Building the QML Run-time

Building the run-time
by Johan Thelin
Pelagicore

Copyright©2012 Johan Thelin
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Bio
● Johan Thelin @e8johan

● Worked with Qt
since 10+ years
● Author
● FoQD
● Articles
● Tutorials
● Embedded Linux
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Work

● Produces an In-Vehicle Infotainment (IVI) framework
● Open Source / Linux / Qt
● GENIVI / LinuxFoundation / Ubuntu Core

● We're hiring!
● http://www.pelagicore.com/career.html
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Demo Screens

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QML is Productive
● Developed over 2.5 months for CES 2012
● Interaction and graphics design
● Run-time and QML development
● ~5 full time persons involved (team of 15)

● The code consists of
● 5986 loc (4871 / 1115 according to cloc)
● 10312 lines of QML (not loc)

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QML in Five Slides

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QML 1(5) – Creating and Naming
import QtQuick 1.0
Rectangle {
id: root
Rectangle {
id: red
}
Rectangle {
id: yellow
}
}
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QML 2(5) – Basic Items
Rectangle {
width: …
● Rectangle height: …
color: “#abcdef”
}

Image {
● Image }
source: “...”

Text {
text: “abc 123”
● Text abc 123 font.family: “helvetica”
font.pixelSize: 25
color: “black”
}

● MouseArea
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QML 3(5) – Binding and Actions
Rectangle {
id: red
width: 100; height: yellow.height
color: “red”
MouseArea {
anchors.fill: parent
onClicked: console.log(“I'm clicked!”)
}
}

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QML 4(5) - Components
// Button.qml
Rectangle {
id: root
signal clicked
property string text
Text { anchors.centerIn: parent; text: root.text }
MouseArea {
anchors.fill: parent
onClicked: root.clicked()
}
}

// main.qml
Button { text: “Click me!”; onClicked: { … } }

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QML 5(5) – States and Transitions
Rectangle {
id: r
states: [
State { name: “either”
PropertyChanges { target: r; opacity: 1.0 } },
State { name: “or”
PropertyChanges { target: r; opacity: 0.2 } }
]
state: “either”
transitions: [
Transition {
PropertyAnimation { properties: “opacity”; duration: 3000 }
}
]
}

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QML, Declarative and QtQuick
● QtQuick consists of
● QML – the Qt Meta Language
● QtDeclarative – the Qt module for executing QML
● Tooling – Visual
designer, profiler,
viewer, etc

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The Demonstrator

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Taking one Step Back

User Experience

Run-time

Environment

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QML HTML5

Qt and C++

Linux

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WebKit WebKit2
QML
Qt 4.x Qt 5.x

Deep
Integration WebChannels

Qt and C++

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The Interface

QML

Values
Classes
Objects

Qt and C++

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A Basic Run-Time
int main(int argc, char **argv)
{
QApplication a(argc, argv);
QDeclarativeView view;
view.setSource(“main.qml”);
view.show();
return a.exec();
}
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Exposing a Value
● Expose it as a root context property, i.e. a
global variable
view.rootContext()->
setContextProperty(“itemWidth”, 350);

● Bind to it in QML
Rectangle {
width: itemWidth
}
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Exposing an Object
class ClimateControl : public QObject {
Q_OBJECT
Q_PROPERTY(int fanSpeed READ fanSpeed
WRITE setFanSpeed NOTIFY fanSpeedChanged)

public:
Q_INVOKABLE resetClimate();

int fanSpeed() const;
void setFanSpeed(int);
Image {
signals: source: “fan-image-” +
void climateReset(); climate.fanSpeed + “.png”
void fanSpeedChanged(); }
};

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Exposing an Item
● Inherit QDeclarativeItem
● Position and size
● Anchoring
● Keyboard focus

● For Qt 5, inherit QQuickPaintedItem
● Slightly different signature, but nothing dramatical
● http://bit.ly/y17W1n (Zchydem)

● Register using qmlRegisterType
qmlRegisterType<MapItem>(“com.pelagicore.navigation”,
1, 0, “Map”);

