YooAsset源码阅读-Downloader篇

YooAsset源码阅读-Downloader

继续 YooAsset 的 Downloader ，本文将详细介绍如何创建下载器相关代码

CreateResourceDownloaderByAll

关键类

• PlayModeImpl.cs
• ResourceDownloaderOperation.cs
• DownloaderOperation.cs
• BundleInfo.cs

CreateResourceDownloaderByAll 方法用于创建下载所有需要更新资源的下载器。

关键源代码

// PlayModeImpl.cs Line 110-115
ResourceDownloaderOperation IPlayMode.CreateResourceDownloaderByAll(int downloadingMaxNumber, int failedTryAgain)
{
    List<BundleInfo> downloadList = GetDownloadListByAll(ActiveManifest);
    var operation = new ResourceDownloaderOperation(PackageName, downloadList, downloadingMaxNumber, failedTryAgain);
    return operation;
}

// PlayModeImpl.cs Line 245-264
public List<BundleInfo> GetDownloadListByAll(PackageManifest manifest)
{
    if (manifest == null)
        return new List<BundleInfo>();

    List<BundleInfo> result = new List<BundleInfo>(1000);
    foreach (var packageBundle in manifest.BundleList)
    {
        var fileSystem = GetBelongFileSystem(packageBundle);
        if (fileSystem == null)
            continue;

        if (fileSystem.NeedDownload(packageBundle))
        {
            var bundleInfo = new BundleInfo(fileSystem, packageBundle);
            result.Add(bundleInfo);
        }
    }
    return result;
}

流程图

CreateResourceDownloaderByTags

关键类

• PlayModeImpl.cs
• ResourceDownloaderOperation.cs
• PackageBundle.cs

CreateResourceDownloaderByTags 方法用于创建下载指定标签资源的下载器，支持DLC（可下载内容）场景。

关键源代码

// PlayModeImpl.cs Line 116-121
ResourceDownloaderOperation IPlayMode.CreateResourceDownloaderByTags(string[] tags, int downloadingMaxNumber, int failedTryAgain)
{
    List<BundleInfo> downloadList = GetDownloadListByTags(ActiveManifest, tags);
    var operation = new ResourceDownloaderOperation(PackageName, downloadList, downloadingMaxNumber, failedTryAgain);
    return operation;
}

// PlayModeImpl.cs Line 265-297
public List<BundleInfo> GetDownloadListByTags(PackageManifest manifest, string[] tags)
{
    if (manifest == null)
        return new List<BundleInfo>();

    List<BundleInfo> result = new List<BundleInfo>(1000);
    foreach (var packageBundle in manifest.BundleList)
    {
        var fileSystem = GetBelongFileSystem(packageBundle);
        if (fileSystem == null)
            continue;

        if (fileSystem.NeedDownload(packageBundle))
        {
            // 如果未带任何标记，则统一下载
            if (packageBundle.HasAnyTags() == false)
            {
                var bundleInfo = new BundleInfo(fileSystem, packageBundle);
                result.Add(bundleInfo);
            }
            else
            {
                // 查询DLC资源
                if (packageBundle.HasTag(tags))
                {
                    var bundleInfo = new BundleInfo(fileSystem, packageBundle);
                    result.Add(bundleInfo);
                }
            }
        }
    }
    return result;
}

流程图

CreateResourceDownloaderByPaths

关键类

• PlayModeImpl.cs
• ResourceDownloaderOperation.cs
• AssetInfo.cs
• PackageManifest.cs

CreateResourceDownloaderByPaths 方法用于创建下载指定资源路径及其依赖的下载器，支持递归下载选项。

关键源代码

// PlayModeImpl.cs Line 122-127
ResourceDownloaderOperation IPlayMode.CreateResourceDownloaderByPaths(AssetInfo[] assetInfos, bool recursiveDownload, int downloadingMaxNumber, int failedTryAgain)
{
    List<BundleInfo> downloadList = GetDownloadListByPaths(ActiveManifest, assetInfos, recursiveDownload);
    var operation = new ResourceDownloaderOperation(PackageName, downloadList, downloadingMaxNumber, failedTryAgain);
    return operation;
}

// PlayModeImpl.cs Line 298-359 (核心逻辑)
public List<BundleInfo> GetDownloadListByPaths(PackageManifest manifest, AssetInfo[] assetInfos, bool recursiveDownload)
{
    if (manifest == null)
        return new List<BundleInfo>();

