source: trunk/doc/src/getting-started/gettingstartedqml.qdoc@ 846

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/*!
    \page gettingstartedqml.html
    \title Getting Started Programming with QML
    \ingroup gettingStarted

    Welcome to the world of QML, the declarative UI language. In this Getting
    Started guide, we will create a simple text editor application using QML.
    After reading this guide, you should be ready to develop your own applications
    using QML and Qt C++.

    \section1 Installation

    First, we would need to install the latest version of Qt that includes \l{Qt
    Quick}, which is Qt 4.7. The \l{Installation} {installation} guide contains
    installation instructions and requirements for different platforms.

    Qt Quick includes a declarative language called
    \l{Introduction to the QML language}{QML}, the \l{Qt Declarative Module}, and
    \l{QML Viewer}.

    \section1 QML to Build User Interfaces

    The application we are building is a simple text editor that will load, save,
    and perform some text manipulation. This guide will consist of two parts. The
    first part will involve designing the application layout and behaviors using
    declarative language in QML. For the second part, file loading and saving will
    be implemented using Qt C++. Using
    \l {The Meta-Object System}{Qt's Meta-Object System}, we can expose C++ functions
    as properties that QML elements can use. Utilizing QML and Qt C++, we can
    efficiently decouple the interface logic from the application logic.

    \image qml-texteditor5_editmenu.png

    The final source code is in the \c{examples/tutorials/gettingStarted/gsQml}
    directory. You may need to compile the C++ plugin in the
    \c{examples/tutorials/gettingStarted/gsQml/} first. This will put the
    C++ plugin in a directory where the QML files may find it.

    To launch the text editor, merely provide the included \l{QML Viewer}{qmlviewer}
    tool with the QML file as the argument. The C++ portion of this tutorial assumes
    that the reader possesses basic knowledge of Qt's compilation procedures.

    Tutorial chapters:
    \list 1
        \o \l {Defining a Button and a Menu}{Defining a Button and a Menu}
        \o \l {Implementing a Menu Bar}{Implementing a Menu Bar}
        \o \l {Building a Text Editor}{Building a Text Editor}
        \o \l {Decorating the Text Editor}{Decorating the Text Editor}
        \o \l {Extending QML using Qt C++}{Extending QML using Qt C++}
    \endlist

    \section1 Defining a Button and a Menu

    \section2 Basic Component - a Button

    We start our text editor by building a button. Functionally, a button has a mouse
    sensitive area and a label. Buttons perform actions when a user presses the button.

    In QML, the basic visual item is the \l {Rectangle}{Rectangle} element. The
    \c Rectangle element has properties to control the element's appearance and location.

    \snippet examples/tutorials/gettingStarted/gsQml/parts/part0/Button.qml document

    First, the \c { import QtQuick 1.0 } allows the qmlviewer tool to import the QML elements
    we will later use. This line must exist for every QML file. Notice that the version
    of Qt modules is included in the import statement.

    This simple rectangle has a unique identifier, \c simplebutton, which is bound to the
    id property. The \c Rectangle element's properties are bound to values by listing the
    property, followed by a colon, then the value. In the code sample, the color \c grey
    is bound to the the Rectangle's \c color property. Similarly, we bind the \c width
    and \c height of the Rectangle.

    The \l {Text}{Text} element is a non-editable text field. We name this \c Text element
    \c buttonLabel. To set the string content of the Text field, we bind a value to the
    \c text property. The label is contained within the Rectangle and in order to center
    it in the middle, we assign the \c anchors of the Text element to its parent, which
    is called \c simplebutton. Anchors may bind to other items' anchors, allowing layout
    assignments simpler.

    We shall save this code as \c SimpleButton.qml. Running qmlviewer with the file as the
    argument will display the grey rectangle with a text label.

    \image qml-texteditor1_simplebutton.png

    To implement the button click functionality, we can use QML's event handling. QML's event
    handling is very similar to \l {Signals & Slots}{Qt's signal and slot} mechanism. Signals
    are emitted and the connected slot is called.

    \code
        Rectangle{
            id:simplebutton
            ...

            MouseArea{
                id: buttonMouseArea

                anchors.fill: parent //anchor all sides of the mouse area to the rectangle's anchors
                        //onClicked handles valid mouse button clicks
                onClicked: console.log(buttonLabel.text + " clicked" )
            }
        }
    \endcode

    We include a \l{MouseArea} element in our simplebutton. \c MouseArea elements describe
    the interactive area where mouse movements are detected. For our button, we anchor the
    whole MouseArea to its parent, which is \c simplebutton. The \c anchors.fill syntax is
    one way of accessing a specific property called \c fill inside a group of properties
    called \c anchors. QML uses \l {Anchor-based Layout in QML}{anchor based layouts} where
    items can anchor to another item, creating robust layouts.

