Amarok Blog

I was asked today if LGPL licensed code could be used in a commercial application. Trying to answer the question, I came to realise I couldn’t accurately answer the specifics. Is somebody able to provide a clear and simple explanation on where LGPL code can be used, and under what conditions? Every time I read the license I convince myself of another interpretation.

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Ian writes some more:

In yesterday’s blog I talked about my issue of wanting to use (animated) GraphicsView’s in an Interview system for Amarok’s playlist. Ivan ?uki? pointed me to a 2006 Trolltech Developer Days presentation about Advanced Item View. The presenter did show a pretty brute force way of doing animations in delegates, by just firing a timer every 40 ms. I tried this out two different ways.

First was similar to how he did it, firing a timer. It was not to my great surprise that this resulted in 30% CPU time for just one row needing to be animated, and it went up from there as additional rows were added It actually still looked and reacted fine though, so before I noticed the CPU usage I was feeling like I had cracked the case.

Then I tried it by connecting (indirectly, since I had to translate a QGraphicsScene into a QModelIndex) QGraphicsScene::sceneRectChanged to QAbstractItemView::update. However this didn’t work. Apparently QGraphicsScene doesn’t consider a flashing cursor as a scene change worth mentioning?

So now I’m basically back to where I was. Except the presentation did hold out another possibility of subclassing QAbstractItemView and then doing setViewport. This is what he did for the QGLWidget views that he made (instead of just writing a view from scratch). However its not quite clear to me what the QAbstractItemView provides when you override the viewport. More importantly none of the examples had scrolling, so they just painted the whole area all the time. And figuring that out is the biggest issue with perhaps using a QGraphicsView (whether by itself or as a viewport of a QAbstractItemView.)



More Playlist Details

Benjamin Meyer had a few questions about the playlist that I’ll address here:

• I think Ben you know what the playlist does. The user queues up music to listen to and can also edit their tags.


• I see no reason for multiple views.


• Animations - yes. I don’t know if I’ll have time to do cool animations this summer, but I do want to have a framework in place where someone could implement nifty animations easily.


• Editing - currently the reason I want to get animations to work. Users can do some basic bulk tag editing using the playlist.


• As far as what the playlist looks like, its the thing on the right in Nikolaj’s most recent blog. In the future we want to do more complicated things such as grouping tracks from the same albums together.


• Each row is a single cell, so the entire row is drawn in a single delegate paint. So the view itself is a fairly basic subclass of QListView.

One of the Amarok developers, Ian, has run into some trouble in his summer of code project for the summer. I’m posting a copy of his blog at his request to reach a greater readership. Please, lend your eyes for just a minute!

Background Info

My Summer of Code project is to spruce up the playlist. My idea was to use Qt’s new Interview system to make Amarok more memory efficient and less laggy even with large number of tracks in the playlist - performance of Amarok 1.x suffers noticeably with large playlists. This has largely worked out well.

To make the playlist look good I decided to use GraphicsView. So for the past month I’ve been using QGraphicsScene to paint the items in a QListView using a QAbstractItemDelegate subclass. The subclass overrides the ::paint method, which provides a QPainter. I send the painter to QGraphicsScene::render and it paints the item.

The problem

What I’ve come to find out is that QGraphicsScene render is really meant for printing, not for screen display. Since (you might have noticed) there’s no QGraphicsView involved, there’s nothing to translate normal QWidget events into their GraphicsView counterparts. After looking at qgraphicsview.cpp, I found it

wasn’t hard to simply put that sort of functionality into QListView. So currently double clicking works in QListView and you can initiate edits this way.

What initiating edits has made quite clear is that there animation’s do not work (the cursor doesn’t blink), and unlike with the event-translation issue, there is no clear solution. Without a QGraphicsView and with Delegate’s simplistic painting system (which is all-or-nothing) I don’t see how to do it easily.

Possible Solutions

Widgets Everywhere

Using some hack or another (setItemWidget, abusing the editor widget stuff, dark magic) I could make each item on the playlist a widget. Andrew of Skype (who attended aKademy) apparently has similar issues (wanting to use Interview, but with interactive custom widgets which you can see if you use Skype). He made it pretty clear to me that this was a life-sucking endeavour. It also has obvious efficiency issues… playlist items can’t be QObjects due to memory issues, let alone QWidgets (though if there was some smart way to only have 100 qwidgets at a time that would work).

Overlay Editor Widget

Somehow overlay a text edit line just over the text that the user is editing. No idea if this is possible. And it doesn’t allow any other animations to be used.

Pixie-dust the delegate or QListView to display the animations

Under the current regime, somehow let the delegate or the QListView know when and where it needs to repaint the QGraphicsScene to show the animations.

Replace QListView with a QGraphicsView

…and implement everything that QListView does in a QGraphicsView subclass. Really this is what I’m leaning towards at the moment. This would be very similar to what the Drill Down Example does, except with 100x the data.

One of the main benefits of Model/View is that it only has in memory the data being displayed. It doesn’t really store any data, on every paint operation the

View asks the Model for the data.  I really don’t know where to start on replicating this sort of functionality in QGraphicsView. Would I just constantly delete and add items as the user scrolls?

I would appreciate any suggestions. Whether its something (else) I’ve missed or how I would go about implementing the last solution.

Thanks!

Driving around Sydney city can be a right headache, but I stumbled upon this number plate this week:

For the colour coded out there, #CC26CC is the RGB hex value for purple, the colour of the plate and car. I admit, it is a little difficult to tell, so blame the crappy camera on my phone.

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