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<std::string, Manifest::AssetDiff> 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);
}
}
}