    The \c MouseArea has many signal handlers that are called during mouse movements within
    the specified \c MouseArea boundaries. One of them is \c onClicked and it is called
    whenever the acceptable mouse button is clicked, the left click being the default. We
    can bind actions to the onClicked handler. In our example, \c console.log() outputs text
    whenever the mouse area is clicked. The function \c console.log() is a useful tool for
    debugging purposes and for outputting text.

    The code in \c SimpleButton.qml is sufficient to display a button on the screen and
    output text whenever it is clicked with a mouse.

    \code
        Rectangle {
            id: button
            ...

            property color buttonColor: "lightblue"
            property color onHoverColor: "gold"
            property color borderColor: "white"

            signal buttonClick()
            onButtonClick: {
                console.log(buttonLabel.text + " clicked" )
            }

            MouseArea{
                onClicked: buttonClick()
                hoverEnabled: true
                onEntered: parent.border.color = onHoverColor
                onExited:  parent.border.color = borderColor
            }

            //determines the color of the button by using the conditional operator
            color: buttonMouseArea.pressed ? Qt.darker(buttonColor, 1.5) : buttonColor
        }
    \endcode

    A fully functioning button is in \c Button.qml. The code snippets in this article
    have some code omitted, denoted by ellipses because they were either introduced
    earlier in the previous sections or irrelevant to the current code discussion.

    Custom properties are declared using the \c {property type name} syntax. In the
    code, the property \c buttonColor, of type \c color, is declared and bound to
    the value \c{"lightblue"}. The \c buttonColor is later used in a conditional
    operation to determine the buttons's fill color. Note that property value
    assignment is possible using the \c= equals sign, in addition to value binding
    using the \c : colon character. Custom properties allow internal items to be
    accessible outside of the Rectangle's scope. There are basic
    \l{QML Basic Types}{QML types} such as \c int, \c string, \c real, as well as
    a type called \c variant.

    By binding the \c onEntered and \c onExited signal handlers to colors, the
    button's border will turn yellow when the mouse hovers above the button and
    reverts the color when the mouse exits the mouse area.

    A \c buttonClick() signal is declared in \c Button.qml by placing the \c signal
    keyword in front of the signal name. All signals have their handlers automatically
    created, their names starting with \c on. As a result, the \c onButtonClick is
    \c buttonClick's handler. The \c onButtonClick is then assigned an action to
    perform. In our button example, the \c onClicked mouse handler will simply call
    \c onButtonClick, which displays a text. The \c onButtonClick enables outside
    objects to access the \c {Button}'s mouse area easily. For example, items may
    have more than one \c MouseArea declarations and a \c buttonClick signal can
    make the distinction between the several \c MouseArea signal handlers better.

    We now have the basic knowledge to implement items in QML that can handle
    basic mouse movements. We created a \c Text label inside a \c Rectangle,
    customized its properties, and implemented behaviors that respond to mouse
    movements. This idea of creating elements within elements is repeated
    throughout the text editor application.

    This button is not useful unless used as a component to perform an action.
    In the next section, we will soon create a menu containing several of these
    buttons.

    \image qml-texteditor1_button.png

    \section2 Creating a Menu Page

    Up to this stage, we covered how to create elements and assign behaviors inside
    a single QML file. In this section, we will cover how to import QML elements and how
    to reuse some of the created components to build other components.

    Menus display the contents of a list, each item having the ability to perform an action.
    In QML, we can create a menu in several ways. First, we will create a menu containing
    buttons which will eventually perform different actions. The menu code is in
    \c FileMenu.qml.

    \code
        import QtQuick 1.0                \\import the main Qt QML module
        import "folderName"            \\import the contents of the folder
        import "script.js" as Script        \\import a Javascript file and name it as Script
    \endcode

    The syntax shown above shows how to use the \c import keyword. This is required to
    use JavaScript files, or QML files that are not within the same directory. Since
    \c Button.qml is in the same directory as \c FileMenu.qml, we do not need to import
    the \c Button.qml file to use it. We can directly create a \c Button element by declaring
    \c Button{}, similar to a \c Rectangle{} declaration.

