Montag, 4. Oktober 2010

Qt: Write once, #ifdef everywhere?

Despite what the title of this post might suggest, I really like Qt. But as a developer, I also know that "write once, run everywhere" isn't realistic without writing some special-cased platform-specific code. Qt does take care of many platform-specific things, and I think it's the closest you can get to "write once, run everywhere" right now.

This post should serve two purposes: To provide a real-world example of what needs to be done in the code for the app to work on both Maemo 5 and Symbian^3 (with example code), and to get suggestions on what parts I could rewrite in a more platform-agnostic manner with existing APIs (so please comment if you are in the know!).

Here's the story: In early September, I've rewritten my game "That Rabbit Game" to use QGraphicsView on the N900, and in the last weeks, I've ported it to Symbian^3. I use the macro Q_OS_SYMBIAN to check for Symbian and Q_WS_MAEMO_5 to check for Fremantle. Here's the current main menu on a N900 and N8 (different screen resolutions and aspect ratios):

Qt modules: In the qmake project file, I can use linux-g++-maemo5 to add Maemo-specific configuration, and symbian for Symbian-specific settings. I use D-Bus module on Maemo, but obviously not on Symbian, so my project file contains something like this:

linux-g++-maemo5 {
QT += dbus

This is nice, because I only have to maintain one project file, and the block structure is very readable (and I can even use different Qt submodules for each platform).

Screen orientation: Symbian has auto-rotation for Qt apps by default, and Maemo 5 has landscape-only mode by default. For my game, portrait mode does not make sense, so I have to request landscape-only on Symbian (similarly, if I want auto-rotation everywhere, I have to request it on Maemo 5 and do nothing in Symbian). For this specific case, I need some libs in Symbian, so I add this to the project file:

symbian {
LIBS += -lcone -leikcore -lavkon

I also need to add this to the top of my main source file:

#include <AknAppUi.h>

And finally, I have to copy'n'paste a code block into my main() function before I create the first window:

CAknAppUi* appUi = dynamic_cast<CAknAppUi*> (CEikonEnv::Static()->AppUi());
if (appUi) {
// Lock application orientation into landscape

It would be nice if Qt (or Qt Mobility?) has some generic API for this, where I just need to do something along the lines of QRotation::setMode(QRotation::LandscapeOnly); and let it take care of the platform-specific stuff.

Accelerated QGraphicsView: On Symbian^3, QGraphicsView is automatically accelerated via OpenVG (I think), but on Maemo 5, it can use OpenGL for this (but does not by default), so in the code where I create my QGraphicsView, I have to have this at the top:

#ifdef Q_WS_MAEMO_5
# include <QGLWidget>

And then somewhere down that file where I create the QGraphicsView, this code goes there:

#ifdef Q_WS_MAEMO_5
QGLWidget *glw = new QGLWidget(QGLFormat(QGL::DoubleBuffer));

It's not that difficult, and is already cross-platform (on platforms where OpenGL is available), but maybe QGraphicsView can be OpenGL-accelerated on Maemo 5 by default. Maybe there is some non-obvious side effect that using an OpenGL viewport has that I'm not aware of, and that's why one has to do it manually.

Accelerometer readings: This might actually be a bug in Qt Mobility. For my game, I have to read the accelerometer's Y axis on Symbian, and its X axis on Maemo to determine the rotation for the same holding position of the device, so there's another platform-specific #ifdef there. Again, this will hopefully be fixed in a future release of Qt Mobility, and will most likely be fixed for MeeGo as well.

Task switcher: It's customary on Maemo to have a task switcher button in the upper left corner. There's no platform-agnostic way of going to the task switcher, so I needed to special-case it for Maemo 5, and have not bothered implementing it on Symbian^3 (suggestions welcome!). First, I need to include D-Bus headers for this (I've already mentioned how to request the D-Bus module in the qmake project file above) :

#if defined(Q_WS_MAEMO_5)
# include <QDBusConnection>
# include <QDBusMessage>

When I'm in the handler code where I need to do the actual task switching, I can utilize it to activate Maemo 5's task switcher:

