Making QAbstractListModel items individually accessible

[romsharkov]

Hi again,

Writing a simple message list viewer, where the message has a title, a message text, and a unique identifier I faced a problem trying to make the individual messages stored in the QAbstractListModel derived MessageList accessible by their identifier from QML.

I tried to implement the QML invokable index operator on the list model which would perform a lookup on a map that's associating identifier strings to QList::iterator of the certain object, but that requires the Message class to derive from QObject, which by design is non-copyable and what happens if the list model changes, but the map is untouched and the iterators are corrupted? I have that feeling of doing it totally wrong again...

Could someone help me design a working structure to elegantly solve this kind of problem? The complete source code is attached below

main.qml:

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

ApplicationWindow {
	visible: true
	width: 640
	height: 480
	title: qsTr("Messages")

	Rectangle {
		id: menu
		color: Qt.rgba(0.9, 0.9, 0.9, 1)
		height: 32
		anchors.top: parent.top
		anchors.left: parent.left
		anchors.right: parent.right
		Text {
			anchors.left: parent.left
			anchors.margins: 8
			anchors.verticalCenter: parent.verticalCenter
			//THIS DOESNT YET WORK
			//text: "at id 'a' there is: " + MessageList["a"].title
		}
	}

	ListView {
		model: MessageList
		anchors.top: menu.bottom
		anchors.left: parent.left
		anchors.right: parent.right
		anchors.bottom: parent.bottom
		clip: true
		delegate: Rectangle {
			height: 32
			Column {
				anchors.fill: parent
				anchors.margins: 8
				spacing: 2
				Text {
					text: title + " (" + identifier + ")"
					font.bold: true
				}
				Text {
					text: message
				}
			}
		}
	}
}

MessageList.cpp:

#ifndef MESSAGELIST_HPP
#define MESSAGELIST_HPP

#include "Message.hpp"
#include <QObject>
#include <QAbstractListModel>
#include <QModelIndex>
#include <QList>
#include <QHash>
#include <QVariant>
#include <QMap>

class MessageList : public QAbstractListModel {
	Q_OBJECT
public:
	enum Roles {
		IdentifierRole,
		TitleRole,
		MessageRole,
	};
	typedef QMap<QString, QList<Message>::iterator> IdMap;

protected:
	QList<Message> _list;
	IdMap _byId;
public:
	MessageList();
	int rowCount(const QModelIndex &parent) const;
	QHash<int, QByteArray> roleNames() const;
	QVariant data(const QModelIndex &index, int role) const;
	bool insert(
		const QList<Message>& messages,
		int position = 0
	);
	Q_INVOKABLE QVariant operator[](const QString& identifier) const;
	bool reset();
};
#endif // MESSAGELIST_HPP

MessageList.cpp:

#include "MessageList.hpp"
#include <QObject>
#include <QModelIndex>
#include <QVariant>
#include <QHash>
#include <QByteArray>

MessageList::MessageList() :
	QAbstractListModel(nullptr)
{

}

int MessageList::rowCount(const QModelIndex &parent) const {
	Q_UNUSED(parent)
	return _list.size();
}

QHash<int, QByteArray> MessageList::roleNames() const {
	QHash<int, QByteArray> roles;
	roles[IdentifierRole] = "identifier";
	roles[TitleRole] = "title";
	roles[MessageRole] = "message";
	return roles;
}

QVariant MessageList::data(const QModelIndex &index, int role) const {
	if(!index.isValid()
		|| index.row() >= _list.size()
		|| index.row() < 0
	) {
		return QVariant();
	}
	switch(role) {
	case IdentifierRole:
		return _list.at(index.row()).identifier();
		break;
	case TitleRole:
		return _list.at(index.row()).title();
		break;
	case MessageRole:
		return _list.at(index.row()).message();
		break;
	default:
		return QVariant();
	}
}

bool MessageList::insert(
	const QList<Message>& messages,
	int position
) {
	beginInsertRows(QModelIndex(), position, position + messages.size() - 1);
	for(int row = 0; row < messages.size(); ++row) {
		_list.insert(position, messages.at(row));
		QList<Message>::iterator listItr(_list.begin());
		listItr += position;
		_byId.insert(messages.at(row).identifier(), listItr);
	}
	endInsertRows();
	return true;
}

bool MessageList::reset() {
	beginResetModel();
	_list.clear();
	endResetModel();
	return true;
}

