QT/WebKit::diskCache 淘汰机制

当当前的cache文件大小之和大于MAX_DISKCACHE_SIZE的90%时，older cache文件会被删除直到cache文件大小之和小于MAX_DISKCACHE_SIZE的90%，其余的10%相当于备用，这样就不会出现某一条 cache记录装不进去的现象，除非MAX_DISKCACHE_SIZE非常小。这里的older cache是由系统时钟决定的。cache被创建时会有一个时间信息，而当被引用时，不会另外去创建新的cache文件，而仅仅是更新该cache文件的时间信息。那些时间信息越早的cache文件会被优先淘汰。

qint64 QNetworkDiskCache::expire()
{
    Q_D(QNetworkDiskCache);
    if (d->currentCacheSize >= 0 && d->currentCacheSize < maximumCacheSize())
        return d->currentCacheSize;

    if (cacheDirectory().isEmpty()) {
        qWarning() << "QNetworkDiskCache::expire() The cache directory is not set";
        return 0;
    }

    QDir::Filters filters = QDir::AllDirs | QDir:: Files | QDir::NoDotAndDotDot;
    QDirIterator it(cacheDirectory(), filters, QDirIterator::Subdirectories);

    QMultiMap cacheItems;
    qint64 totalSize = 0;
    while (it.hasNext()) {
        QString path = it.next();
        QFileInfo info = it.fileInfo();
        QString fileName = info.fileName();
        if (fileName.endsWith(CACHE_POSTFIX) && fileName.startsWith(CACHE_PREFIX)) {
            cacheItems.insert(info.created(), path);
            totalSize += info.size();
        }
    }

    int removedFiles = 0;
    qint64 goal = (maximumCacheSize() * 9) / 10;
    QMultiMap::const_iterator i = cacheItems.constBegin();
    while (i != cacheItems.constEnd()) {
        if (totalSize < goal)
            break;
        QString name = i.value();
        QFile file(name);
        qint64 size = file.size();
        file.remove();
        totalSize -= size;
        ++removedFiles;
        ++i;
    }
#if defined(QNETWORKDISKCACHE_DEBUG)
    if (removedFiles > 0) {
        qDebug() << "QNetworkDiskCache::expire()"
                << "Removed:" << removedFiles
                << "Kept:" << cacheItems.count() - removedFiles;
    }
#endif
    if (removedFiles > 0)
        d->lastItem.reset();
    return totalSize;
}