    // 获取资源对象的资源包和所有依赖资源包
    List<PackageBundle> checkList = new List<PackageBundle>();
    foreach (var assetInfo in assetInfos)
    {
        if (assetInfo.IsInvalid)
        {
            YooLogger.Warning(assetInfo.Error);
            continue;
        }

        // 获取主资源包
        PackageBundle mainBundle = manifest.GetMainPackageBundle(assetInfo.Asset);
        if (checkList.Contains(mainBundle) == false)
            checkList.Add(mainBundle);

        // 获取依赖资源包
        List<PackageBundle> mainDependBundles = manifest.GetAssetAllDependencies(assetInfo.Asset);
        foreach (var dependBundle in mainDependBundles)
        {
            if (checkList.Contains(dependBundle) == false)
                checkList.Add(dependBundle);
        }

        // 递归下载主资源包内所有资源对象依赖的资源包
        if (recursiveDownload)
        {
            foreach (var otherMainAsset in mainBundle.IncludeMainAssets)
            {
                var otherMainBundle = manifest.GetMainPackageBundle(otherMainAsset.BundleID);
                if (checkList.Contains(otherMainBundle) == false)
                    checkList.Add(otherMainBundle);

                List<PackageBundle> otherDependBundles = manifest.GetAssetAllDependencies(otherMainAsset);
                foreach (var dependBundle in otherDependBundles)
                {
                    if (checkList.Contains(dependBundle) == false)
                        checkList.Add(dependBundle);
                }
            }
        }
    }

    // 筛选需要下载的资源包
    List<BundleInfo> result = new List<BundleInfo>(1000);
    foreach (var packageBundle in checkList)
    {
        var fileSystem = GetBelongFileSystem(packageBundle);
        if (fileSystem == null)
            continue;

        if (fileSystem.NeedDownload(packageBundle))
        {
            var bundleInfo = new BundleInfo(fileSystem, packageBundle);
            result.Add(bundleInfo);
        }
    }
    return result;
}

流程图

下载器核心机制

ResourceDownloaderOperation 继承关系

下载器状态机

DownloaderOperation 使用状态机管理下载流程：

下载器池管理机制

DownloaderOperation 采用动态下载器池来优化下载性能，主要特性：

1. 最大并发限制：通过 MAX_LOADER_COUNT = 64 和 _downloadingMaxNumber 控制同时下载的文件数量
2. 失败重试机制：支持 _failedTryAgain 参数控制失败重试次数
3. 动态调度：当有下载器完成时，自动从待下载列表中选择新的文件开始下载
4. 暂停/恢复：支持 PauseDownload() 和 ResumeDownload() 控制下载状态
5. 进度回调：提供详细的下载进度、错误、开始等事件回调

需要注意 DownloaderOperation.InternalUpdate 方法中的关键逻辑：

// 动态创建新的下载器到最大数量限制
// 注意：如果期间有下载失败的文件，暂停动态创建下载器
if (_bundleInfoList.Count > 0 && _failedList.Count == 0)
{
    if (_isPause)
        return;

    if (_downloaders.Count < _downloadingMaxNumber)
    {
        int index = _bundleInfoList.Count - 1;
        var bundleInfo = _bundleInfoList[index];
        var downloader = bundleInfo.CreateDownloader(_failedTryAgain);
        downloader.StartOperation();
        this.AddChildOperation(downloader);

        _downloaders.Add(downloader);
        _bundleInfoList.RemoveAt(index);
    }
}

从用户调用到UnityWebRequest的完整调用链

看完上面的分析，我觉得还需要把整个调用链梳理清楚，这样更容易理解整个下载流程。

调用链概述

用户调用下载器到最终发起UnityWebRequest的完整调用链是这样的：

用户代码 → downloader.BeginDownload() → OperationSystem.Update() → DownloaderOperation.InternalUpdate() 
→ BundleInfo.CreateDownloader() → DefaultCacheFileSystem.DownloadFileAsync() → DownloadPackageBundleOperation
→ DownloadCenter.DownloadFileAsync() → UnityDownloadFileOperation → UnityWebFileRequestOperation → UnityWebRequest

