/****************************************************************************
**
** Copyright (C) 2017 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
\example chattutorial
\title Qt Quick Controls 2 - Chat Tutorial
\brief Tutorial about writing a basic chat client using Qt Quick Controls 2.
\ingroup qtquickcontrols2-examples

This tutorial shows how to write a basic chat application using Qt Quick
Controls 2. It will also explain how to integrate an SQL database into a Qt
application.

\section1 Chapter 1: Setting Up

When setting up a new project, it's easiest to use
\l {Qt Creator Manual}{Qt Creator}. For this project, we chose the
\l {Qt Creator: Creating Qt Quick Projects}{Qt Quick application} template, which creates a
basic "Hello World" application with all of the necessary files.

\section2 main.cpp

As we created a Qt Quick application, our \c main.cpp has two includes:

\quotefromfile chattutorial/chapter1-settingup/main.cpp
\skipto include
\printline include
\printline include

The first gives us access to QGuiApplication. All Qt applications require
an application object, but the precise type depends on what the application
does. QCoreApplication is sufficient for non-graphical applications.
QGuiApplication is sufficient for graphical applications that do not use
\l {Qt Widgets}, while QApplication is required for those that do.

The second include makes QQmlApplicationEngine available, along with
some useful functions required for making C++ types accessible from QML.

Within \c main(), we set up the application object and QML engine:

\skipto main
\printuntil }

To enable Qt's support for \l {High DPI Displays}{high DPI scaling}, it
is necessary to set an attribute before the application object is constructed.

After that's done, we construct the application object, passing any application
arguments provided by the user.

Next, the QML engine is created. \l QQmlApplicationEngine is a convenient
wrapper over QQmlEngine, providing the \l {QQmlApplicationEngine::load}{load()}
function to easily load QML for an application. It also adds some convenience
for using \l {Using File Selectors with Qt Quick Controls 2}{file selectors}.

Once we've set up things in C++, we can move on to the user interface in QML.

\section2 main.qml

\quotefromfile chattutorial/chapter1-settingup/main.qml
\skipto import
\printuntil import QtQuick.Controls 2.1

First, we import the \l {Qt Quick} module. This gives us
access to graphical primitives such as \l Item, \l Rectangle, \l Text, and so
on.
For the full list of types, see the \l {Qt Quick QML Types} documentation.

Next, we import the Qt Quick Controls 2 module. Amongst other things, this
makes \l ApplicationWindow available:

\skipto ApplicationWindow
\printuntil visible: true
\printuntil }
\printuntil }
\printuntil }

ApplicationWindow is a \l Window with some added convenience for creating a
\l {ApplicationWindow::}{header} and a \l {ApplicationWindow::}{footer}.
It also provides the foundation for \l {Popup}{popups} and supports some
basic styling, such as the background \l {Window::}{color}.

There are three properties that are almost always set when using
ApplicationWindow: \l {Window::}{width}, \l {Window::}{height}, and
\l {Window::}{visible}.
Once we've set these, we have a properly sized, empty window ready to be
filled with content.

The first \e "screen" in our application will be a list of contacts. It would
be nice to have some text at the top of each screen that describes its purpose.
The header and footer properties of ApplicationWindow could work in
this situation. They have some characteristics that make them ideal for
items that should be displayed on every screen of an application:

\list
\li They are anchored to the top and bottom of the window, respectively.
\li They fill the width of the window.
\endlist

However, when the contents of the header and footer varies depending on
which screen the user is viewing, it can be much easier to use \l Page.
For now, we'll just add one page, but in the next chapter, we'll demonstrate
how to navigate between several pages.

Now that we have a Page, we can assign a \l Label to its \l {Page::}{header}
property. Label extends the primitive \l Text item from the Qt Quick module by
adding \l {Styling Qt Quick Controls 2}{styling} and \l {Control::}{font}
inheritance. This means that a Label can look different depending on which
style is in use, and can also propagate its pixel size to its children.

We want some distance between the top of the application window and the text,
so we set the \l {Text::padding}{padding} property. This will allocate extra
space on each side of the label (within its bounds). We could have also set the
\l {Text::}{topPadding} and \l {Text::}{bottomPadding} properties explicitly.

