Changes between Version 3 and Version 4 of QtWebKitTiling


Ignore:
Timestamp:
Jun 22, 2010 2:16:54 PM (11 years ago)
Author:
kenneth@webkit.org
Comment:

--

Legend:

Unmodified
Added
Removed
Modified
  • QtWebKitTiling

    v3 v4  
    11Tiling with QtWebKit
    22
    3 On touch-based mobile devices a feature known as tiling is often used. It is used due to the interaction model (touch) as well as a scrolling "optimization".
     3Add info about
     4 * how to use it correctly
     5 * issues with fixed contents / solutions
     6 * view port meta tag
     7 * frame flattening
     8
     9----
     10
     11QtWebView goes Mobile
     12
     13There is a lot of effort being put into QtWebKit in order to make it attractive on the mobile front.
     14
     15Among a tons of bug fixes and good performance improvements there are also lots of new features being developed, many geared toward mobile deployment.
     16
     17The goal with this tutorial is to help you understand some of these new features and how to make the best of them. Or said in other words; how to create a good mobile web view that can be used on touch devices.
     18
     19First we should make it clear that QGraphicsWebView is the way forward, so if you want to target mobile devices, it is bye bye QWebView. Why is this? Well, the QWebView is based on the QWidget system, thus it cannot easily support rotation, overlays, hardware accelerated compositing and tiling. If you need a QWidget anyway, you can always construct a QGraphicsView (which is a QWidget) with a QGraphicsWebView inside. This is more or less what we will start with.
     20
     21Let's start with the most simple QGraphicsWebView based "browser" ever:
     22
     23{{{
     24#include <QApplication>
     25#include <QGraphicsScene>
     26#include <QGraphicsView>
     27#include <QGraphicsWebView>
     28#include <QWebSettings>
     29int main(int argc, char **argv)
     30{
     31    QApplication app(argc, argv);
     32
     33    const int width = 640;
     34    const int height = 480;
     35
     36    QGraphicsScene scene;
     37
     38    QGraphicsView view(&scene);
     39    view.setFrameShape(QFrame::NoFrame);
     40    view.setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
     41    view.setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
     42
     43    QGraphicsWebView webview;
     44    webview.resize(width, height);
     45    webview.load(QUrl("http://www.wouwlabs.com/blogs/jeez"));
     46
     47    scene.addItem(&webview);
     48    view.resize(width, height);
     49    view.show();
     50
     51    return app.exec();
     52}
     53}}}
     54
     55Here we just bootstrap a QGraphicsView (QGV) application and add a QGraphicsWebView to the scene.
     56
     57It might seem a bit useless as you can only navigate through one website but it serves well as a simple example. Notice that I'm disabling the scrollbars on the QGV because QtWebKit handles scrolling and scrollbars automatically. This is due to scrolling optimizations and due to the fact that web authors can interact with the scrollbars for instance style them differently.
     58
     59On touch-based mobile devices a feature known as tiling is often used. It is used due to the interaction model (touch) as well as a scrolling "optimization". With this optimization we will have to deal with scrolling ourselves and we thus will have to say good bye to the scrollbar styling. Not a big things, as mobile browsers usually do not even show scrollbars, but use scroll indicators instead.
    460
    561Tiling basically means that the contents of the viewport is separated into a grid of tiles, so that when you update some area, instead of just updating the area you actually update the whole tile. This gives a few advantages for scrolling as when you scroll you do not need to repaint the new visible area for each scroll step, as you update a row of tiles each time; tiles that are often only partly on the screen. This minimized all the paint calls that we have to do and makes it possible to make nicely kinetic scrolling a possibility.
     
