In-Depth Model/View with QML

Integrated Computer Solutions Inc. www.ics.com
Qt Model / View with QML
July 25th Webinar With Chris Probst & Ryan Hampton
Visit us at http://www.ics.com
Produced by Integrated Computer Solutions
Material based on Qt 5.12
1

What is MVC?
- Stands for Model/View/Controller
- Architectural Design Pattern used for developing user interfaces
- Separates data interactions into three separate parts
- Allows for User Interface being independently developed from business logic.
In general terms:
- Model manages the data of the user interface
- View displays the model
- The controller responds to the user input and manages the model accordingly
2

Simple QML Example
- In Qt/QML, the controller is combined with the view
- A simple approach of leveraging Model/View in Qt/QML would be
- For every value displayed, create a Q_PROPERTY
- Provide a read, write and notify signal
- Export the properties to QML, using void QQmlContext::setContextProperty
3
class simpleModel : public QObject
{
Q_OBJECT
Q_PROPERTY(int speed READ speed WRITE setSpeed NOTIFY speedChanged)
Q_PROPERTY(int temperature READ temperature WRITE setTemperature NOTIFY temperatureChanged)
QQmlApplicationEngine engine;
simpleModel aSimpleModel;
engine.rootContext()->setContextProperty("asimplemodel",&asimpleModel);
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

Model View Delegate
- Preserves Business-Logic/View separation
- Model provides data to the view
- View is the visual container displaying the data
- Delegates renders each of the instances of the
data
- Available for both Widgets and QML
4

Other Simple Approaches
- Creating the Model in QML
- Standard QML models
- ListModel - simple hierarchy of QML types
- ObjectModel - Visual items, no delegate needed
- XmlListModel - XML data source
- Integer - read only, the model has no data roles
- Object instances - properties of the object are the roles
- JavaScript array - read only, no need for dynamic model updates
5
ListModel {
id: fruitModel
ListElement {
name: "Apple"
cost: 2.45 }
ListElement {
name: "Orange"
cost: 3.25 }
… }

Example: Contact List
● Create a ListModel
○ Fill it with ListElement objects, each with:
■ A name property
■ A file property referring to an image
● Add a ListView and a Component
to use as a delegate
● Add header, footer and highlight
properties to the view
6

Examples of QML Views - ListView
7
- Based on a Flickable area
- It uses a model, instantiates a delegate and
between the delegates, there can be spacing
- Most commonly used
- Can be horizontal or vertical
ListView {
model: mymodel
delegate: numberDelegate
spacing: 5
}
}

Examples of QML Views - GridView
8
- Based on a Flickable area
- It uses a model, instantiates a delegate and
between the delegates, there can be spacing
- Places the data into a two-dimensional grid,
left-to-right or top-to-bottom
See $QTDIRExamplesQt-5.11.2quickviews

Examples of QML Views - PathView
9
- Lays out the data along an arbitrary path
- Most powerful, but also the most complex
- Requires the path property to be set
- Paths are speciﬁed using the Path QML
object

What about the Delegate?
10
- Appearance of each individual item
- Can be a Component or a QML ﬁle
Column {
spacing: 2
Repeater {
model: 10
delegate: BlueBox {
width: 100
height: 32
text: index
}
}
}

A Model in C++ : A Simple Approach
11
QStringList dataList;
dataList.append("Item 1");
engine.rootContext()
QQmlContext *ctxt = view.rootContext();
ctxt->setContextProperty("myModel",
QVariant::fromValue(dataList));
ListView {
width: 100; height: 100
model: myModel
delegate: Rectangle {
height: 25
width: 100
Text { text: modelData }
}
}
● Model can be speciﬁed QStringList, QVariantList, QObjectList

QAbstractListModel - What to reimplement?
12
● provide implementations of the QAbstractListModel::rowCount() and
QAbstractListModel::data() functions.
QVariant Mp3ListModel::data(const QModelIndex &index, int role) const
{
if (!index.isValid())
return QVariant();
Mp3Item *mp3Item = m_mp3Items.at(index.row());
switch (role) {
case DoneRole:
return QVariant(mp3Item->download.finished());
case UrlRole:
return QVariant(mp3Item->download.url().toString());
case ProgressRole:
return QVariant(mp3Item->download.progress());
case ErrorRole:
return QVariant(mp3Item->download.error());
return QVariant();
}
int Mp3ListModel::rowCount(const QModelIndex &parent)
const
{
if (parent.isValid())
return 0;
return m_mp3Items.size();
}

