Modern mobile applications with Qt and QML

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English [French]

Modern Mobile Applications with Qt and QML

Qt is a flexible and powerful framework for creating cross-platform applications. QML is now part of Qt, providing a markup language which gives complete freedom in development of user interfaces.

A good way to see the power of Qt is to start coding with it. Let's write a simple application using Qt and QML. This will be an application I called 4Toddler; the application will start in full-screen mode and it has just two buttons placed in the top right corner of the window: About and Close. The main function is to display a random image with a fireworks effect and with a random sound effect.

The User interface will be written with QML; the backbone is written in Qt C+.

Create new project

If you do not have Qt already installed, visit Qt download page to download Qt SDK for your platform and install it.

Launch Qt Creator, select File -> New File or Project. In the new dialog select Qt C++ Project-> Qt Gui Application, and then click Choose…

screenshot

In the new window: type project name, select path to the project folder and then click Next.

screenshot

In the next window: uncheck Generate form option. We do not need to generate a form. Click Next

screenshot

In the last window: just click Finish.

The application skeleton is ready, now we can proceed to code the logic and design the UI.

Code core logic in C+

First we need to add the qt-declarative module to our project. This is the module that provides a widget in which a QML interface is displayed.

Open project file (4Toddler.pro) and append the line

QT ''= core gui<code>

with ''declarative''

QT= core gui declarative

Now we need to change the base class for our main window. Replace QMainWindow with QDeclarativeView and include QDeclarativeView

#include <QDeclarativeView>
class MainWindow : public QDeclarativeView
{
 …
}

Also we need to cut off QMainWindow(parent) from the MainWindow constructor; we do not need this initialization anymore.

MainWindow::MainWindow(QWidget *parent)
{
 Init();
}

If you launch the application right now you will see an empty window. This is because we have not initialized or created our QML interface yet.

Add QML interface

Let's add a new file to our project: right click on the project in the projects explorer window.

Add New then select Qt section and Qt QML File in the templates list and click Choose…

Type the file name in the Name field, click Next then Finish in the next window.

The wizard will create and open the new QML file in the editor. There is just one Rectangle element inside it. This will be the root element for our user interface. Let's add some new properties to this element.

Note: some versions of Qt Creator will add a Hello World text element centered in the Rectangle. Remove the text element before proceeding.

Rectangle
{
 // ID of this element. Using this ID we can access the element and its properties from other elements
 id: canvas
 // Background color, black in this case
 color: "black"
 // Change sizes of this element to fill the parent
 anchors.fill: parent
 // Element can receive focus
 focus: true
}

There is nothing special for now, just black background.

Multilingual

During the remainder of this tutorial we will shift back and forth between QML and C++ development.

We need to add some initialization code for our QML interface by defining a new method inside the mainwindow.h file …

void Init();<code>

.. And implementing it in the '''mainwindow.cpp''' file

void MainWindow::Init() {

// Path to the content folder
QString contentPath;

1. ifdef QT_DEBUG

// In the debug version of our project this is a path to the project folder
contentPath = "D:/MyProjects/QT/4Toddler";

1. else

// In the release version - path to the application folder
contentPath = QApplication::applicationDirPath();

1. endif

setFocusPolicy(Qt::StrongFocus);
// Change QML document sizes to fit the main window
setResizeMode(QDeclarativeView::SizeRootObjectToView);
// Load QML file
setSource(QUrl::fromLocalFile(contentPath + "/main.qml"));

}

Now we need to replace one line in the '''main.cpp''' file

int main(int argc, char *argv[]) {

…
w.show();
…

}

With

int main(int argc, char *argv[])

{

…
w.showFullScreen();
…

}

This will force our window to open in full-screen mode.

== Structure code with components ==

Before we launch our application, let's add two buttons. One is to display '''About''' dialog and the second to '''Close''' our window. We do not need to implement buttons twice, we going to create a [http://doc.qt.nokia.com/4.7-snapshot/qml-extending-types.html component] and use it every time we need to add a new button by overriding just a few properties.

Let's add the new Qml file - '''WindowButton.qml''' to our project. ''Note the filename begins with a capital letter — this signifies it defines a QML component.''

Image {

// ID of this element
id: button
// The MouseArea item enables simple mouse handling
MouseArea
{
anchors.fill: parent
id: mouseArea
// On click call callback() handler
onClicked: callback()
}

}

Now we can add two buttons to our window

// Place items in row

Row {

// Right side of the item is anchored to the right side of the parent element
anchors.right: parent.right
// Right margin
anchors.rightMargin: 4
// Top side of the item is anchored to the top side of the parent element
anchors.top: parent.top
// Top margin
anchors.topMargin: 4
// Margins for children elements
spacing: 4
WindowButton
{
// Button to display the About dialog
id: about
// Path to background picture. This is a relative path to the path of this QML file
source: "about.png"
// Callback method, which will be called on mouse click
// onClicked: callback()
function callback()
{
}
}
WindowButton
{
// Button to close window
id: exit
source: "exit.png"
function callback()
{
}
}

}

== Communicate between QML and C++ ==

Now we should implement both '''callback()''' methods. To close the window we will call the '''Quit''' function of the main window. Now we will see how Qt and QML communicate. Methods will be implemented inside Qt and then called from the QML file.