    \code
    In FileMenu.qml:

        Row{
            anchors.centerIn: parent
            spacing: parent.width/6

            Button{
                id: loadButton
                buttonColor: "lightgrey"
                label: "Load"
            }
            Button{
                buttonColor: "grey"
                id: saveButton
                label: "Save"
            }
            Button{
                id: exitButton
                label: "Exit"
                buttonColor: "darkgrey"

                onButtonClick: Qt.quit()
            }
        }
    \endcode

    In \c FileMenu.qml, we declare three \c Button elements. They are declared
    inside a \l {Row}{Row} element, a positioner that will position its children
    along a vertical row. The \c Button declaration resides in Button.qml,
    which is the same as the \c Button.qml we used in the previous section.
    New property bindings can be declared within the newly created buttons,
    effectively overwriting the properties set in \c Button.qml. The button
    called \c exitButton will quit and close the window when it is clicked.
    Note that the signal handler \c onButtonClick in \c Button.qml will be
    called in addition to the \c onButtonClick handler in \c exitButton.

    \image qml-texteditor1_filemenu.png

    The \c Row declaration is declared in a \c Rectangle, creating a rectangle
    container for the row of buttons. This additional rectangle creates an indirect
    way of organizing the row of buttons inside a menu.

    The declaration of the edit menu is very similar at this stage. The menu has
    buttons that have the labels: \c Copy, \c Paste, and \c {Select All}.

    \image qml-texteditor1_editmenu.png

    Armed with our knowledge of importing and customizing previously made
    components, we may now combine these menu pages to create a menu bar,
    consisting of buttons to select the menu, and look at how we may structure
    data using QML.

    \section1 Implementing a Menu Bar

    Our text editor application will need a way to display menus using a menu bar.
    The menu bar will switch the different menus and the user can choose which menu
    to display. Menu switching implies that the menus need more structure than
    merely displaying them in a row. QML uses models and views to structure data
    and display the structured data.

    \section2  Using Data Models and Views

    QML has different \l{QML Data Models}{data views} that display
    \l{QML Data Models}{data models}. Our menu bar will display the menus in a list,
    with a header that displays a row of menu names. The list of menus are declared
    inside a \c VisualItemModel. The \l{VisualItemModel}{\c VisualItemModel}
    element contains items that already have views such as \c Rectangle elements
    and imported UI elements. Other model types such as the \l{ListModel}{\c ListModel}
    element need a delegate to display their data.

    We declare two visual items in the \c menuListModel, the \c FileMenu and the
    \c EditMenu. We customize the two menus and display them using a
    \l {ListView}{ListView}. The \c MenuBar.qml file contains the QML declarations
    and a simple edit menu is defined in \c EditMenu.qml.

    \code
        VisualItemModel{
            id: menuListModel
            FileMenu{
                width: menuListView.width
                height: menuBar.height
                color: fileColor
            }
            EditMenu{
                color: editColor
                width:  menuListView.width
                height: menuBar.height
            }
        }
    \endcode

    The \l {ListView}{ListView} element will display a model according to a delegate.
    The delegate may declare the model items to display in a \c Row element or display
    the items in a grid. Our \c menuListModel already has visible items, therefore,
    we do not need to declare a delegate.

    \code
        ListView{
            id: menuListView

            //Anchors are set to react to window anchors
            anchors.fill:parent
            anchors.bottom: parent.bottom
            width:parent.width
            height: parent.height

            //the model contains the data
            model: menuListModel

            //control the movement of the menu switching
            snapMode: ListView.SnapOneItem
            orientation: ListView.Horizontal
            boundsBehavior: Flickable.StopAtBounds
            flickDeceleration: 5000
            highlightFollowsCurrentItem: true
            highlightMoveDuration:240
            highlightRangeMode: ListView.StrictlyEnforceRange
        }
    \endcode

    Additionally, \c ListView inherits from \l{Flickable}{\c Flickable}, making
    the list respond to mouse drags and other gestures. The last portion of the
    code above sets \c Flickable properties to create the desired flicking movement
    to our view. In particular,the property \c highlightMoveDuration changes the
    duration of the flick transition. A higher \c highlightMoveDuration value
    results in slower menu switching.

    The \c ListView maintains the model items through an \c index and each visual
    item in the model is accessible through the \c index, in the order of the
    declaration. Changing the \c currentIndex effectively changes the highlighted
    item in the \c ListView. The header of our menu bar exemplify this effect.
    There are two buttons in a row, both changing the current menu when clicked.
    The \c fileButton changes the current menu to the file menu when clicked,
    the \c index being \c 0 because \c FileMenu is declared first in the
    \c menuListModel. Similarly, the \c editButton will change the current
    menu to the \c EditMenu when clicked.

    The \c labelList rectangle has \c z value of \c 1, denoting that it is displayed
    at the front of the menu bar. Items with higher \c z values are displayed in front
    of items with lower \c z values. The default \c z value is \c 0.