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From QML
import com.pelagicore.navigation 1.0

Map {
pitch: 30
zoom: 128
position: vehicle.position
}

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Exposing a Class
● An item is just another QObject – you can
expose more!
● Functional building blocks
● Dynamic control elements
● etc

● Just qmlRegisterType any type derived from
QObject and let QML handle the instantiation

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Models
● The QAbstractItemModel interface provides a
standardized way to expose data models in Qt
● Can be exposed both as objects and classes

● An ideal way to expose lists of data to QML
● The current playlist
● The locations of restaurants in the vicinity
● A list of tweets

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A Basic Model
class PlayListModel : public QAbstractListModel {
Q_OBJECT
public:
int rowCount() const;
QVariant data(const QModelIndex&, int) const;
QVariant headerData(int,
Qt::Orientation, int) const;
};

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Models in QML

Model

View

Separates the presentation
Delegate from the visualization!

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Available Views

GridView

PathView
ListView

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And from QML...
● Exposed as an object
ListView {
model: media.playlist
delegate: PlayListDelegate {}
}

● Exposed as a class
MediaSearchModel { id: mediaModel }
ListView {
model: mediaModel
delegate: MediaDelegate {}
}

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Exposing Different Roles
● For each element of a model, there can be
multiple roles
● Using QAbstractItemModel::setRoleNames
more roles can be named
● Allows for easy-to-read delegates
Text { text: albumName }
Text { text: songDuration }
Image { source: albumArtUrl }

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Asynchronous Data Retrieval
● Use canFetchMore and fetchMore to request more data
bool canFetchMore(const QModelIndex&);
void fetchMore(const QModelIndex&);

● Use beginInsertRows and endInsertRows when the data
has been retrieved
void MyModel::gotMoreData() {
beginInsertRows(parent, first, last);
updateModelWithNewData();
endInserRows();
}

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Prototyping Models in QML
ListModel {
id: musicModel

ListElement {
albumName: “The Wall”
songTitle: “Empty Spaces”
}
ListElement {
…
}
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When to do what?

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The Goals
● QML controls
● Appearance
● Behaviour
● The run-time provides
● Functionality CC-BY ekkebus
http://www.flickr.com/photos/ekkebus/5020840511/
● Access to state and data
● A well defined interface allows designers and
run-time developers to work in parallel

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The State of the System
● Use system-wide singletons per function, e.g.
● vehicle, climate, media

● Rationale
● There is only a single vehicle, so only one state
● Dividing them among functional areas
– gives small, clean interfaces
– allows limited system access for sandboxed elements

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The State of the System
● Provide properties
● media.masterVolume
● Provide signals for events
● navigation.onDestinationReached
● Provide methods for common functions
● media.mute

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The State from QML
VolumeIndicator { volume: media.masterVolume }

Slider { onValueChanged: {
media.masterVolume = value;
}
}

MuteButton { onClicked: media.mute(); }

Connections {
target: navigation
onDestinationReached: navigation.resetDestination();
}

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Data From the System
● Use models for all data that is not a state
● What is a state and what is a model?
● Climate zone states?
– driverTemperature, passengerTemperature, rearTemperature
● Climate zone as a model?
zoneName temperature
Driver 20.5
Passenger 20.0
Rear 22.0

● How dynamic is your system?
● How dynamic is your design?
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Object or Class
● Exposing a model as an object
● There is only one playlist, use media.playlist, e.g.
ListView { model: media.playlist }

● Exposing a model as a class
● There can be any number of search results, use MediaSearchModel, e.g.
MediaSearchModel {
id: search
filter: “artist=Pink Floyd”
}
PathView {
model: search
}

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The Engineering Choice
● QML forces you to separate form and function
● It also gives you new choices
● Limiting the run-time environment saves
development time short term
● Generalizing the run-time improves reuseability

● How do you work?
● What are you building?

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Thank you!
@e8johan
johan.thelin@pelagicore.com

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Building the QML Run-time

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