Let's add a new function to the '''mainwindow.h''' header file

Q_INVOKABLE void Quit();

And implementation in mainwindow.cpp file

void MainWindow::Quit()

{

QApplication::quit();

}

Now we need to "tell" QML about this method. Inside Init we should to add just one line:

rootContext()->setContextProperty("window", this);<code>

Now we can access '''window''' object's functions declared as '''Q_INVOKABLE''' from QML. '''Window''' here is just an example, you could use any object name you want.

Let's add '''callback()''' function implementation to close button

function callback() {

window.Quit();

}

== Visualize state and add animation ==
Ok, the application can be launched. Launch it and click the '''Close''' button. You see? State of the button is not changed after click; it looks like the button is just inactive. Let's add state changes for normal and clicked states.

Image {

…
states:[
State
{
// Name of the state
name: "hovered"
// When condition. Item will go to this state if condition is true
// In this case on mouse click
when: mouseArea.pressed
// Which properties will be changed in that state
// In this case- opacity
PropertyChanges { target: button; opacity: 1}
},

State

{
name: "normal"
// This state will be activated if mouse button is not pressed
when: mouseArea.pressed == false
PropertyChanges { target: button; opacity: 0.7; }
}
]

}

Item will change state if the condition described in ''when'' property is true. You could change state manually by assigning state property.
Let's launch the application again. Ok, this time it looks more active. We can make our button nicer by adding some animation.

Image {

…
Behavior on opacity
{
// Animaion step is a 100 milliseconds
// On every iteration opacity will increase or descrease with step equals to 0,1
NumberAnimation { duration: 100 }
}

}

The ''Behavior'' is a simple and flexible way to create animations. This element allows you to specify a default animation for a property change.

Launch the application and try to click on '''About''' or '''Close''' buttons. This looks much better!

[[Image:http://appdeveloper.intel.com/sites/files/8-qt-qml-art.png|http://appdeveloper.intel.com/sites/files/8-qt-qml-art.png]]

We will implement the '''About''' dialog only using QML. Dialog will appear on the screen when '''About''' button is clicked and disappear when the user clicks inside the button or on the background.

Let's add '''About.qml''' file to our project.

// Parent element for dialog window Rectangle {

id: about
// Function to display dialog window
// This function just changes opacity of the main element to 1
function show()
{
about.opacity = 1
}
// Function to hide dialog window
// This function just changes opacity of the main element to 0
function hide()
{
about.opacity = 0
}
// Transparent background
color: "transparent"
// Opacity is 0 by default, dialog not visible
opacity: 0
// Anchors to parent width and height.
width: parent.width
height: parent.height
// Element is visible if opacity > 0
// opacity > 0
visible: opacity > 0
// Child element to create semitransparent background
Rectangle
{
anchors.fill: parent
opacity: 0.5
color: "gray"
}
// Body of the dialog window
Rectangle
{
id: dialog

// Fixed width and height

width: 360
height: 230
// To make dialog centered inside parent we need to calculate its x and y coordinates
x: parent.width / 2 - dialog.width / 2
y: parent.height / 2 - dialog.height / 2
// Place dialog on top of other elements
z: 10
border.color: "gray"
Text
{
text: "4 Toddler"
font.bold: true
font.pixelSize: 22

anchors.horizontalCenter: parent.horizontalCenter

anchors.verticalCenter: parent.verticalCenter
}
}
Behavior on opacity
{
NumberAnimation { duration: 100 }
}
MouseArea
{
anchors.fill: parent
// Hide dialog on mouse click
onClicked: hide()
}

}

Take a look at the line

visible: opacity > 0

As you can see, property can be assigned and can be calculated.

Let's add About dialog and callback() function implementation for the button About. In Main.qml file we need to declare the new About element.

Rectangle
{
 id: canvas
 ..
 About
 {
 id: aboutDlg
 }
}

And callback() function implementation

aboutDlg.show();<code>

to

WindowButton {

id: about
…
function callback()
{
aboutDlg.show()
}

}

Finally we need to implement the main functionality for our application.

The element to display a random icon on the screen will be an '''Image''' element. Let's add a new file for the new element - ''Block.qml''

Image {

id: block;
// New custom properties to change visibility of this element
property bool remove: false
property bool show: false
opacity: 0
fillMode: Image.Stretch
states: [
State
{
// State is used to remove element from the screen and destroy it
name: "remove"; when: remove  true
     PropertyChanges { target: block; opacity: 0 }
     StateChangeScript { script: block.destroy(1000) }
   },
   State
   {
     // State is used to display element
     name: "show"; when: show  true
PropertyChanges { target: block; opacity: 1 }
}
]
Behavior on opacity { NumberAnimation { duration: 300 } }

}

Now we need to implement the keyboard handler. Handler will be implemented with '''JavaScript'''. Let's add new ''main.js'' to our project.