详细调用链分析

1. 用户入口点

// 用户代码
var downloader = package.CreateResourceDownloaderByAll(10, 3);
downloader.BeginDownload(); // 开始下载

2. BeginDownload → InternalUpdate

// DownloaderOperation.cs
public void BeginDownload()
{
    // 开始下载会调用 OperationSystem.StartOperation
    OperationSystem.StartOperation(PackageName, this);
}

// OperationSystem每帧会调用
internal override void InternalUpdate()
{
    // 在Loading状态时会动态创建下载器
    if (_bundleInfoList.Count > 0 && _failedList.Count == 0)
    {
        var bundleInfo = _bundleInfoList[index];
        var downloader = bundleInfo.CreateDownloader(_failedTryAgain); // 关键调用
        downloader.StartOperation();
        this.AddChildOperation(downloader);
    }
}

3. BundleInfo.CreateDownloader

// BundleInfo.cs Line 39-44
public FSDownloadFileOperation CreateDownloader(int failedTryAgain)
{
    DownloadFileOptions options = new DownloadFileOptions(failedTryAgain);
    options.ImportFilePath = _importFilePath;
    return _fileSystem.DownloadFileAsync(Bundle, options); // 委托给FileSystem
}

4. DefaultCacheFileSystem.DownloadFileAsync

// DefaultCacheFileSystem.cs Line 172-191
public virtual FSDownloadFileOperation DownloadFileAsync(PackageBundle bundle, DownloadFileOptions options)
{
    // 获取下载地址
    string mainURL = RemoteServices.GetRemoteMainURL(bundle.FileName);
    string fallbackURL = RemoteServices.GetRemoteFallbackURL(bundle.FileName);
    options.SetURL(mainURL, fallbackURL);

    // 创建具体的下载操作
    var downloader = new DownloadPackageBundleOperation(this, bundle, options);
    return downloader;
}

5. DownloadPackageBundleOperation

// DownloadPackageBundleOperation.cs Line 58-72
if (_steps == ESteps.CreateRequest)
{
    string url = GetRequestURL();
    // 委托给DownloadCenter创建Unity下载器
    _unityDownloadFileOp = _fileSystem.DownloadCenter.DownloadFileAsync(Bundle, url);
    _steps = ESteps.CheckRequest;
}

6. DownloadCenter → UnityDownloadFileOperation

// DownloadCenterOperation.cs
public UnityDownloadFileOperation DownloadFileAsync(PackageBundle bundle, string url)
{
    // 根据断点续传等条件创建不同的下载器
    if (bundle.FileSize >= _fileSystem.ResumeDownloadMinimumSize)
    {
        return new ResumeDownloadOperation(_fileSystem, bundle, url);
    }
    else
    {
        return new NormalDownloadOperation(_fileSystem, bundle, url);
    }
}

7. UnityDownloadFileOperation → UnityWebRequest

// UnityWebFileRequestOperation.cs Line 72-81
private void CreateWebRequest()
{
    DownloadHandlerFile handler = new DownloadHandlerFile(_fileSavePath);
    handler.removeFileOnAbort = true;
    
    // 最终创建UnityWebRequest
    _webRequest = DownloadSystemHelper.NewUnityWebRequestGet(_requestURL);
    _webRequest.timeout = _timeout;
    _webRequest.downloadHandler = handler;
    _webRequest.disposeDownloadHandlerOnDispose = true;
    
    // 发起网络请求
    _requestOperation = _webRequest.SendWebRequest();
}

完整调用链流程图

关键调用点解析

1. 责任分离设计

• DownloaderOperation：管理下载器池和并发控制
• BundleInfo：适配器，统一不同FileSystem的接口
• DefaultCacheFileSystem：处理URL获取和参数配置
• DownloadPackageBundleOperation：状态机管理，重试逻辑
• DownloadCenter：并发限制和下载器复用
• UnityDownloadFileOperation：Unity网络请求的具体实现

2. 异步调用链

整个调用链是异步的，通过 OperationSystem 的 Update 循环驱动：

// 主要更新循环
YooAsset.Update() → OperationSystem.Update() → AsyncOperationBase.Update() → InternalUpdate()

3. 错误处理和重试

在调用链的每一层都有相应的错误处理：

• DownloaderOperation：失败隔离策略
• DownloadPackageBundleOperation：重试逻辑和超时处理
• UnityWebRequest：网络层错误处理