBundleOptions.ChunkBasedCompression：LZ4压缩方式，加载速度和包体大小更均衡。
+
+### 1.3 AssetBundle下载
+从服务器下载bundle的方式主要以下四种。
+**注意：WWW和WWW.LoadFromCacheOrDownload已不被Unity推荐使用**
+
+- UnityWebRequestAssetBundle.GetAssetBundle
+  ```c#
+  UnityWebRequest request = UnityWebRequestAssetBundle.GetAssetBundle(uriPath);
+  yield return request.SendWebRequest();
+  if (request.isHttpError)
+  {
+      Debug.LogError(GetType() + "/ERROR/" + request.error);
+  }
+  else
+  {
+      AssetBundle ab = (request.downloadHandler as DownloadHandlerAssetBundle).assetBundle;
+  }
+  ```
+- UnityWebRequest
+  ```c#
+  UnityWebRequest www = new UnityWebRequest(uriPath);
+  DownloadHandlerAssetBundle handler = new DownloadHandlerAssetBundle(www.uri.ToString(), 0);
+  www.downloadHandler = handler;
+  yield return www.Send();
+  if (www.isHttpError)
+  {
+      Debug.LogError(GetType() + "/ERROR/" + www.error);
+  } else
+  {
+      AssetBundle ab = handler.assetBundle
+  }
+  ```
+- WWW.LoadFromCacheOrDownload
+  ```c#
+  WWW www = WWW.LoadFromCacheOrDownload(uriPath, 2);
+  yield return www;
+  if (www.error != null)
+  {
+      Debug.Log(www.error);
+      yield break;
+  }
+  AssetBundle ab = www.assetBundle;
+  ```
+- WWW
+  ```c#
+  WWW www = new WWW(uriPath);
+  yield return www;
+  if (www.error != null)
+  {
+      Debug.Log(www.error);
+      yield break;
+  }
+  AssetBundle ab = www.assetBundle;
+  ```
+
+界面如下
+<image src="../image/assetbundle/assetbundle-scene.png" width="600" />
+
+按钮从上往下加载方式分别为使用`UnityWebRequestAssetBundle.GetAssetBundle`, `UnityWebRequest`, `WWW.LoadFromCacheOrDownload`, `WWW`。
+
+### 1.4 AssetBundle内资源加载
+下载完毕后，加载bundle内资源可使用`AssetBundle.LoadAsset`代码示意如下
+```c#
+StartCoroutine(UnityWebRequestLoad(string.Format("http://127.0.0.1:8080/StreamingAssets/AssetBundles/{0}", bundleName), (ab) => {
+    Sprite img = ab.LoadAsset<Sprite>("deer");
+    image.GetComponent<SpriteRenderer>().sprite = img;
+    ab1 = ab;
+    img1Loaded = true;
+}));
+```
+
+## 二、AssetBundle下载API内存分析
+
+加载的bundle文件大小为5403162字节
+```bash
+-rw-r--r--  1 junkunzhang  TENCENT\Domain Users  5403162  6 22 16:07 image1_751427a66d22b184c95342d556354150
+```
+通过加载前后snapshot来对比总内存变化
+- UnityWebRequestAssetBundle.GetAssetBundle/UnityWebRequest/WWW
+   这三种加载方式加载前后内存无变化
+   <image src="../image/assetbundle/normal-assetbundle-memory.png" />
+
+- WWW.LoadFromCacheOrDownload
+   加载bundle后内存增加了5403162字节，增加了bundle大小的内存，是因为WWW.LoadFromCacheOrDownload除了会将文件存入IndexDB外，还会存入内存中的UnityCache
+   <image src="../image/assetbundle/assetbundle-unnormal-memory.png" />
+
+**因此需要注意业务中不要使用已淘汰的WWW类，尤其WWW.LoadFromCacheOrDownload，当bundle数量多时，会浪费不少内存。**
+
+## 三、参考资料
+1. Introduction to Asset Bundles
+  https://learn.unity.com/tutorial/introduction-to-asset-bundles#6028bab6edbc2a750bf5b8a7
+2. Unity WebGL Memory Optimization: Part Deux
+  https://blog.kongregate.com/unity-webgl-memory-optimization-part-deux/
+3. Understanding Memory in Unity WebGL
+  https://blog.unity.com/technology/understanding-memory-in-unity-webgl
+4. AssetBundle API
+  https://docs.unity3d.com/2019.4/Documentation/ScriptReference/AssetBundle.html
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