AssetsManager 的怪异行为

@wangfuguii 写道：

最近测试 AssetsManagerEx 的时候发现有几个无法理解的行为，想请教引擎组的大大。

1，下载资源清单文件失败后，state 不会变化到 UNCHECKED，导致不能重试 update 方法：

可以看到如果读取不到资源清单的 url 会切换状态到 UNCHECKED 并且发送 ERROR_DOWNLOAD_MANIFEST 事件：

void AssetsManagerEx::downloadManifest()
{
    if (_updateState != State::PREDOWNLOAD_MANIFEST)
        return;

    std::string manifestUrl = _localManifest->getManifestFileUrl();
    if (manifestUrl.size() > 0)
    {
        _updateState = State::DOWNLOADING_MANIFEST;
        // Download version file asynchronously
        _downloader->createDownloadFileTask(manifestUrl, _tempManifestPath, MANIFEST_ID);
    }
    // No manifest file found
    else
    {
        CCLOG("AssetsManagerEx : No manifest file found, check update failed\n");
        dispatchUpdateEvent(EventAssetsManagerEx::EventCode::ERROR_DOWNLOAD_MANIFEST);
        _updateState = State::UNCHECKED;
    }
}

但是在下载错误的回调方法中只是发送了 ERROR_DOWNLOAD_MANIFEST 事件后并没有切换状态，也没有接着发送UPDATE_FAILED 事件，其设计是下载资源清单失败后 hang 在这里吗？

void AssetsManagerEx::onError(const network::DownloadTask& task,
                              int errorCode,
                              int errorCodeInternal,
                              const std::string& errorStr)
{
    // Skip version error occurred
    if (task.identifier == VERSION_ID)
    {
        CCLOG("AssetsManagerEx : Fail to download version file, step skipped\n");
        _updateState = State::PREDOWNLOAD_MANIFEST;
        downloadManifest();
    }
    else if (task.identifier == MANIFEST_ID)
    {
        dispatchUpdateEvent(EventAssetsManagerEx::EventCode::ERROR_DOWNLOAD_MANIFEST, task.identifier, errorStr, errorCode, errorCodeInternal);
    }
    else
    {
        auto unitIt = _downloadUnits.find(task.identifier);
        // Found unit and add it to failed units
        if (unitIt != _downloadUnits.end())
        {
            _totalWaitToDownload--;
            
            DownloadUnit unit = unitIt->second;
            _failedUnits.emplace(unit.customId, unit);
        }
        dispatchUpdateEvent(EventAssetsManagerEx::EventCode::ERROR_UPDATING, task.identifier, errorStr, errorCode, errorCodeInternal);
    }
}

2，我在 checkUpdate 后如果收到 NEW_VERSION_FOUND 后会立即调用 update 方法，在 Mac 平台上，会出现提示 More than one download file task write to same file: ...，也能收到 ERROR_DOWNLOAD_MANIFEST 通知，但是到了这一步之后更新流程并没有中断，随后又会下载资源清单文件，这是我的日志：

Simulator: check update 尚未检查是否存在新版本
Simulator: get state 正在下载版本manifest文件
Simulator: get event code 更新过程的进度事件
Simulator: Processing 100.00%
Simulator: get state 检查到新版本，需要更新
Simulator: get event code 找到新版本
Simulator: New version found
Simulator: found new version, call update
Simulator: Fail to download manifest file, hot update skipped.
Simulator: update finished， null More than one download file task write to same file:/Applications/CocosCreator.app/...
Simulator: get state 正在下载资源manifest文件
Simulator: get event code 更新过程的进度事件

通过阅读源码发现是 DownloadTaskCURL 的

// include platform specific implement class
#if (CC_TARGET_PLATFORM == CC_PLATFORM_MAC)

#include "network/CCDownloader-apple.h"
#include "network/CCDownloader-curl.h"
#define DownloaderImpl  DownloaderCURL

...

     if (_sStoragePathSet.end() != _sStoragePathSet.find(_tempFileName))
            {
                // there is another task uses this storage path
                _errCode = DownloadTask::ERROR_FILE_OP_FAILED;
                _errCodeInternal = 0;
                _errDescription = "More than one download file task write to same file:";
                _errDescription.append(_tempFileName);
                return false;
            }
            _sStoragePathSet.insert(_tempFileName);

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