QVariant MessageList::operator[](const QString& identifier) const {
	IdMap::const_iterator itr(_byId.find(identifier));
	if(itr != _byId.constEnd()) {
		return QVariant(QVariant::fromValue(*(*itr)));
	}
	return QVariant();
}

Message:

#ifndef MESSAGE_HPP
#define MESSAGE_HPP

#include <QObject>
#include <QString>

class Message : public QObject {
	Q_OBJECT
	Q_PROPERTY(QString identifier READ identifier NOTIFY identifierChanged)
	Q_PROPERTY(QString title READ title NOTIFY titleChanged)
	Q_PROPERTY(QString message READ message NOTIFY messageChanged)

protected:
	QString _identifier;
	QString _title;
	QString _message;

public:
	Message();
	Message(
		const QString& identifier,
		const QString& title,
		const QString& message
	);
	const QString& identifier() const;
	const QString& title() const;
	const QString& message() const;

signals:
	void identifierChanged();
	void titleChanged();
	void messageChanged();
};

Q_DECLARE_METATYPE(Message)

#endif // MESSAGE_HPP

Message.cpp:

#include "Message.hpp"
#include <QObject>
#include <QString>

Message::Message() :
	QObject(nullptr)
{

}

Message::Message(
	const QString& identifier,
	const QString& title,
	const QString& message
) :
	QObject(nullptr),
	_identifier(identifier),
	_title(title),
	_message(message)
{

}

const QString& Message::identifier() const {
	return _identifier;
}

const QString& Message::title() const {
	return _title;
}

const QString& Message::message() const {
	return _message;
}

[romsharkov]

After hours of google, trial and error I'm proud to finally have solved it myself as I am still pretty new to Qt!

To help people with a similar problem I'll quickly sum up all I had to do to make it work:

1. turn the internal QList<Message> into a QList<QSharedPointer<Message>>

2. define a QMap<QString, QSharedPointer<Message>> to hold index ids to the individual messages

3. when inserting, insert in the indexMap as well

4. define a method in the QAbstractListModel derived MessageList which simply returns a QVariantMap

P.S. Below is the new working version of the example app, in case anyone notices any misconceptions, please let me know

MessageList.hpp:

#ifndef MESSAGELIST_HPP
#define MESSAGELIST_HPP

#include "Message.hpp"
#include <QObject>
#include <QAbstractListModel>
#include <QModelIndex>
#include <QList>
#include <QHash>
#include <QVariant>
#include <QMap>
#include <QSharedPointer>

class MessageList : public QAbstractListModel {
	Q_OBJECT
public:
	typedef QSharedPointer<Message> MessagePointer;
	typedef QList<MessagePointer> MessagePointerList;
	typedef QMap<QString, MessagePointer> IndexMap;
	enum Roles {
		IdentifierRole,
		TitleRole,
		MessageRole,
	};

protected:
	MessagePointerList _list;
	IndexMap _indexMap;

public:
	MessageList();
	int rowCount(const QModelIndex& parent) const;
	QHash<int, QByteArray> roleNames() const;
	QVariant data(const QModelIndex& index, int role) const;
	bool insert(
		const QList<Message>& messages,
		int position = 0
	);
	bool reset();
	Q_INVOKABLE QVariantMap get(const QString& identifier) const;
	const Message& at(int index) const;
};

#endif // MESSAGELIST_HPP

MessageList.cpp:

#include "MessageList.hpp"
#include <QObject>
#include <QModelIndex>
#include <QVariant>
#include <QHash>
#include <QByteArray>