    1167Tiles also helps with zooming. Repainting at each zoom level change during a zoom animation is basically impossible on a mobile device (or desktop for that sake) and thus with tiling, you can stop the tiles from being updates and just scale the already existing tiles, and then at the end of the animation update tiles on top of the scaled ones.
    1268
    13 Add info about
    14  * how to use it correctly
    15  * issues with fixed contents / solutions
    16  * view port meta tag
    17  * frame flattening
     69
     70When using tiling, we basically want the QGraphicsWebView to act as our contents, as it supports tiling a.o. things. In order for this we need to make it resize itself to the size of its contents.
     71
     72#1 Magical Step - QGraphicsWebView::resizesToContents
     73http://doc.qt.nokia.com/4.7-snapshot/qgraphicswebview.html#resizesToContents-prop
     74
     75From Qt 4.7 documentation: "If this property is set, the QGraphicsWebView will automatically change its size
     76to match the size of the main frame contents. As a result the top level frame will never have scrollbars.
     77It will also make CSS fixed positioning to behave like absolute positioning with elements positioned
     78relative to the document instead of the viewport."
     79
     80This setting, thus, removes the scrollbars for us on the main frame and makes our QGraphicsWebView resize itself to its content's size. Enabling it:
     81
     82{{{
     83    webview.setResizesToContents(true);
     84}}}
     85
     86If we are going to expand our mobile web view to the size of the contents of its contained page, then that is going to make the view a lot bigger that what can fit on the device's screen!
     87
     88#3 Magical Step - Use a "view" and handle mouse events
     89
     90The idea is to have a custom widget which has a QGraphicsWebView as a class member. Remember that the QGraphicsWebView
     91will be as big as its content's size, so this custom widget will serve as a window, as a viewport.
     92
     93There is not much more to say here, and the following code snippet illustrates it well:
     94
     95{{{
     96class MobileWebView : public QGraphicsWidget
     97{
     98    Q_OBJECT
     99
     100public:
     101    MobileWebView(QGraphicsItem *parent = 0);
     102    ~MobileWebView();
     103
     104    bool mousePress(const QPoint &value);
     105    void mouseMove(const QPoint &value);
     106    void mouseRelease(const QPoint &value);
     107
     108private:
     109    QGraphicsWebView *webView;
     110};
     111}}}
     112
     113In order to properly handle mouse events you must install an event filter on web view or stack it behind its
     114parent object (search for QGraphicsItem::ItemStacksBehindParent). By doing this the mouse events will reach
     115a MobileWebView instance before they reach the member QGraphicsWebView
     116
     117Keep in mind that you'll need to add some logic in order to distinguish different mouse events and gestures
     118like a single click, double click, click-and-pan, etc. That is left as an exercise to the reader.
     119
     120Also keep in mind that scrolling will have to be implemented manually, just as zoom etc.
     121
     122When testing the above on a device, you will notice that many pages do not layout very nicely. In particular the width is larger than the width of the device!
     123
     124The way web contents is laid out, is that the first the viewport width is used for fitting the contents. If the contents doesn't fit due to non-flexible element with a width larger than the viewport width, the min width possible will be used. As most pages are written with a desktop browser in mind, that makes only very few sites fit into the width of a mobile device.
     125
     126Qt WebKit has a way to force a layout to a given width/height. What really matters here is the width. If you layout a size to a given width, it will get that width and images etc might get cut of. The width/height is also used for laying out fixed elements, but when we resize the QGraphicsWebView to the size of the contents, fixed elements will not be relative to the view, which is the behaviour found on most mobile browsers.
     127
     128Qt 4.7 docs also says: "This property should be used in conjunction with the QWebPage::preferredContentsSize property.
     129If not explicitly set, the preferredContentsSize is automatically set to a reasonable value."
     130
     131#2 Magical Step - QWebPage::preferredContentsSize
     132http://doc.qt.nokia.com/4.7-snapshot/qwebpage.html#preferredContentsSize-prop
     133
     134From Qt 4.7 documentation: "If this property is set to a valid size, it is used to lay out the page."
     135
     136We saw that this property is automatically set to a reasonable value when using QGraphicsWebView::resizesToContents.
     137
     138As you can imaging, laying out with a smaller viewport can cause pages to break, and as thus, a default value has been chosen so that it almost breaks no pages which still making the sites fit. This value is 960x800.
     139
     140If the device have a bigger resolution, this value can be changed using:
     141
     142{{{
     143    webview.page()->setPreferredContentsSize(QSize(desiredWidth, desiredHeight));
     144}}}
     145
     146You can play around with this and find your own magic number, but let's stick to this 960px wide for now.
     147
     148As some sites do not work with 980 or want to have control on how the page is laid out, QtWebKit as well as Android, Firefox Mobile and the iPhone Safari supports a meta tag called viewport.
     149
     150#6 Magical Step - Handle viewport meta tag
     151This one also deserves a whole blog post for itself. For now let's just say that this is a meta tag that Apple came up with to make a web page capable of "telling" the browser how it wants to be shown.
     152
     153More information: http://developer.apple.com/safari/library/documentation/appleapplications/reference/safarihtmlref/articles/metatags.html
     154and http://developer.apple.com/safari/library/documentation/appleapplications/reference/safariwebcontent/usingtheviewport/usingtheviewport.html
     155
     156In QtWebKit trunk we already have support for this with a nice API. You must connect the signal from
     157QWebPage::viewportChangeRequested(const ViewportHints& hints) to a slot of your mobile web view and use
     158what is provided by QWebPage::ViewportHints to updated your viewport size, scale range, and so on.
     159
     160This can be tricky and that's why I'm not going deeper on it right now. Since I know you are curious
     161about it I'll leave you with one more exercise! So try to understand how the guys from MicroB and
     162Firefox Mobile dealt with this: http://hacks.mozilla.org/2010/05/upcoming-changes-to-the-viewport-meta-tag-for-firefox-mobile
     163
     164
     165We haven't actually enabled tiling yet, so lets go ahead and do that. That is very simple as it is basically a setting:
     166
     167{{{
     168    QWebSettings::globalSettings()->setAttribute(QWebSettings::TiledBackingStoreEnabled, true);
     169}}}
     170
     171Voila! Mind that if you are going to add animations to your zoom/scale or want to implement a fancy kinetic scrolling
     172you might want to take a look at QGraphicsWebView::setTiledBackingStoreFrozen. With this you can avoid updates to your
     173tiles during an animation, for instance.
     174
     175One big issue with the above is that, iframes and sites using frames can contain scrollable sub elements. That doesn't work well with the touch interaction model, as you want a finger swipe to scroll the whole page and not end up just scrolling a sub frame. Most mobile browser work around this by enabling something called frame flattening.
     176
     177#5 Magical Step - Enable Frame Flattening
     178Going straight to the point:
     179
     180{{{
     181    QWebSettings::globalSettings()->setAttribute(QWebSettings::FrameFlatteningEnable, true);
     182}}}
     183
     184This will make all frames from a web page expand themselves to the size of their contents, keeping us free of scrollable subareas.