    \code
        Rectangle{
            id: labelList
            ...
            z: 1
            Row{
                anchors.centerIn: parent
                spacing:40
                Button{
                    label: "File"
                    id: fileButton
                    ...
                    onButtonClick: menuListView.currentIndex = 0
                }
                Button{
                    id: editButton
                    label: "Edit"
                    ...
                    onButtonClick:    menuListView.currentIndex = 1
                }
            }
        }
    \endcode

    The menu bar we just created can be flicked to access the menus or by clicking
    on the menu names at the top. Switching menu screens feel intuitive and responsive.

    \image qml-texteditor2_menubar.png

    \section1 Building a Text Editor

    \section2 Declaring a TextArea

    Our text editor is not a text editor if it didn't contain an editable text area.
    QML's \l {TextEdit}{TextEdit} element allows the declaration of a multi-line
    editable text area. \l {TextEdit}{TextEdit} is different from a \l {Text}{Text}
    element, which doesn't allow the user to directly edit the text.

    \code
        TextEdit{
            id: textEditor
            anchors.fill:parent
            width:parent.width; height:parent.height
            color:"midnightblue"
            focus: true

            wrapMode: TextEdit.Wrap

            onCursorRectangleChanged: flickArea.ensureVisible(cursorRectangle)
        }
    \endcode

    The editor has its font color property set and set to wrap the text. The
    \c TextEdit area is inside a flickable area that will scroll the text if the
    text cursor is outside the visible area. The function \c ensureVisible() will
    check if the cursor rectangle is outside the visible boundaries and move the
    text area accordingly. QML uses Javascript syntax for its scripts, and as previously
    mentioned, Javascript files can be imported and used within a QML file.

    \code
        function ensureVisible(r){
            if (contentX >= r.x)
                contentX = r.x;
            else if (contentX+width <= r.x+r.width)
                contentX = r.x+r.width-width;
            if (contentY >= r.y)
                contentY = r.y;
            else if (contentY+height <= r.y+r.height)
                contentY = r.y+r.height-height;
        }
    \endcode

    \section2 Combining Components for the Text Editor

    We are now ready to create the layout of our text editor using QML. The text
    editor has two components, the menu bar we created and the text area. QML allows
    us to reuse components, therefore making our code simpler, by importing components
    and customizing when necessary. Our text editor splits the window into two;
    one-third of the screen is dedicated to the menu bar and two-thirds of the screen
    displays the text area. The menu bar is displayed in front of any other elements.

    \code
        Rectangle{

            id: screen
            width: 1000; height: 1000

            //the screen is partitioned into the MenuBar and TextArea. 1/3 of the screen is assigned to the MenuBar
            property int partition: height/3

            MenuBar{
                id:menuBar
                height: partition
                width:parent.width
                z: 1
            }

            TextArea{
                id:textArea
                anchors.bottom:parent.bottom
                y: partition
                color: "white"
                height: partition*2
                width:parent.width
            }
        }
    \endcode

    By importing reusable components, our \c TextEditor code looks much simpler.
    We can then customize the main application, without worrying about properties
    that already have defined behaviors. Using this approach, application layouts
    and UI components can be created easily.

    \image qml-texteditor3_texteditor.png

    \section1 Decorating the Text Editor
    \section2 Implementing a Drawer Interface

    Our text editor looks simple and we need to decorate it. Using QML, we can declare
    transitions and animate our text editor. Our menu bar is occupying one-third of the
    screen and it would be nice to have it only appear when we want it.

    We can add a drawer interface, that will contract or expand the menu bar when clicked.
    In our implementation, we have a thin rectangle that responds to mouse clicks. The
    \c drawer, as well as the application, has two sates: the "drawer is open" state and
    the "drawer is closed" state. The \c drawer item is a strip of rectangle with a small
    height. There is a nested \l {Image}{Image} element declaring that an arrow icon will
    be centered inside the drawer. The drawer assigns a state to the whole application,
    with the identifier \c screen, whenever a user clicks the mouse area.