We set the text of the label using the \c qsTr() function, which ensures that
the text can be translated by \l {Writing Source Code for Translation}{Qt's
translation system}. It's a good idea to do this for text that will
be visible to the end users of your application.

By default, text is vertically aligned to the top of its bounds, while the
horizontal alignment depends on the natural direction of the text; for example,
text that is read from left to right will be aligned to the left. If we
used these defaults, our text would be at the top-left corner of the window.
This is not desirable for a header, so we align the text to the center of its
bounds, both horizontally and vertically.

\section2 The Project File

The \c .pro or \l {Creating Project Files}{project} file contains all of the
information needed by \l {qmake Manual}{qmake} to generate a Makefile, which is
then used to compile and link the application.

\quotefromfile chattutorial/chapter1-settingup/chapter1-settingup.pro
\printline TEMPLATE

The first line tells \c qmake which kind of project this is. We're building an
application, so we use the \c app template.

\printline QT

The next line declares the Qt libraries that we want to use from C++.

\printline CONFIG

This line states that a C++11 compatible compiler is required to build the
project.

\printline SOURCES

The \c SOURCES variable lists all of the source files that should be compiled.
A similar variable, \c HEADERS, is available for header files.

\printline RESOURCES

The next line tells \c qmake that we have a collection of
\l {The Qt Resource System}{resources} that should be built into the
executable.

\printline target.path

This line determines where the example will be copied to when running
\c {make install}.

Now we can build and run the application:

\borderedimage qtquickcontrols2-chattutorial-chapter1.png

\section1 Chapter 2: Lists

In this chapter, we'll explain how to create a list of interactive items using
\l ListView and \l ItemDelegate.

ListView comes from the Qt Quick module, and displays a list of items
populated from a \l {Models and Views in Qt Quick}{model}. ItemDelegate comes from
the Qt Quick Controls 2 module, and provides a standard view item for use in views
and controls such as ListView and \l ComboBox. For example, each ItemDelegate
can display text, be checked on and off, and react to mouse clicks.

Here is our ListView:

\quotefromfile chattutorial/chapter2-lists/main.qml
\dots 8
\codeline
\skipto ListView
\printuntil }
\printuntil }
\printuntil }
\codeline
\dots 8

\section2 Sizing and Positioning

The first thing we do is set a size for the view. It should fill the available
space on the page, so we use \l {Item::anchors}{anchors.fill}. Note that
Page ensures that its header and footer have enough of their own space
reserved, so the view in this case will sit below the header, for example.

Next, we set \l {Flickable::leftMargin}{margins} around the ListView to put
some distance between it and the edges of the window. The margin properties
reserve space within the bounds of the view, which means that the empty areas
can still be \e "flicked" by the user.

The items should be nicely spaced out within the view, so the
\l {ListView::}{spacing} property is set to \c 20.

\section2 Model

In order to quickly populate the view with some items, we've used a JavaScript
array as the model. One of the greatest strengths of QML is its ability to
make prototyping an application extremely quick, and this is an example of
that. It's also possible to simply assign a \l {Integers as Models}{number} to
the model property to indicate how many items you need. For example, if you
assign \c 10 to the \c model property, each item's display text will be a
number from \c 0 to \c 9.

However, once the application gets past the prototype stage, it quickly becomes
necessary to use some real data. For this, it's best to use a proper C++ model
by \l {QAbstractItemModel}{subclassing QAbstractItemModel}.

\section2 Delegate

On to the \l {ListView::}{delegate}. We assign the corresponding text from the
model to the \l {AbstractButton::text}{text} property of ItemDelegate. The exact
manner in which the data from the model is made available to each delegate
depends on the type of model used. See \l {Models and Views in Qt Quick} for
more information.

In our application, the width of each item in the view should be the same
as the width of the view. This ensures that the user has a lot of room with
which to select a contact from the list, which is an important factor on
devices with small touch screens, like mobile phones. However, the width of the
view includes our \c 48 pixel margins, so we must account for that in our
assignment to the width property.