#if defined(Q_WS_MAEMO_5)
QDBusConnection c = QDBusConnection::sessionBus();
QDBusMessage m = QDBusMessage::createSignal("/", "", "exit_app_view");

This is very much Maemo 5-specific, and will very likely not work on MeeGo Harmattan. Again, here it would be nice to have a cross-platform way of doing window management (in Qt or Qt Mobility?). I can imagine this being useful not only on handsets, but also on netbooks and tablets (from a MeeGo PoV). Again, trying to come up with pseudo-APIs here, QWindowManager::showTaskSwitcher(); could be a nice way to handle this in a cross-platform way, hiding platform-specific implementations. From what I've seen of MeeGo Touch, activating the task switcher there simply iconifies the window on the Desktop, so maybe if I would iconify my main window, it should activate the task switcher on Maemo 5 and Symbian. It does not work on Maemo 5 right now, though, and I haven't tested it on Symbian.

Screen resolution: This was (thanks to QGraphicsView) less of a problem than I thought it would be. Symbian^3 (or at least the N8) uses 640x360 as its resolution, and Maemo 5 uses 800x480. Maybe the MeeGo Harmattan device will use yet another resolution. I still configure my QGraphicsScene object manually via #ifdefs to get a good resolution, but the code could just measure the resolution and configure the scene as well. Here's what I use:

#if defined(Q_WS_MAEMO_5)
setSceneRect(0, 0, 800, 480);
#elif defined(Q_OS_SYMBIAN)
setSceneRect(0, 0, 640, 360);

I then use the sceneRect() of my scene to calculate a scale factor for all contents and call setScale() on all root items of the scene to scale the contents to the current screen size. You might have to take care of different aspect ratios on different devices, too - but it was unproblematic for my use case (the game) this time.


Anonym hat gesagt…

You might find yourself a bit happier you drop all of those #ifdefs and make an abstraction everywhere you are tempted to use one. You can then give different implementations living in completely different source files for each version.

e.g. You could have an "orientation" abstraction, so you could do:

#include "orientation.h"



in your main code. Then you have a:

maemo5/orientation.c which for this case can do nothing.

and a

s3/orientation.c which has the code you included before.

It tends to scale much nicer as you keep adding more and more platforms.

thp hat gesagt…

@the-new-andy: Sure, I'd probably put everything into a "Platform" class that I then subclass (and augment with platform-specific code) and use it as a singleton (with an instance of the platform-specific subclass returned) throughout my code. That's not the point, though - Qt should take care of some of these things for me.

Having platform-specific code in a codebase gets tedious if you write (and maintain) multiple applications and want to have similar features in every one of them. Also, code isn't write-once, so when e.g. a bugfix or improvement is made to, say, the orientation-changing code, I have to copy it into every project.

If this is abstracted by the toolkit, every application benefits from the bugfix as soon as it lands in the toolkit (and gets pushed out as an update). Also, if it's in the toolkit, I get support for new platforms for free (the Qt implementation in MeeGo Handset is able to do the right thing if I say "landscape only") instead of having to add even more code every time a new platform/device comes out.

Jani hat gesagt…

Good post and good tips too. I just started to port Qt desktop/Maemo5 application to Symbian and this is actually really helpful info.

I use desktop version as a primary development version and of course I then miss all mobile device specific issues. But yeah, I have done some #ifdefs too already.

And I do bet, that every developer has to go through all kinds of weird issues when writing and porting mobile Qt applications. Qt is great, but especially Symbian side seems to be a bit shaky still.

Alex Brown hat gesagt…

Meego makes this even worse. It's got a lot of Meego-only classes that have to used if you want your app to look like a true native Meego app.

fms hat gesagt…

Funny, how "Qt-based" Symbian^3 still requires you to use Avkon to get things running :)

Harry hat gesagt…

You can use QDesktopWidget to figure out the screen size, no need to #ifdef it.

Thiago hat gesagt…

Not to mention that #ifdefs are now in Qt code examples;

I think that are many ways to avoid than. This Forum Nokia example is not well done IMO.

Machatronik2007 hat gesagt…

Thanks a lot for your solutions to this problem. I wrote a pocket calculator app, which makes sense only in portrait-mode.

i only did, what you explained, and it worked at the first try!!!