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Network caching in Qt

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Article Metadata
Tested with
Devices(s): Tested on Nokia N96,
Nokia 6210 Navigator
Platform(s): S60 3rd Edition, FP1
S60 3rd Edition, FP2
S60 5th Edition
S60 5th Edition
S60 3rd Edition FP2
Keywords: QNetworkDiskCache
Created: User:Kbwiki (30 Nov 2009)
Last edited: hamishwillee (11 Oct 2012)


This article demonstrates how to use network caching in Qt.


The QNetworkDiskCache class provides a very basic disk cache. It can be used with QNetworkAccessManager to store the requested web pages into a specific directory and get the cached content whenever required. Each cached file has a cache_ prefix and .cache extension in the file name.

Network Caching

Required headers

 #include <QNetworkAccessManager>
#include <QUrl>
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QNetworkDiskCache>

Required modules

 QT += network

NetWorkCacheTest is our test class, derived from QWidget:

   class NetWorkCacheTest : public QWidget
NetWorkCacheTest( QWidget *parent = 0 );
// ...
void sendRequest();
public slots:
// Slot to receive finished signal from QNetworkAccessManager
void finishedSlot( QNetworkReply* aReply );
QNetworkAccessManager* m_nam;
QNetworkDiskCache* m_diskCache;
QUrl m_url;

Constructor implementation

  NetWorkCacheTest::NetWorkCacheTest( QWidget *parent )
: QWidget( parent )
m_url.setUrl( <nowiki>""</nowiki> );
m_nam = new QNetworkAccessManager( this );
// Connect the finished signal to receive network replies
connect( m_nam, SIGNAL( finished( QNetworkReply* ) ), this, SLOT( finishedSlot( QNetworkReply* ) ) );
// Create an instance of disk cache and set cache directory
m_diskCache = new QNetworkDiskCache( this );
QString sysCacheDir( "c:\\data" );
m_diskCache->setCacheDirectory( sysCacheDir );
// Assign cache to network manager and send a request
m_nam->setCache( m_diskCache );

sendRequest() implementation

  void NetWorkCacheTest::sendRequest()
QNetworkRequest request( m_url );
// Load from cache if available, otherwise load from network, may return stale data
request.setAttribute( QNetworkRequest::CacheLoadControlAttribute, QNetworkRequest::PreferCache );
// Data obtained should be saved to cache for future uses
request.setAttribute( QNetworkRequest::CacheSaveControlAttribute, true );
// Posts a request to obtain the contents of the target request,
// finished signal will get triggered when network reply is available
m_nam->get( QNetworkRequest( request ) );

Handling the network reply

  void NetWorkCacheEx::finishedSlot( QNetworkReply* reply )
// Get and display the last modified header
QVariant lmod = reply->header( QNetworkRequest::LastModifiedHeader );
if ( lmod.isValid() ) {
QDateTime lmodDate = lmod.toDateTime();
QString lmodStr = lmodDate.toString( "dd.MM.yyyy" );
QMessageBox::information( this, "Last Modified", lmodStr, "ok" );
// Show expiry date
if ( reply->hasRawHeader( "Expires" ) ) {
QString expires = reply->rawHeader( "Expires" );
QMessageBox::information( this, "Expiry date", expires, "ok" );
// Show Cache-Control header
if ( reply->hasRawHeader( "Cache-Control" ) ) {
QString cCntrl = reply->rawHeader( "Cache-Control" );
QMessageBox::information( this, "Cache-Control", cCntrl, "ok" );
// Check if the data was obtained from cache or not
QVariant fromCache = reply->attribute( QNetworkRequest::SourceIsFromCacheAttribute );
if ( fromCache.toBool() ) {
// Data is from cache
else {
// Data is from network
if ( reply->error() == QNetworkReply::NoError ) {
// Display received data in web view (needs webkit module added)
// QWebView view;
// Reading bytes form the reply
// QByteArray bytes = reply->readAll();
// view.setHtml(bytes);
else { // Error handling
int error = reply->error();
QMessageBox::information( this, "Error", reply->errorString(), "ok" );
This page was last modified on 11 October 2012, at 01:17.
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