MessageList::MessageList() :
	QAbstractListModel(nullptr)
{

}

int MessageList::rowCount(const QModelIndex& parent) const {
	Q_UNUSED(parent)
	return _list.size();
}

QHash<int, QByteArray> MessageList::roleNames() const {
	QHash<int, QByteArray> roles;
	roles[IdentifierRole] = "identifier";
	roles[TitleRole] = "title";
	roles[MessageRole] = "message";
	return roles;
}

QVariant MessageList::data(const QModelIndex& index, int role) const {
	if(!index.isValid()
		|| index.row() >= _list.size()
		|| index.row() < 0
	) {
		return QVariant();
	}
	switch(role) {
	case IdentifierRole:
		return _list.at(index.row())->identifier();
		break;
	case TitleRole:
		return _list.at(index.row())->title();
		break;
	case MessageRole:
		return _list.at(index.row())->message();
		break;
	default:
		return QVariant();
	}
}

bool MessageList::insert(
	const QList<Message>& messages,
	int position
) {
	beginInsertRows(QModelIndex(), position, position + messages.size() - 1);
	for(int row = 0; row < messages.size(); ++row) {
		IndexMap::const_iterator indexMapItr(_indexMap.constFind(messages.at(row).identifier()));
		if(indexMapItr == _indexMap.constEnd()) {
			MessagePointer newMessage(new Message(messages.at(row)));
			_list.insert(position, newMessage);
			_indexMap.insert(messages.at(row).identifier(), newMessage);
		}
	}
	endInsertRows();
	return true;
}

bool MessageList::reset() {
	beginResetModel();
	_list.clear();
	__indexMap.clear();
	endResetModel();
	return true;
}

QVariantMap MessageList::get(const QString& identifier) const {
	QVariantMap result;
	IndexMap::const_iterator indexMapItr(_indexMap.constFind(identifier));
	if(indexMapItr != _indexMap.constEnd()) {
		result["identifier"] = QVariant(indexMapItr->data()->identifier());
		result["title"] = QVariant(indexMapItr->data()->title());
		result["message"] = QVariant(indexMapItr->data()->message());
	}
	return result;
}

const Message& MessageList::at(int index) const {
	return *(_list.at(index).data());
}

main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include "Message.hpp"
#include "MessageList.hpp"

int main(int argc, char *argv[])
{
	QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
	QGuiApplication app(argc, argv);

	MessageList messageList;
	messageList.insert(QList<Message> {
		Message("a", "first message", "this is a sample text message of the first message"),
		Message("b", "second message", "another sample text message of the second message"),
		Message("c", "third message", "yet a third text message sample"),
		Message("d", "fouth message", "last sample message")
	});
	QQmlApplicationEngine engine;
	engine.rootContext()->setContextProperty("MessageList", &messageList);
	engine.load(QUrl(QLatin1String("qrc:/main.qml")));

	return app.exec();
}

main.qml:

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

ApplicationWindow {
	visible: true
	width: 640
	height: 480
	title: qsTr("Messages")

	Rectangle {
		id: menu
		color: Qt.rgba(0.9, 0.9, 0.9, 1)
		height: 32
		anchors.top: parent.top
		anchors.left: parent.left
		anchors.right: parent.right
		Text {
			anchors.left: parent.left
			anchors.margins: 8
			anchors.verticalCenter: parent.verticalCenter
			text: "at id 'b' there is: " + MessageList.get("b").title
		}
	}

	ListView {
		model: MessageList
		anchors.top: menu.bottom
		anchors.left: parent.left
		anchors.right: parent.right
		anchors.bottom: parent.bottom
		clip: true
		delegate: Rectangle {
			height: 32
			Column {
				anchors.fill: parent
				anchors.margins: 8
				spacing: 2
				Text {
					text: title + " (" + identifier + ")"
					font.bold: true
				}
				Text {
					text: message
				}
			}
		}
	}
}