Next, we define an \l Image. This will display a picture of the user's contact.
The image will be \c 40 pixels wide and \c 40 pixels high. We'll base the
height of the delegate on the image's height, so that we don't have any empty
vertical space.

\borderedimage qtquickcontrols2-chattutorial-chapter2.png

\section1 Chapter 3: Navigation

In this chapter, you'll learn how to use \l StackView to navigate between pages
in an application. Here's the revised \c main.qml:

\quotefromfile chattutorial/chapter3-navigation/main.qml
\skipto import
\printuntil }
\printuntil }
\printuntil }

\section2 StackView

As its name suggests, StackView provides stack-based navigation. The last item
to be \e "pushed" onto the stack is the first one to be removed, and the
top-most item is always the one that is visible.

In the same manner as we did with Page, we tell the StackView to fill the
application window. The only thing left to do after that is to give it an item
to display, via \l {StackView::}{initialItem}. StackView accepts
\l {Item}{items}, \l {Component}{components} and \l [QML]{url}{URLs}.

You'll notice that we moved the code for the contact list into
\c ContactPage.qml. It's a good idea to do this as soon as you have a general
idea of which screens your application will contain. Doing so not only makes
your code easier to read, but ensures that items are only instantiated from
a given component when completely necessary, reducing memory usage.

\note Qt Creator provides several convenient \l {http://doc.qt.io/qtcreator/creator-editor-refactoring.html#refactoring-qml-code}{refactoring options for QML},
one of which allows you to move a block of code into a separate file
 (\c {Alt + Enter > Move Component into Separate File}).

Another thing to consider when using ListView is whether to refer to it by
\c id, or use the attached \l {ListView::view}{ListView.view}
property. The best approach depends on a few different factors. Giving the
view an id will result in shorter and more efficient binding expressions, as
the attached property has a very small amount of overhead. However, if you plan
on reusing the delegate in other views, it is better to use the attached
properties to avoid tying the delegate to a particular view. For example, using
the attached properties, the \c width assignment in our delegate becomes:

\code
width: ListView.view.width - ListView.view.leftMargin - ListView.view.rightMargin
\endcode

In chapter 2, we added a ListView below the header. If you run the application
for that chapter, you'll see that the contents of the view can be scrolled over
the top of the header:

\borderedimage qtquickcontrols2-chattutorial-chapter2-listview-header.gif

This is not that nice, especially if the text in the
delegates is long enough that it reaches the text in the header. What we
ideally want to do is to have a solid block of color under the header text, but
\e {above} the view. This ensures that the listview contents can't visually
interfere with the header contents. Note that it's also possible to achieve
this by setting the \l {Item::}{clip} property of the view to \c true, but
doing so \l {Clipping}{can affect performance}.

\l ToolBar is the right tool for this job. It is a container of both
application-wide and context-sensitive actions and controls, such as navigation
buttons and search fields. Best of all, it has a background color that, as
usual, comes from the application style. Here it is in action:

\quotefromfile chattutorial/chapter3-navigation/ContactPage.qml
\skipto header
\printuntil }
\printuntil }

\borderedimage qtquickcontrols2-chattutorial-chapter3-listview-header.gif

It has no layout of its own, so we center the label within it ourselves.

The rest of the code is the same as it was in chapter 2, except that we've
taken advantage of the \l {AbstractButton::}{clicked} signal to push the next
page onto the stackview:

\skipto onClicked
\printline onClicked

When pushing a \l Component or \l [QML] url onto StackView, it's often
necessary to initialize the (eventually) instantiated item with some variables.
StackView's \l {StackView::push}{push()} function accounts for this, by taking a JavaScript object
as the second argument. We use this to provide the next page with a contact's
name, which it then uses to display the relevant conversation. Note the
\c {root.StackView.view.push} syntax; this is necessary because of how
\l {A Note About Accessing Attached Properties and Signal Handlers}
{attached properties} work.

Let's step through \c ConversationPage.qml, beginning with the imports:

\quotefromfile chattutorial/chapter3-navigation/ConversationPage.qml
\skipto import
\printline import
\printline import
\printline import

These are the same as before, except for the addition of the \c QtQuick.Layouts
import, which we'll cover shortly.