    \code
        Rectangle{
            id:drawer
            height:15

            Image{
                id: arrowIcon
                source: "images/arrow.png"
                anchors.horizontalCenter: parent.horizontalCenter
            }

            MouseArea{
                id: drawerMouseArea
                anchors.fill:parent
                onClicked:{
                    if (screen.state == "DRAWER_CLOSED"){
                        screen.state = "DRAWER_OPEN"
                    }
                    else if (screen.state == "DRAWER_OPEN"){
                        screen.state = "DRAWER_CLOSED"
                    }
                }
                ...
            }
        }
    \endcode

    A state is simply a collection of configurations and it is declared in a
    \l{State}{State} element. A list of states can be listed and bound to the
    \c states property. In our application, the two states are called
    \c DRAWER_CLOSED and \c DRAWER_OPEN. Item configurations are declared in
    \l {PropertyChanges}{PropertyChanges} elements. In the \c DRAWER_OPEN state,
    there are four items that will receive property changes. The first target,
    \c menuBar, will change its \c y property to \c 0. Similarly, the \c textArea
    will lower to a new position when the state is \c DRAWER_OPEN. The \c textArea,
    the \c drawer, and the drawer's icon will undergo property changes to meet the
    current state.

    \snippet examples/tutorials/gettingStarted/gsQml/texteditor.qml states

    State changes are abrupt and needs smoother transitions. Transitions between states
    are defined using the \l {Transition}{Transition} element, which can then bind to
    the item's \c transitions property. Our text editor has a state transition whenever
    the state changes to either \c DRAWER_OPEN or \c DRAWER_CLOSED. Importantly, the
    transition needs a \c from and a \c to state but for our transitions, we can use
    the wild card \c * symbol to denote that the transition applies to all state changes.

    During transitions, we can assign animations to the property changes. Our
    \c menuBar switches position from \c {y:0} to \c {y:-partition} and we can animate
    this transition using the \l {NumberAnimation}{NumberAnimation} element. We declare
    that the targets' properties will animate for a certain duration of time and using
    a certain easing curve. An easing curve controls the animation rates and
    interpolation behavior during state transitions. The easing curve we chose is
    \l{PropertyAnimation::easing.type}{Easing.OutQuint}, which slows the movement near
    the end of the animation. Please read \l {qdeclarativeanimation.html}{QML's Animation}
    article.

    \snippet examples/tutorials/gettingStarted/gsQml/texteditor.qml transitions

    Another way of animating property changes is by declaring a \l {Behavior}{Behavior}
    element. A transition only works during state changes and \c Behavior can set an
    animation for a general property change. In the text editor, the arrow has a
    \c NumberAnimation animating its \c rotation property whenever the property changes.

    \code
    In TextEditor.qml:

        Behavior{
            NumberAnimation{property: "rotation";easing.type: Easing.OutExpo }
        }
    \endcode

    Going back to our components with knowledge of states and animations, we can improve
    the appearances of the components. In \c Button.qml, we can add \c color and \c scale
    property changes when the button is clicked. Color types are animated using
    \l {ColorAnimation}{ColorAnimation} and numbers are animated using
    \l {NumberAnimation}{NumberAnimation}. The \c {on propertyName} syntax displayed below
    is helpful when targeting a single property.

    \code
    In Button.qml:
        ...

        color: buttonMouseArea.pressed ? Qt.darker(buttonColor, 1.5) : buttonColor
        Behavior on color { ColorAnimation{ duration: 55} }

        scale: buttonMouseArea.pressed ? 1.1 : 1.00
        Behavior on scale { NumberAnimation{ duration: 55} }
    \endcode

    Additionally, we can enhance the appearances of our QML components by adding color
    effects such as gradients and opacity effects. Declaring a \l {Gradient}{Gradient}
    element will override the \c color property of the element. You may declare a color
    in the gradient using the \l {GradientStop}{GradientStop} element. The gradient is
    positioned using a scale, between \c 0.0 and \c 1.0.

    \code
    In MenuBar.qml
        gradient: Gradient {
            GradientStop { position: 0.0; color: "#8C8F8C" }
            GradientStop { position: 0.17; color: "#6A6D6A" }
            GradientStop { position: 0.98;color: "#3F3F3F" }
            GradientStop { position: 1.0; color: "#0e1B20" }
        }
    \endcode

    This gradient is used by the menu bar to display a gradient simulating depth.
    The first color starts at \c 0.0 and the last color is at \c 1.0.


    \section3 Where to Go from Here

        We are finished building the user interface of a very simple text editor.
        Going forward, the user interface is complete, and we can implement the
        application logic using regular Qt and C++. QML works nicely as a prototyping
        tool, separating the application logic away from the UI design.

    \image qml-texteditor4_texteditor.png

    \section2 Extending QML using Qt C++

    Now that we have our text editor layout, we may now implement the text editor
    functionalities in C++. Using QML with C++ enables us to create our application
    logic using Qt. We can create a QML context in a C++ application using the
    \l {Using QML in C++ Applications}{Qt's Declarative} classes and display the QML
    elements using a Graphics Scene. Alternatively, we can export our C++ code into
    a plugin that the \l {QML Viewer}{qmlviewer} tool can read. For our application,
    we shall implement the load and save functions in C++ and export it as a plugin.
    This way, we only need to load the QML file directly instead of running an executable.

