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Item Views Next Gen

Published on Tuesday April 07, 2009 by Thomas Zander in Labs Graphics View Itemviews | Comments

In recent years the graphics capabilities of desktops and handhelds have been growing more rapidly than their central processors. This is a revolution that is being accelerated by animating user interfaces. This in turn is based on the fact that you can make more usable and more attractive user interfaces using animations.
Qt has been steadily working on solutions that allow more dynamic user interfaces and animations for core features to actually use this available graphics power. Providing support via a framework like Qt sets a baseline and thus allows developers to create usable apps faster.
In Qt this all started with QGraphicsView and we are seeing a coherent vision progress steadily. And now its time for itemviews to be looked at :)
Itemviews is the collective name for the widgets that show lists, tables and trees. Qt4 has a solution for these widgets already, which works fine for a large set of usecases. There are also usecases where they lack somewhat. Dynamic and fluid user interfaces are a major part of that.
We have started a research project to address all these shortcomings and provide a solution that makes it easier and faster to make beautiful and usable lists, tables and trees. In this blog I want to give an overview of what we have done and what our plans are going forward. Additionally we have put our research project online for everyone to download and play with.

Who is it for?

At this time of development we focussed on great design, good APIs and making easy the common usecases and possible the complex ones. For this reason there will be missing concepts, broken features and in general things may just fall apart. You have been warned :)
Who we are looking for are developers that want to kick the tires or our new baby, especially useful are developers that have specific usecases in mind of lists and tables that are a challenge to do in the Qt Itemviews. Having running examples build on top of the ItemViewsNG will help us a lot going forward and it will shape the direction we are going.
The actual repository where we work will be public and so people can just follow our progress as we go but we also encourage developers to contribute their patches back to our main repo.

Design

For the next generation of item views, we choose to use QGraphicsView concepts. The obvious advantage is reuse of concepts and features. SimpleListIf you look at the simple list example the individual list items (each row of text, really) is an individual graphics item (QGraphicsWidget, specifically). GraphicsView has various features that are very useful for us. It is very cheap graphics wise to move around the individual items, for example. This allows animations of individual items to use hardware acceleration automatically. GraphicsView also gives us features like being able to click on an item and that item having code to handle it so you can just use the existing concepts and code for making your individual items be drawn and behave the way you want. For old-graphicsview coders; this mirrors the concept of delegates in many ways.

All this talking about graphicsview may raise the question on how to integrate this with traditional user interfaces with QWidgets. A good question, and we created specific widgets for each of the 3 types of item views with proper defaults and simple to use APIs. These are the QtListWidgetNG, the QtTableWidgetNG and the QtTreeWidgetNG. These widgets come with several replaceable components that make it easy to change the look or the interaction separately and so mix and match to your liking.

Let me focus on the QtListWidgetNG here; behind the widget are various classes worth mentioning;

itemviewNGDesign

  • The QtGraphicsListView allows customizers to decide how to show the individual list items. The default class uses separate graphics view items which has the advantage that we can cheaply animate them.
  • The view decides which items to show by asking the QtListModelInterface. As the name states that is just an interface and thus has no implementation. We provide a simple implementation as the QtListDefaultModel.
  • All the communication goes via the QtListController class. This includes things like handling keyboard and mouse input received by the QtListWidgetNG but also things like when the active item in the list is changed for some reason then the scrollbar moves to the correct position.

In the ItemViewNG repository today you will find several implementations of at least the listview which allows for an easy way to customize your list.
Here are some usages;


    // Simples case; show a complete list
    QtListWidgetNG widget1;
    widget1.defaultModel()->appendItem(new QtListDefaultItem("Simple!"));
    widget1.show();


    // Use an alternative view that shows the items as
    // icons in a flow and we also use a custom model here.
    QtListWidgetNG widget2;
    QtListController *controller = widget2.controller();
    // custom model comes from the iconFlow example.
    controller->setModel(new IconsModel(controller));
    controller->setView(new QtGraphicsFlowView());
    widget2.resize(QSize(240, 200));
    widget2.show();

I started this blog stating that we created the new itemviews based on the idea that we needed pretty user interfaces, and here I am pasting code and showing really boring and standard widgets :) So lets show a bit more action; I made a screencast of the photoAlbum example that we made on top of the itemviews.

The demo (source-file) really shows a list of images quite similar to the boring list screenshot above, only instead of using the default QGraphicsListView we use a tweaked one to show the items vertically and almost full screen.
Next to that we changed the controller to a QtKineticController which provides the scrolling and flicklist behavior.

Where is it again?

On gitweb/labs.
You'll appreciate the API docs too. Both are very clearly unfinished as this is research-in-progress!
Compilation requires Qt4.5 (works fine) or the kinetic branch (for some extra functionality, like the photo album).

Lets see who makes the coolest usage example list based on this ;)

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