\skipto Page
\printuntil }
\printuntil }
\printuntil }
\dots 4

The root item of this component is another Page, which has a custom property
called \c inConversationWith. For now, this property will simply determine what
the label in the header displays. Later on, we'll use it in the SQL query that
populates the list of messages in the conversation.

To allow the user to go back to the Contact page, we add a \l ToolButton that
calls \l {StackView::pop}{pop()} when clicked. A \l ToolButton is functionally
similar to \l Button, but provides a look that is more suitable within a
ToolBar.

There are two ways of laying out items in QML: \l {Item Positioners}
and \l {Qt Quick Layouts}. Item positioners (\l Row, \l Column, and so on) are
useful for situations where the size of items is known or fixed, and all that
is required is to neatly position them in a certain formation. The layouts in
Qt Quick Layouts can both position and resize items, making them well suited
for resizable user interfaces. Below, we use \l ColumnLayout to vertically
lay out a ListView and a \l Pane:

\skipto ColumnLayout
\printto Layout.margins
\codeline
\dots 12
\codeline
\skipuntil ScrollBar
\printline }
\codeline
\dots 8
\codeline
\printuntil Layout.fillWidth: true
\dots 12
\skipuntil }
\skipuntil }
\skipuntil }
\skipuntil }
\printline }

Pane is basically a rectangle whose color comes from the application's style.
It is similar to \l Frame, with the only difference being that it has no stroke
around its border.

Items that are direct children of a layout have various
\l {Layout}{attached properties} available to them. We use
\l {Layout::fillWidth}{Layout.fillWidth} and
\l {Layout::fillHeight}{Layout.fillHeight} on the ListView to ensure
that it takes as much space within the ColumnLayout as it can. The
same is done for the Pane. As ColumnLayout is a vertical layout, there
aren't any items to the left or right of each child, so this will result in
each item consuming the entire width of the layout.

On the other hand, the \l {Layout::fillHeight}{Layout.fillHeight} statement in
the ListView will enable it to occupy the remaining space that is left after
accommodating the Pane.

Let's look at the listview in detail:

\quotefromfile chattutorial/chapter3-navigation/ConversationPage.qml
\skipto ListView
\printuntil ScrollBar
\printuntil }

After filling the width and height of its parent, we also set some margins on
the view. This gives us a nice alignment with the placeholder text in the
"compose message" field:

\borderedimage qtquickcontrols2-chattutorial-chapter3-view-margins.png

Next, we set \l {ListView::}{displayMarginBeginning} and \l
{ListView::}{displayMarginEnd}. These properties ensure that the delegates
outside the bounds of the view do not disappear while scrolling at the edges of
the view. It's easiest to understand this by commenting out the properties and
seeing what happens when scrolling the view.

We then flip the vertical direction of the view, so that first items are at the
bottom. The delegates are spaced out by 12 pixels, and a \e "dummy" model is
assigned for testing purposes, until we implement the real model in chapter 4.

Within the delegate, we declare a \l Row as the root item, as we want the
avatar to be followed by the message contents, as shown in the image above.

Messages sent by the user should be distinguished from those sent by a contact.
For now, we set a dummy property \c sentByMe, which simply uses the index
of the delegate to alternate between different authors. Using this property,
we distinguish between different authors in three ways:

\list
\li Messages sent by the user are aligned to the right side of the screen
by setting \c anchors.right to \c parent.right.

\li By setting the \c visible property of the avatar (which is simply a
Rectangle for now) based on \c sentByMe, we only show it if the message was
sent by a contact.

\li We change the color of the rectangle depending on the author. Since we
do not want to display dark text on a dark background, and vice versa, we also
set the text color depending on who the author is. In chapter 5, we'll see how
styling takes care of matters like this for us.
\endlist

At the bottom of the screen, we place a \l TextArea item to allow multi-line
text input, and a button to send the message. We use Pane to cover the area
under these two items, in the same way that we use ToolBar to prevent the
contents of the listview from interfering with the page header:

\skipto Pane
\printuntil }
\printuntil }
\printuntil }
\printuntil }

The TextArea should fill the available width of the screen. We assign some
placeholder text to provide a visual cue to the user as to where they should
begin typing. The text within the input area is wrapped to ensure that it
does not go outside of the screen.