    \section3 Exposing C++ Classes to QML

    We will be implementing file loading and saving using Qt and C++. C++ classes
    and functions can be used in QML by registering them. The class also needs to be
    compiled as a Qt plugin and the QML file will need to know where the plugin is located.

    For our application, we need to create the following items:
    \list 1
    \o \c Directory class that will handle directory related operations
    \o \c File class which is a QObject, simulating the list of files in a directory
    \o plugin class that will register the class to the QML context
    \o Qt project file that will compile the plugin
    \o A \c qmldir file telling the qmlviewer tool where to find the plugin
    \endlist

    \section3 Building a Qt Plugin

        To build a plugin, we need to set the following in a Qt project file. First,
        the necessary sources, headers, and Qt modules need to be added into our
        project file. All the C++ code and project files are in the \c filedialog
        directory.

    \code
    In filedialog.pro:

        TEMPLATE = lib
        CONFIG += qt plugin
        QT += declarative

        DESTDIR +=  ../plugins
        OBJECTS_DIR = tmp
        MOC_DIR = tmp

        TARGET = FileDialog

        HEADERS +=     directory.h \
                file.h \
                dialogPlugin.h

        SOURCES +=    directory.cpp \
                file.cpp \
                dialogPlugin.cpp
    \endcode

    In particular, we compile Qt with the \c declarative module and configure it as a
    \c plugin, needing a \c lib template. We shall put the compiled plugin into the
    parent's \c plugins directory.


    \section3 Registering a Class into QML

    \code
    In dialogPlugin.h:

        #include <QtDeclarative/QDeclarativeExtensionPlugin>

        class DialogPlugin : public QDeclarativeExtensionPlugin
        {
            Q_OBJECT

            public:
            void registerTypes(const char *uri);

        };

    \endcode

    Our plugin class, \c DialogPlugin is a subclass of \l{QDeclarativeExtensionPlugin}.
    We need to implement the inherited function, \l {QDeclarativeExtensionPlugin::}{registerTypes()}.
    The \c dialogPlugin.cpp file looks like this:

    \code
    DialogPlugin.cpp:

        #include "dialogPlugin.h"
        #include "directory.h"
        #include "file.h"
        #include <QtDeclarative/qdeclarative.h>

        void DialogPlugin::registerTypes(const char *uri){

            qmlRegisterType<Directory>(uri, 1, 0, "Directory");
            qmlRegisterType<File>(uri, 1, 0,"File");
        }

        Q_EXPORT_PLUGIN2(FileDialog, DialogPlugin);
    \endcode

    The \l{QDeclarativeExtensionPlugin::}{registerTypes()} function registers
    our File and Directory classes into QML. This function needs the class name
    for its template, a major version number, a minor version number, and a name
    for our classes.

    We need to export the plugin using the \l {Q_EXPORT_PLUGIN2}{Q_EXPORT_PLUGIN2}
    macro. Note that in our \c dialogPlugin.h file, we have the \l {Q_OBJECT}{Q_OBJECT}
    macro at the top of our class. As well, we need to run \c qmake on the project
    file to generate the necessary meta-object code.


    \section3 Creating QML Properties in a C++ class

        We can create QML elements and properties using C++ and
        \l {The Meta-Object System}{Qt's Meta-Object System}. We can implement
        properties using slots and signals, making Qt aware of these properties.
        These properties can then be used in QML.

    For the text editor, we need to be able to load and save files. Typically,
    these features are contained in a file dialog. Fortunately, we can use
    \l {QDir}{QDir}, \l {QFile}{QFile}, and \l {QTextStream}{QTextStream} to
    implement directory reading and input/output streams.

    \code
        class Directory : public QObject{

            Q_OBJECT

            Q_PROPERTY(int filesCount READ filesCount CONSTANT)
            Q_PROPERTY(QString filename READ filename WRITE setFilename NOTIFY filenameChanged)
            Q_PROPERTY(QString fileContent READ fileContent WRITE setFileContent NOTIFY fileContentChanged)
            Q_PROPERTY(QDeclarativeListProperty<File> files READ files CONSTANT )

            ...
    \endcode

    The \c Directory class uses Qt's Meta-Object System to register properties it
    needs to accomplish file handling. The \c Directory class is exported as a plugin
    and is useable in QML as the \c Directory element. Each of the listed properties
    using the \l {Q_PROPERTY()}{Q_PROPERTY} macro is a QML property.

