Revision as of 19:41, 23 October 2011 by gnuton (Talk | contribs)

Rotating QWidget based applications on MeeGo Harmattan

From Nokia Developer Wiki
Jump to: navigation, search

This article explains how to rotate QtWidget-based applications on Harmattan...

Article Metadata
Code ExampleTested with
Devices(s): Nokia N9, Nokia N950
Platform(s): MeeGo 1.2 Harmattan
Keywords: QWidget, Orientation
Created: gnuton (11 Nov 2011)
Last edited: gnuton (23 Oct 2011)



Qt is a framework used to develop application software with a graphical user interface (GUI). It allows developers to write the application GUI using QWidget(pure C++), QtComponents(C++/QML/JS) or HTML(C++/HTML/JS).

MeeGo 1.2 Harmattan, Nokia's latest platform in the market, offers to application developers a set of QML Components which can be used to design amazing interfaces where touch input, fluid animations and user experience are crucial. QML Components have been optimized to squeeze out all the GPU power, providing developers with an easy way of coding which doesn't cause performance loss as QWidget does. For this reason, Harmattan doesn't really support QWidgets. Porting a Symbian or Maemo QWidget-based application to the new platform requires the re-writing of the entire UI. Writing a UI with QML is much easier than with QWidget and the porting doesn't really take so much time. However, some developers don't really want to spend time on QML and they want to run the app on MeeGo as it is.

Limitations of QWidgets on Harmattan

QWidgets has many limitations on Harmattan; the most important are:

  • Application orientation locked in landscape
  • No MeeGo Style

In this article we will talk about the first limitation and how it can be worked around.

Why don't QWidget-based apps rotate on Harmattan?

A mobile application can be locked in landscape or potrait or even be left free to change according to the device orientation value provided by the accelerometer sensor. QWidget class provides a setAttribute method that was largely used on Symbian and Maemo to set the application orientation. QWidget::setAttribute(orientation) on Harmattan doesn't work anymore because X11 server has been built without RandR extension ([X Resize, Rotate and Reflect http://en.wikipedia.org/wiki/RandR]). This means that the screen cannot rotate anymore and therefore it is the application itself which has to rotate. This is an important feature, allowing MeeGo to have a fancy and smooth transition effect when an application switches from one orientation mode to another.

How to work around this problem

In order to make the application work in portrait mode QWidgets need to be rotated. Unfortuantely QWidgets, originally designed for Desktop, are not able to rotate. The only way to rotate a QWidget is to paint it in a canvas and rotate it. Qt canvas is called QGraphicsScene. The scene is displayed to the user by a QGraphicsView and the items in the QGraphicsScene are QGraphicsItems. In order to paint a QWidget in a QGraphicsScene we need a QGraphicsProxyWidget. The easiest solution to rotate the app would be to paint the main window on QGraphicsScene and then rotate the scene. Unfortunately this is not really the best solution because developers would need to write the animation code of the transition between the two orientation modes and to adapt the application so that it shows the system bar on the top part of the screen.

A better option is to make the graphics item which paints the QWidget a QML element.

The solution: "Embedding" a QWidget in a MeeGo Component based application

As it has been said before, MeeGo application uses MeeGo QML components and our goal is to have our application working with different orientation. A MeeGo component application is made by a [Window] and some [Pages]. So to make our application scalable, we will to make the main window of our application child of a Page. For this purpose we have to "transform" our MainWindow widget in QDeclarataiveItem as the code below does.


Then the new object is registered and made available to the declarative view

 qmlRegisterType<ProxyWidget>("ProxyWidget", 1, 0, "ProxyWidgetItem");

As said before we could comment the following line since it doesn't really work:

import ProxyWidget 1.0
Page { tools: commonTools Label { id: label anchors.centerIn: parent text: qsTr("Hello world!") visible: false } Button{ anchors { horizontalCenter: parent.horizontalCenter top: label.bottom topMargin: 10 } text: qsTr("Click here!") onClicked: label.visible = true } ProxyWidgetItem { anchors.fill: parent }}

Canned Solution

To make your application working in easier I made a template which you could reuse. The instruction to follow in order to use it are [here]. Basically the canned example works in this way. Your application is built as static library and its main widget is loaded by the proxy widget we have seen before.


Biggest limitation of this solution are:

  • QTreeWidget::setItemWidget doesn't work as expected: the bug has been reported here
  • If a WebView microfocus doesn't work correctly. The virtual keyboard doesn't disappear when input html element lose the focus: SOLUTION

So far those are the current issue. Please edit this page or leave a comment in case you find new ones. Thanks.

Add categories below. Remove Category:Draft when the page is complete or near complete


164 page views in the last 30 days.

Was this page helpful?

Your feedback about this content is important. Let us know what you think.


Thank you!

We appreciate your feedback.