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/*!
    \page qml-textEditor.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 create a simple text editor application using QML.
	After reading this guide, you should be ready to start developing your own
	applications using QML and Qt C++.

	\example tutorials/gettingStarted/gsQml
	
	\section1 QML to Build User Interfaces

	Here we are building is a simple text editor that con load, save,
	and perform some text manipulation. This guide consists of two parts. The
	first part involves designing the application layout and behaviors using
	declarative language in QML. For the second part, file loading and saving is
	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. By utilizing QML and Qt C++,
	we can efficiently decouple the interface logic from the application logic.
	
	\image qml-texteditor5_editmenu.png
	
	To run the QML example code, we 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.
	
	\omit
	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
	\endomit

	\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. 
	
	\code
		import Qt  4.7
		Rectangle {
			id: simplebutton
			color: "grey"
			width: 150; height: 75
			
			Text{
				id: buttonLabel
				anchors.centerIn: parent
				text: "button label"	
			}
		}
	\endcode
	
	First, the \c { import Qt 4.7 } 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 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 specfied \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 Qt 4.7				\\import the main Qt QML module
		import “folderName”			\\import the contents of the folder	
		import “Button.qml”			\\import a QML file
		import “NewButton.qml” as ButtonModule	\\import a QML file and give it a name
		import “script.js” as Script		\\import a Javascript file and name it as Script
	\endcode

	To use the \c Button element in \c FileMenu.qml, we need to import \c Button.qml.
	The syntax shown above, shows how to use the \c import keyword. However, the
	\c {import Button.qml} is not necessary; qmlviewer will import all the contents
	of the current directory. 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 {Data Models}{data views} that display
	\l {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
		
	\section1 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.

	\code

		states:[
			State{
				name: "DRAWER_OPEN"
				PropertyChanges { target: menuBar; y:0}
				PropertyChanges { target: textArea; y: partition + drawer.height}
				PropertyChanges { target: drawer; y: partition}
				PropertyChanges { target: arrowIcon; rotation: 180} 
			},
			State{
				name: "DRAWER_CLOSED"
				PropertyChanges { target: menuBar; y:-partition}
				PropertyChanges { target: textArea; y: drawer.height; height: screen.height - drawer.height}
				PropertyChanges { target: drawer; y: 0}
				PropertyChanges { target: arrowIcon; rotation: 0} 
			}

		]

	\endcode

	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. Pleae read \l {qdeclarativeanimation.html}{QML's Animation}
	article.

	\code
		transitions: [
			Transition{
				to: "*"
				NumberAnimation { target: textArea; properties: "y, height"; duration: 100; easing.type: Easing.OutQuint }
				NumberAnimation { target: menuBar; properties: "y"; duration: 100;easing.type: Easing.OutQuint }
				NumberAnimation { target: drawer; properties: "y"; duration: 100;easing.type: Easing.OutQuint }
			}
		]
	\endcode

	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.


	\section2 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  
	
	\section1 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.

	\section2 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

	\section2 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 cppPlugins.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.


	\section2 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}{QDeclarativeExtensionPlugin}. We need to implement the inherited function, \l {QDeclarativeExtensionPlugin::registerTypes}{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}{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.


	\section2 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 {QLists}{QLists} can be
	used in QML files by declaring them as a \c QDeclarativeListProperty in C++.
	The templated object needs to inherit from a \l {QObject}{QObject}, therefore,
	the \c File class must also inherit from \c QObject. In the \c Directory class,
	the list of \c File objects is stored in a \c 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 consructor 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 cppPlugins.pro project file, then run
	\c make to build and transfer the plugin to the \c plugins directory.


	\section2 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. 


	\section2 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 sloadButton 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

	\section1 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. 
		
		
*/