    The \l {Q_PROPERTY()} {Q_PROPERTY} declares a property as well as its read and
    write functions into Qt's Meta-Object System. For example, the \c filename
    property, of type \l {QString}{QString}, is readable using the \c filename()
    function and writable using the function \c setFilename(). Additionally, there
    is a signal associated to the filename property called \c filenameChanged(),
    which is emitted whenever the property changes. The read and write functions
    are declared as \c public in the header file.

    Similarly, we have the other properties declared according to their uses. The
    \c filesCount property indicates the number of files in a directory. The filename
    property is set to the currently selected file's name and the loaded/saved file
    content is stored in \c fileContent property.

    \code
        Q_PROPERTY(QDeclarativeListProperty<File> files READ files CONSTANT )
    \endcode

    The \c files list property is a list of all the filtered files in a directory.
    The \c Directory class is implemented to filter out invalid text files; only
    files with a \c .txt extension are valid. Further, \l{QList}s can be
    used in QML files by declaring them as a QDeclarativeListProperty in C++.
    The templated object needs to inherit from a QObject, therefore,
    the \c File class must also inherit from QObject. In the \c Directory class,
    the list of \c File objects is stored in a QList called \c m_fileList.

    \code
        class File : public QObject{

            Q_OBJECT
            Q_PROPERTY(QString name READ name WRITE setName NOTIFY nameChanged)

            ...
        };
    \endcode

    The properties can then be used in QML as part of the \c Directory element's
    properties. Note that we do not have to create an identifier \c id property
    in our C++ code.

    \code
        Directory{
            id: directory

            filesCount
            filename
            fileContent
            files

            files[0].name
        }

    \endcode

    Because QML uses Javascript's syntax and structure, we can iterate through
    the list of files and retrieve its properties. To retrieve the first file's
    name property, we can call \c { files[0].name }.

    Regular C++ functions are also accessible from QML. The file loading and saving
    functions are implemented in C++ and declared using the
    \l {Q_INVOKABLE}{Q_INVOKABLE} macro. Alternatively, we can declare the functions
    as a \c slot and the functions will be accessible from QML.

    \code
    In Directory.h:

        Q_INVOKABLE void saveFile();
        Q_INVOKABLE void loadFile();
    \endcode

    The \c Directory class also has to notify other objects whenever the directory
    contents change. This feature is performed using a \c signal. As previously
    mentioned, QML signals have a corresponding handler with their names prepended
    with \c on. The signal is called \c directoryChanged and it is emitted whenever
    there is a directory refresh. The refresh simply reloads the directory contents
    and updates the list of valid files in the directory. QML items can then be
    notified by attaching an action to the \c onDirectoryChanged signal handler.

    The \c list properties need to be explored further. This is because list
    properties use callbacks to access and modify the list contents. The list
    property is of type \c QDeclarativeListProperty<File>. Whenever the list
    is accessed, the accessor function needs to return a
    \c QDeclarativeListProperty<File>. The template type, \c File, needs to be a
    \c QObject derivative. Further, to create the
    \l {QDeclarativeListProperty}{QDeclarativeListProperty}, the list's accessor
    and modifiers need to be passed to the constructor as function pointers. The list,
    a \c QList in our case,  also needs to be a list of \c File pointers.

    The constructor of \l {QDeclarativeListProperty}{QDeclarativeListProperty}
    constructor and the \c Directory implementation:
    \code
        QDeclarativeListProperty  ( QObject * object, void * data, AppendFunction append, CountFunction count = 0, AtFunction at = 0, ClearFunction clear = 0 )
        QDeclarativeListProperty<File>( this, &m_fileList, &appendFiles, &filesSize, &fileAt,  &clearFilesPtr );
    \endcode

    The constructor passes pointers to functions that will append the list, count
    the list, retrieve the item using an index, and empty the list. Only the append
    function is mandatory. Note that the function pointers must match the definition
    of \l {QDeclarativeListProperty::AppendFunction}{AppendFunction},
    \l {QDeclarativeListProperty::CountFunction}{CountFunction},
    \l {QDeclarativeListProperty::AtFunction}{AtFunction}, or
    \l {QDeclarativeListProperty::ClearFunction}{ClearFunction}.

    \code
        void appendFiles(QDeclarativeListProperty<File> * property, File * file)
        File* fileAt(QDeclarativeListProperty<File> * property, int index)
        int filesSize(QDeclarativeListProperty<File> * property)
        void clearFilesPtr(QDeclarativeListProperty<File> *property)
    \endcode

    To simplify our file dialog, the \c Directory class filters out invalid text
    files, which are files that do not have a \c .txt extension. If a file name
    doesn't have the \c .txt extension, then it won't be seen in our file dialog.
    Also, the implementation makes sure that saved files have a \c .txt extension in
    the file name. \c Directory uses \l {QTextStream}{QTextStream} to read the file
    and to output the file contents to a file.

