Cocos 更新时反复杀进程时，差异更新失效的问题：

问题复现步骤：

1、在project.manifest.temp 文件下载成功后，下载Assets资源的时候杀掉进程

2、重启游戏，继续更新时会使用上次下载成功的project.manifest.temp文件，这个时候因为没有将文件下载状态保存，而更新的时候又判断没有下再成功就去下载，就导致将所有文件都下载了。

下载流程分析

1、 project.manifest.temp 文件下载成功之前如果kill进程，下次进入就会删除这个文件，重新下载；PS：调用update

接口开始更新时kill进程，再次启动都会引起project.manifest.temp的重新下载，这也是导致bug的原因之一。

// 这个方法会在更新之前调用void AssetsManagerEx::initManifests(const std::string& manifestUrl){    _inited = true;    // Init and load local manifest    _localManifest = new (std::nothrow) Manifest();    if (_localManifest)    {        loadLocalManifest(manifestUrl);        // Init and load temporary manifest        _tempManifest = new (std::nothrow) Manifest();        if (_tempManifest)        {            _tempManifest->parse(_tempManifestPath);            // 这里判断如果文件不是完整的，并且存在就删除它；            if (!_tempManifest->isLoaded() && _fileUtils->isFileExist(_tempManifestPath))                _fileUtils->removeFile(_tempManifestPath);        }        else        {            _inited = false;        }        // Init remote manifest for future usage        _remoteManifest = new (std::nothrow) Manifest();        if (!_remoteManifest)        {            _inited = false;        }    }    else    {        _inited = false;    }    if (!_inited)    {        CC_SAFE_DELETE(_localManifest);        CC_SAFE_DELETE(_tempManifest);        CC_SAFE_DELETE(_remoteManifest);    }}

2、开始下载根据project.manifest.temp 临时文件是否完整存在来决定是恢复之前的下载状态，还是根据重新下载回来的manifest文件与本地文件对比差异度，决定下载那些。

void AssetsManagerEx::startUpdate(){    if (_updateState != State::NEED_UPDATE)        return;    _updateState = State::UPDATING;    // Clean up before update    _failedUnits.clear();    _downloadUnits.clear();    _compressedFiles.clear();    _totalWaitToDownload = _totalToDownload = 0;    _percent = _percentByFile = _sizeCollected = _totalSize = 0;    _downloadedSize.clear();    _totalEnabled = false;    // Temporary manifest exists, resuming previous download    if (_tempManifest->isLoaded() && _tempManifest->versionEquals(_remoteManifest))    {        // 文件是完整的，直接从文件恢复下载，并且从文件中读取下载状态，来判断是否需要下载        // 更新完成之前kill进程和每次启动程序下载project.manifest.temp文件都会导致下载状态被清空        // 恢复下载是根据manifest临时文件中保存的下载状态来决定，详细见3        _tempManifest->genResumeAssetsList(&_downloadUnits);        _totalWaitToDownload = _totalToDownload = (int)_downloadUnits.size();        this->batchDownload();        std::string msg = StringUtils::format("Resuming from previous unfinished update, %d files remains to be finished.", _totalToDownload);        CCLOG(msg);        // this time , the remoteManifest(has no DownloadState) file overwrite tempManifest file. when we kill process, and restart, it will uses error file.      // 为了避免被重新下载的manifest文件覆盖掉下载状态，我们这里需要将当前状态再次写入文件备份        _tempManifest->saveToFile(_tempManifestPath);        dispatchUpdateEvent(EventAssetsManagerEx::EventCode::UPDATE_PROGRESSION, "", msg);    }    // Check difference    else    {        // Temporary manifest not exists or out of date,        // it will be used to register the download states of each asset,        // in this case, it equals remote manifest.        _tempManifest->release();        _tempManifest = _remoteManifest;        std::unordered_map
    
      diff_map = _localManifest->genDiff(_remoteManifest);        std::string log = StringUtils::format("AssetsManagerEx : Diff file size %d", diff_map.size());        CCLOG(log);        if (diff_map.size() == 0)        {            CCLOG("AssetsManagerEx updateSucceed");            updateSucceed();        }        else        {            // Generate download units for all assets that need to be updated or added            std::string packageUrl = _remoteManifest->getPackageUrl();            for (auto it = diff_map.begin(); it != diff_map.end(); ++it)            {                Manifest::AssetDiff diff = it->second;                if (diff.type == Manifest::DiffType::DELETED)                {                    CCLOG("AssetsManagerEx DELETED " + diff.asset.path);                    _fileUtils->removeFile(_storagePath + diff.asset.path);                }                else                {                    std::string path = diff.asset.path;                    CCLOG("AssetsManagerEx " + diff.asset.path + " type "+ diff.type);                    // Create path                    _fileUtils->createDirectory(basename(_storagePath + path));                    DownloadUnit unit;                    unit.customId = it->first;                    unit.srcUrl = packageUrl + path;                    unit.storagePath = _storagePath + path;                    _downloadUnits.emplace(unit.customId, unit);                }            }            // Set other assets' downloadState to SUCCESSED            auto &assets = _remoteManifest->getAssets();            for (auto it = assets.cbegin(); it != assets.cend(); ++it)            {                const std::string &key = it->first;                auto diffIt = diff_map.find(key);                if (diffIt == diff_map.end())                {                    // 根据文件对比，将不需要下载的文件的状态设置为下载成功                    _tempManifest->setAssetDownloadState(key, Manifest::DownloadState::SUCCESSED);                }            }            _totalWaitToDownload = _totalToDownload = (int)_downloadUnits.size();            this->batchDownload();            std::string msg = StringUtils::format("Start to update %d files from remote package.", _totalToDownload);                       // 为了避免进程在更新完成之前被kill导致下载状态丢失，这里先保存文件，备份一次            _tempManifest->saveToFile(_tempManifestPath);            dispatchUpdateEvent(EventAssetsManagerEx::EventCode::UPDATE_PROGRESSION, "", msg);        }    }}
    

3、恢复下载是，需要根据之前保存在临时文件中的下载状态来决定下载那些文件

void Manifest::genResumeAssetsList(DownloadUnits *units) const{    for (auto it = _assets.begin(); it != _assets.end(); ++it)    {        Asset asset = it->second;        if (asset.downloadState != DownloadState::SUCCESSED)        {            DownloadUnit unit;            unit.customId = it->first;            unit.srcUrl = _packageUrl + asset.path;            unit.storagePath = _manifestRoot + asset.path;            units->emplace(unit.customId, unit);        }    }}