Finally, the button is only enabled when there is actually a message to send.

\borderedimage qtquickcontrols2-chattutorial-chapter3.gif

\section1 Chapter 4: Models

In chapter 4, we'll take you through the process of creating both read-only and
read-write SQL models in C++ and exposing them to QML to populate views.

\section2 QSqlQueryModel

In order to keep the tutorial simple, we've chosen to make the list of user
contacts non-editable. \l QSqlQueryModel is the logical choice for this
purpose, as it provides a read-only data model for SQL result sets.

Let's take a look at our \c SqlContactModel class that derives from
QSqlQueryModel:

\quotefromfile chattutorial/chapter4-models/sqlcontactmodel.h
\skipto #include
\printuntil };

There's not much going on here, so let's move on to the \c .cpp file:

\quotefromfile chattutorial/chapter4-models/sqlcontactmodel.cpp
\skipto #include
\printuntil }
\printuntil }
\printuntil }

We include the header file of our class and those that we require from Qt. We
then define a static function named \c createTable() that we'll use to create
the SQL table (if it doesn't already exist), and then populate it with some
dummy contacts.

The call to \l {QSqlDatabase::database}{database()} might look a little bit
confusing because we have not set up a specific database yet. If no connection
name is passed to this function, it will return a \e {"default connection"},
whose creation we will cover soon.

\skipto SqlContactModel
\printuntil }

In the constructor, we call \c createTable(). We then construct a query that
will be used to populate the model. In this case, we are simply interested in
all rows of the \c Contacts table.

\section2 QSqlTableModel

\c SqlConversationModel is more complex:

\quotefromfile chattutorial/chapter4-models/sqlconversationmodel.h
\skipto #include
\printuntil };

We use both the \c Q_PROPERTY and \c Q_INVOKABLE macros, and therefore we must
let \l {Using the Meta-Object Compiler (moc)}{moc} know by using the \c
Q_OBJECT macro.

The \c recipient property will be set from QML to let the model know which
conversation it should retrieve messages for.

We override the \l {QSqlTableModel::data}{data()} and
\l {QAbstractItemModel::}{roleNames()} functions so that we can use our
custom roles in QML.

We also define the \c sendMessage() function that we want to call from
QML, hence the \c Q_INVOKABLE macro.

Let's take a look at the \c .cpp file:

\quotefromfile chattutorial/chapter4-models/sqlconversationmodel.cpp
\skipto #include
\printuntil }
\printuntil }
\printuntil }

This is very similar to \c sqlcontactmodel.cpp, with the exception that we are
now operating on the \c Conversations table. We also define
\c conversationsTableName as a static const variable, as we use it in a couple
of places throughout the file.

\skipto SqlConversationModel
\printuntil }

As with \c SqlContactModel, the first thing that we do in the constructor is
create the table. We tell QSqlTableModel the name of the table we'll be using
via the \l {QSqlTableModel::setTable}{setTable()} function. To ensure that the
latest messages in the conversation are shown first, we sort the query results
by the \c timestamp field in descending order. This goes hand in hand with
setting ListView's \l {ListView::}{verticalLayoutDirection} property to
\c ListView.BottomToTop (which we covered in chapter 3).

\skipto ::recipient(
\printuntil }
\printuntil }

In \c setRecipient(), we set a filter over the results returned from
the database.

\skipto ::data(
\printuntil }

The \c data() function falls back to QSqlTableModel's implementation if the
role is not a custom user role. If the role is a user role, we can subtract
Qt::UserRole from it to get the index of that field and then use that to find
the value that we need to return.

\skipto ::roleNames(
\printuntil }

In \c roleNames(), we return a mapping of our custom role values to role names.
This enables us to use these roles in QML. It can be useful to declare an enum
to hold all of the role values, but since we don't refer to any specific value
in code outside of this function, we don't bother.

\skipto ::sendMessage(
\printuntil }

The \c sendMessage() function uses the given \c recipient and a \c message to
insert a new record into the database. Due to our usage
of \l QSqlTableModel::OnManualSubmit, we must manually call
\l {QSqlTableModel::submitAll}{submitAll()}.