// Template for new elements

var component = Qt.createComponent("block.qml") // Max number of items on the screen var maxBlocksCount = 10 // Array for the items var blocksArray = new Array() // Keyboard handler function handleKey() {

// x coordinate is a random number from 0 to window with in pixels
var x = Math.floor(Math.random() * canvas.width)
// y coordinate is a random number from 0 to window height in pixels
var y = Math.floor(Math.random() * canvas.height)
// This function will create a new element on each call with random x and y coordinates
createNewBlock(x, y)

} // Function to create a new element function createNewBlock(x, y) {

if(component.status != Component.Ready)
{
return false
}
// Remove items if number of items is more than maxBlocksCount
if(blocksArray.length > maxBlocksCount)
{
removeAllBlocks()
}
var newBlock = component.createObject(canvas)
if(newBlock == null)
{
return false
}
// Path to image is available via randomIcon property of the main window
var iconFile = window.randomIcon
newBlock.source = ("Icons/" + iconFile)
newBlock.x = x
newBlock.y = y
// Change state to show
newBlock.show = true
blocksArray.push(newBlock)
// Play random sound effect
window.PlaySound()
return true

} // Function do remove all existings items function removeAllBlocks() {

for(var i = 0; i < blocksArray.length; +i)
{
blocksArray[i].remove = true
}
while(blocksArray.length != 0)
{
blocksArray.pop()
}

}

As you see in code above we should implement new property '''randomIcon''' and method '''PlaySound'''.

Let's add property declaration and method to access it to our ''mainwindow.h'' file.

Q_PROPERTY(QString randomIcon READ RandomIcon)

QString RandomIcon();<code>

Implementation in ''mainwindow.cpp'' file

QString MainWindow::RandomIcon() {

QStringList iconFilesList;
QString searchPath = m_ContentPath "/Icons/";
QDir directory = QDir(searchPath);
QStringList filters;
filters << ".png";
directory.setNameFilters(filters);
// Get the list of the png files inside Icons directory
iconFilesList = directory.entryList(QDir::AllEntries);
// Generate random index of the element
int fileIdx = qrand() % iconFilesList.count();

// Return file name
return iconFilesList.at(fileIdx);

}

Now we need to declare the method to play a random sound effect in ''mainwindow.h'' file

Q_INVOKABLE void PlaySound();

And implementation in mainwindow.cpp file

void MainWindow::PlaySound()
{
 QStringList soundFilesList;
 QDir directory = QDir(m_ContentPath + "/Sounds/");
 QStringList filters;
 filters << "'''.wav";
 directory.setNameFilters(filters);
 // Wav files list inside Sounds directory
 soundFilesList = directory.entryList(QDir::AllEntries);
 // Generate random index of the element
 int fileIdx = qrand() % soundFilesList.count();
 // File name
 QString soundFile = m_ContentPath + "/Sounds/" + soundFilesList.at(fileIdx);
 // Play file asynchronously
 QSound::play(soundFile);
}

Almost done. All we need is to add the keyboard handler to our parent QML element. But first we should include main.js file to main.qml file

import Qt 4.7
import "main.js" as Main

Keyboard event handler for root element

Rectangle
{
 id: canvas
 …
 Keys.onPressed: {
 if(event.isAutoRepeat == false) {
 Main.handleKey()
 }
 }
}

That's all! Now you can launch the application and try to press any key on the keyboard. But… I almost forgot about the fireworks effect.

Let's add a new file called Fireworks.qml. Why do we need a new file? Because this will be a reusable element, this will save time in the future.

import Qt.labs.particles 1.0
Particles
{
 id: particles
 width: 1
 height: 1
 anchors.centerIn: parent
 emissionRate: 0
 lifeSpan: 700
 lifeSpanDeviation: 600
 angle: 0
 angleDeviation: 360
 velocity: 100
 velocityDeviation: 30
 source: randomImage()
 // Get random image path
 function randomImage()
 {
 // Array of the image files
 var images = ["red.png", "blue.png", "green.png", "white.png", "yellow.png"]
 // Get random index of the array element
 var idx = Math.floor((Math.random() * 100)) % images.length
 // Return the relative image file path
 return ("Stars/" + images[idx])
 }
}

Now we need to add our Firework element to the Block element. Open Block.qml file and add declaration of the new element

Image
{
 id: block
 …
 Firework
 {
 id: firework
 }
 …
}

To launch fireworks on item going to visible state we need to add just one line of code

StateChangeScript { script: firework.burst(50); }<code>

to "show" state

State {

name: "show"; when: show == true
StateChangeScript { script: firework.burst(50)}
PropertyChanges { target: block; opacity: 1 }

}

Launch application and press any key. Image appears on the screen with fireworks and sound effect.

http://appdeveloper.intel.com/sites/files/9-qt-qml-art.png

This is a simple application, but this is a good playground to understand the principles of QML.

To learn more about QML you can extend this application by adding features. Who knows, maybe someday this sample will grow to a commercial app? Idea from me: educational software with letters, numbers, shapes, colors, etc.




---

This article originally appeared on the Intel AppUp(SM) developer program blog. A Russian version is available here

Thanks to Dmitry for allowing us to copy his tutorial.