    With our \c Directory element, we can retrieve the files as a list, know how many
    text files is in the application directory, get the file's name and content as a
    string, and be notified whenever there are changes in the directory contents.

    To build the plugin, run \c qmake on the \c filedialog.pro project file, then run
    \c make to build and transfer the plugin to the \c plugins directory.


    \section3 Importing a Plugin in QML

        The qmlviewer tool imports files that are in the same directory as the
        application. We can also create a \c qmldir file containing the locations of
        QML files we wish to import. The \c qmldir file can also store locations of
        plugins and other resources.

    \code
    In qmldir:

        Button ./Button.qml
        FileDialog ./FileDialog.qml
        TextArea ./TextArea.qml
        TextEditor ./TextEditor.qml
        EditMenu ./EditMenu.qml

        plugin FileDialog plugins
    \endcode

    The plugin we just created is called \c FileDialog, as indicated by the
    \c TARGET field in the project file. The compiled plugin is in the \c plugins directory.


    \section3 Integrating a File Dialog into the File Menu

        Our \c FileMenu needs to display the \c FileDialog element, containing a list of
        the text files in a directory thus allowing the user to select the file by
        clicking on the list. We also need to assign the save, load, and new buttons
        to their respective actions. The FileMenu contains an editable text input to
        allow the user to type a file name using the keyboard.

    The \c Directory element is used in the \c FileMenu.qml file and it notifies the
    \c FileDialog element that the directory refreshed its contents. This notification
    is performed in the signal handler, \c onDirectoryChanged.

    \code
    In FileMenu.qml:

        Directory{
            id:directory
            filename: textInput.text
            onDirectoryChanged: fileDialog.notifyRefresh()
        }
    \endcode

    Keeping with the simplicity of our application, the file dialog will always be
    visible and will not display invalid text files, which do not have a \c .txt
    extension to their filenames.

    \code
    In FileDialog.qml:

        signal notifyRefresh()
        onNotifyRefresh: dirView.model = directory.files
    \endcode

    The \c FileDialog element will display the contents of a directory by reading its
    list property called \c files. The files are used as the model of a
    \l {GridView}{GridView} element, which displays data items in a grid according
    to a delegate. The delegate handles the appearance of the model and our file
    dialog will simply create a grid with text centered in the middle. Clicking on
    the file name will result in the appearance of a rectangle to highlight the file
    name. The \c FileDialog is notified whenever the \c notifyRefresh signal is emitted,
    reloading the files in the directory.

    \code
    In FileMenu.qml:

        Button{
            id: newButton
            label: "New"
            onButtonClick:{
                textArea.textContent = ""
            }
        }
        Button{
            id: loadButton
            label: "Load"
            onButtonClick:{
                directory.filename = textInput.text
                directory.loadFile()
                textArea.textContent = directory.fileContent
            }
        }
        Button{
            id: saveButton
            label: "Save"
            onButtonClick:{
                directory.fileContent = textArea.textContent
                directory.filename = textInput.text
                directory.saveFile()
            }
        }
        Button{
            id: exitButton
            label: "Exit"
            onButtonClick:{
                Qt.quit()
            }
        }
    \endcode

    Our \c FileMenu can now connect to their respective actions. The \c saveButton
    will transfer the text from the \c TextEdit onto the directory's \c fileContent
    property, then copy its file name from the editable text input. Finally, the button
    calls the \c saveFile() function, saving the file. The \c loadButton has a similar
    execution. Also, the \c New action will empty the contents of the \c TextEdit.

    Further, the \c EditMenu buttons are connected to the \c TextEdit functions to copy,
    paste, and select all the text in the text editor.

    \image qml-texteditor5_filemenu.png

    \section2 Text Editor Completion

    \image qml-texteditor5_newfile.png

    The application can function as a simple text editor, able to accept text
    and save the text into a file. The text editor can also load from a file and
    perform text manipulation.

    \section1 Running the Text Editor

    We need to compile the file dialog C++ plugin before the text editor can run.
    To compile, enter the \c gsQml directory, then run \c qmake and compile
    using \c make or \c nmake, depending on your platform. To run, launch
    qmlviewer and open the \c texteditor.qml file.

    The source code is in the \c{examples/tutorials/gettingStarted/gsQml}
    directory.
*/