\section2 Connecting to the Database and Registering Types With QML

Now that we've established the model classes, let's take a look at \c main.cpp:

\quotefromfile chattutorial/chapter4-models/main.cpp
\skipto #include
\printuntil return app.exec();
\printuntil }

\c connectToDatabase() creates the connection to the SQLite database, creating
the actual file if it doesn't already exist.

Within \c main(), we call \l {qmlRegisterType}{qmlRegisterType()} to
register our models as types within QML.

\section2 Using the Models in QML

Now that we have the models available as QML types, there are some minor
changes to be done to \c ContactPage.qml. To be able to use the types,
we must first import them using the URI we set in \c main.cpp:

\quotefromfile chattutorial/chapter4-models/ContactPage.qml
\skipto import io.qt.examples.chattutorial 1.0
\printline import io.qt.examples.chattutorial 1.0

We then replace the dummy model with the proper one:

\skipto model: SqlContactModel {}
\printline model: SqlContactModel {}

Within the delegate, we use a different syntax for accessing the model data:

\skipto text: model.display
\printline text: model.display

In \c ConversationPage.qml, we add the same \c chattutorial import, and replace
the dummy model:

\quotefromfile chattutorial/chapter4-models/ConversationPage.qml
\skipto model: SqlConversationModel {
\printuntil }

Within the model, we set the \c recipient property to the name of the contact
for which the page is being displayed.

The root delegate item changes from a Row to a Column, to accommodate the
timestamp that we want to display below every message:

\skipto delegate: Column {
\printuntil Label {
\printuntil }
\printuntil }
\printuntil }
\printuntil }
\printuntil }

\borderedimage qtquickcontrols2-chattutorial-chapter4-message-timestamp.png

Now that we have a proper model, we can use its \c recipient role in the
expression for the \c sentByMe property.

The Rectangle that was used for the avatar has been converted into an Image.
The image has its own implicit size, so we don't need to specify it explicitly.
As before, we only show the avatar when the author isn't the user, except this
time we set the \c source of the image to an empty URL instead of using the
\c visible property.

We want each message background to be slightly wider (12 pixels each side) than
its text. However, if it's too long, we want to limit its width to the edge
of the listview, hence the usage of \c Math.min(). When the message wasn't sent
by us, an avatar will always come before it, so we account for that by
subtracting the width of the avatar and the row spacing.

For example, in the image above, the implicit width of the message text is the
smaller value. However, in the image below, the message text is quite long, so
the smaller value (the width of the view) is chosen, ensuring that the text
stops at the opposite edge of the screen:

\borderedimage qtquickcontrols2-chattutorial-chapter4-long-message.png

In order to display the timestamp for each message that we discussed earlier,
we use a Label. The date and time are formatted with
\l {QtQml::Qt::formatDateTime}{Qt.formatDateTime()}, using a custom format.

The \e "send" button must now react to being clicked:

\skipto Button
\printuntil }
\printuntil }

First, we call the invokable \c sendMessage() function of the model, which
inserts a new row into the Conversations database table. Then, we clear the
text field to make way for future input.

\borderedimage qtquickcontrols2-chattutorial-chapter4.gif

\section1 Chapter 5: Styling

Styles in Qt Quick Controls 2 are designed to work on any platform. In this
chapter, we'll do some minor visual tweaks to make sure our application
looks good when run with the \l {Default Style}{Default},
\l {Material Style}{Material}, and \l {Universal Style}{Universal} styles.

So far, we've just been testing the application with the Default style. If we
run it with the \l {Material Style}, for example, we'll immediately see some issues.
Here is the Contacts page:

\borderedimage qtquickcontrols2-chattutorial-chapter5-contacts-material-test.png

The header text is black on a dark blue background, which is very difficult to
read. The same thing occurs with the Conversations page:

\borderedimage qtquickcontrols2-chattutorial-chapter5-conversations-material-test.png

The solution is to tell the toolbar that it should use the \e "Dark" theme, so
that this information is propagated to its children, allowing them to switch
their text color to something lighter. The simplest way of doing so is to
import the Material style directly and use the Material attached property:

\code
    import QtQuick.Controls.Material 2.1

    // ...

    header: ToolBar {
        Material.theme: Material.Dark

        // ...
    }
\endcode

However, this brings with it a hard dependency to the Material style; the
Material style plugin \e must be deployed with the application, even if the
target device doesn't use it, otherwise the QML engine will fail to find the
import.