QAbstractListModel - If user can change data?
13
● Provide implementations of the QAbstractListModel::setData() functions.
bool Mp3ListModel::setData(const QModelIndex &index, const QVariant &value, int role)
{
if (role == SelectedRole) {
if (index.row() < 0 || index.row() >= m_mp3Items.size())
return false;
if (m_mp3Items.at(index.row())->selected == value.toBool())
return false;
m_mp3Items.at(index.row())->selected = value.toBool();
dataChanged(index, index);
return true;
}
return false;
}

QAbstractListModel - A Note on Roles
14
● Along with the QModelIndex, a role is simply an additional selector to
access data in your model.
class Mp3ListModel : public QAbstractListModel
{
Q_OBJECT
public:
explicit Mp3ListModel(QObject *parent = nullptr)
noexcept;
enum {
SelectedRole = Qt::UserRole,
DoneRole,
UrlRole,
... };
QHash<int, QByteArray>
Mp3ListModel::roleNames() const
{
QHash<int, QByteArray> roles;
roles[SelectedRole] = "selected";
roles[DoneRole] = "done";
roles[ProgressRole] = "progress";
roles[UrlRole] = "url";
roles[ErrorRole] = "error";
return roles;
}
ListView {
….
delegate: RowLayout {
spacing: 10
CheckBox {
checked: model.selected
onClicked: model.selected = checked
}
Label {
Layout.fillWidth: true
text: model.error ? qsTr("Error on
Download") : model.url
elide: Text.ElideLeft
...
}

QAbstractListModel - Notifying the View of
Changes
15
● Emit the signal:
void QAbstractItemModel::dataChanged(const QModelIndex&topLeft,
const QModelIndex &bottomRight, const QVector<int> &roles = ...)
m_mp3Items.at(index.row())->selected = value.toBool();
emit dataChanged(index, index);

Hooking the Model up to the View
16
#include <mp3listmodel>
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_DisableHighDpiScaling);
Mp3ListModel mp3ListModel(downloadDirUrl);
engine.rootContext()->setContextProperty("mp3ListModel"), &mp3ListModel);
qmlRegisterUncreatableType<Mp3ListModel>("DownloadManager", 1, 0,
"Mp3ListModel", "Mp3ListModel should not be created in QML");
engine.load("qrc:/main.qml"); // Load blocks
return app.exec();
}
ListView {
anchors.fill: parent
model: mp3ListModel
delegate: Mp3Item {
width: parent.width
}
}

Example: Download Manager
17

Summary: Model / View
Qt Model / View in a nutshell
● A model provides data for a view
● A view displays the data from the model
● Similar to the Model / View / Controller design pattern
○ But combine the controller with the view
Why Model / View?
● Isolate business logic from UI logic
● Create UI components that are independent
○ Easier Development
○ Easier Testing
○ Easier Maintenance
● More Reuse
○ Reuse your UI logic across multiple views
○ Models can be reused for different views
18

Summary: Model / View Components
Model:
● Interface to business logic data
● Emits signals when data changes or is appended
● Many common model types are provided by Qt
● C++: QStringList, QVariantList, QAbstractItemModel subclass
● QML: an integer, JavaScript array, ListModel, XmlListModel
View:
● Displays the data structure
● Handles the user interaction with the data
● Qt provides many view types with model support
● QML: ListView, GridView, PathView, TableView (new)
Delegate:
● Items in a view are rendered and edited by delegates
● The view expects a Component for its delegate property
● Model items are accessible through the index property
19

Summary: QML Views
20
ListView GridView TableViewPathView

Summary: Model Types
21
● Standard QML models
○ ListModel - simple hierarchy of QML types
○ ObjectModel - Visual items, no delegate needed
○ XmlListModel - XML data source
○ Integer - read only, the model has no data roles
○ Object instances - properties of the object are the roles
○ JavaScript array - read only, no need for dynamic model updates
● C++ models (must be implemented in C++ before use)
○ More complex but more ﬁne-tuned control over data access
○ QStringList, QVariantList, QObjectList, QAbstractItemModel (and subclasses)
○ Subclass QAbstractItemModel for complex data-sets

References
● Qt 5 E-book
● See https://qmlbook.github.io/ch07-modelview/modelview.html
● C++ GUI Programming with Qt 4 (see Model/View chapter)
● Examples: https://gitlab.com/RyanHampton/qml-model-view-framework
Don’t know where to start? Go read the docs!
● Hint - start here: https://doc.qt.io/qt-5/qabstractlistmodel.html
22

Integrated Computer Solutions Inc. www.ics.comIntegrated Computer Solutions Inc.
Thank You!
● https://www.ics.com/blog/exploring-model-view-design-qt-quick
● ICS provides QML training for more in-depth learning
23
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In-Depth Model/View with QML

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