Instead, it is better to rely on Qt Quick Controls 2's built-in support for
\l {Using File Selectors with Qt Quick Controls 2}{style-based file selectors}.
To do this, we must move the ToolBar out into its own file. We'll call it
\c ChatToolBar.qml. This will be the \e "default" version of the file, which
means that it will be used when the \l {Default Style}{Default style} is in
use. Here's the new file:

\quotefromfile chattutorial/chapter5-styling/ChatToolBar.qml
\skipto import
\printuntil }

As we only use the ToolBar type within this file, we only need the
Qt Quick Controls 2 import. The code itself has not changed from how it was
in \c ContactPage.qml, which is how it should be; for the default version
of the file, nothing needs to be different.

Back in \c ContactPage.qml, we update the code to use the new type:

\quotefromfile chattutorial/chapter5-styling/ContactPage.qml
\skipto ToolBar
\printuntil }
\printuntil }

Now we need to add the Material version of the toolbar. File selectors expect
variants of a file to be in appropriately named directories that exist
alongside the default version of the file. This means that we need to add a
folder named "+material" in the same directory that ChatToolBar.qml is in:
the root folder. The "+" is required by \l QFileSelector as a way of ensuring
that the selection feature is not accidentally triggered.

Here's \c +material/ChatToolBar.qml:

\quotefromfile chattutorial/chapter5-styling/+material/ChatToolBar.qml
\skipto import
\printuntil }

We'll make the same changes to \c ConversationPage.qml:

\quotefromfile chattutorial/chapter5-styling/ConversationPage.qml
\skipto header: ChatToolBar
\printuntil }
\printuntil }
\printuntil }

Now both pages look correct:

\borderedimage qtquickcontrols2-chattutorial-chapter5-contacts-material.png
\borderedimage qtquickcontrols2-chattutorial-chapter5-conversations-material.png

Let's try out the Universal style:

\borderedimage qtquickcontrols2-chattutorial-chapter5-contacts-universal.png
\borderedimage qtquickcontrols2-chattutorial-chapter5-conversations-universal.png

No issues there. For a relatively simple application such as this one, there
should be very few adjustments necessary when switching styles.

Now let's try each style's dark theme. The Default style has no dark theme, as
it would add a slight overhead to a style that is designed to be as performant
as possible. We'll test out the Material style first, so add an entry to
\c qtquickcontrols2.conf that tells it to use its dark theme:

\code
[material]
Primary=Indigo
Accent=Indigo
Theme=Dark
\endcode

Once this is done, build and run the application. This is what you should see:

\borderedimage qtquickcontrols2-chattutorial-chapter5-contacts-material-dark.png
\borderedimage qtquickcontrols2-chattutorial-chapter5-conversations-material-dark.png

Both pages look fine. Now add an entry for the Universal style:

\code
[universal]
Theme=Dark
\endcode

After building and running the application, you should see these results:

\borderedimage qtquickcontrols2-chattutorial-chapter5-contacts-universal-dark.png
\borderedimage qtquickcontrols2-chattutorial-chapter5-conversations-universal-dark.png

\section1 Summary

In this tutorial, we've taken you through the following steps of writing a
basic application using Qt Quick Controls 2:

\list
\li Creating a new project using Qt Creator.
\li Setting up a basic ApplicationWindow.
\li Defining headers and footers with Page.
\li Displaying content in a ListView.
\li Refactoring components into their own files.
\li Navigating between screens with StackView.
\li Using layouts to allow an application to resize gracefully.
\li Implementing both custom read-only and writable models that integrate an
SQL database into the application.
\li Integrating C++ with QML via \l Q_PROPERTY, \l Q_INVOKABLE, and
\l qmlRegisterType().
\li Testing and configuring multiple styles.
\endlist

*/
