Qml Listview items disappear when scrolling - c++

Question

I have got the following scrollview with listview inside:
ScrollView{
anchors.fill: parent
ListView{
id: lvCommitsBranch
model: git.getCommitsBranch();
clip: true
delegate: Rectangle {
height: 100
width: parent.width
Text {
anchors.left: parent.left
font.bold: true
text:model.author
id:txtName
}
Text{
anchors.left: parent.left
anchors.top:txtName.bottom
font.pixelSize: 10
text:model.email
id: txtEmail
}
Text {
anchors.left: parent.left
anchors.top:txtEmail.bottom
text: model.message + ' ' + model.hash
id: txtMsg
}
MouseArea{
anchors.fill: parent
onClicked: {
lvCommitsBranch.currentIndex = index;
console.log('Msg: ' + model.message);
console.log('Hash: ' + model.hash);
}
acceptedButtons: Qt.LeftButton | Qt.RightButton
}
}
}
}
The issue is that when I scroll some items disappear (each time randomly and sometimes I have to scroll fast but not always).
When I click on the items that have not disappeared, I get undefined on all the model's properties. When Mousearea's onclick is triggered it prints the following:
qml: Msg: undefined
qml: Hash: undefined
I get the model info from a method (QAbstractListModel) that is returned from my git custom component.
This is my QAbstractListModel:
header:
class CommitsBranch : public QAbstractListModel
{
Q_OBJECT
public:
enum Roles {
AuthorRole,
EMailRole,
MsgRole,
DateRole,
HashRole
};
explicit CommitsBranch(QObject *parent = 0);
CommitsBranch(Repository *repo);
public:
virtual int rowCount(const QModelIndex &parent) const override;
virtual QVariant data(const QModelIndex &index, int role) const override;
protected:
// return the roles mapping to be used by QML
virtual QHash<int, QByteArray> roleNames() const override;
private:
QList<Commit> m_data;
QHash<int, QByteArray> m_roleNames;
};
Cpp:
CommitsBranch::CommitsBranch(QObject *parent)
: QAbstractListModel(parent)
{
}
CommitsBranch::CommitsBranch(Repository *repo)
{
m_roleNames[AuthorRole] = "author";
m_roleNames[EMailRole] = "email";
m_roleNames[MsgRole] = "message";
m_roleNames[DateRole] = "date";
m_roleNames[HashRole] = "hash";
/*
here we append the m_data (QList) Items using libgit2 methods
*/
}
int CommitsBranch::rowCount(const QModelIndex &parent) const
{
Q_UNUSED(parent);
return m_data.count();
}
QVariant CommitsBranch::data(const QModelIndex &index, int role) const
{
// this function returns the required data
}
QHash<int, QByteArray> CommitsBranch::roleNames() const
{
return m_roleNames;
}
And git is just a class that inherits from QObject and it has the following method:
Q_INVOKABLE QObject* getCommitsBranch();
QObject *Git::getCommitsBranch()
{
CommitsBranch* files = new CommitsBranch(repo.data());
return files;
}
I get the same behavior without the scrollview.
EDIT:
If I take a repository with a lot of commits (more lines to the listview), even increasing the cacheBuffer won't help, if I scroll a bit fast all the items will disappear.

Answer 1

The problem here is that, by default, if you return a QObject* it will transfer the ownership to QML.
http://doc.qt.io/qt-5/qtqml-cppintegration-data.html#data-ownership
The exception to this rule is when a QObject is returned from an
explicit C++ method call: in this case, the QML engine assumes
ownership of the object, unless the ownership of the object has
explicitly been set to remain with C++ by invoking
QQmlEngine::setObjectOwnership() with QQmlEngine::CppOwnership
specified.
You have to set the returned QObject* ownership manually, so it doesn't get destroyed by the QML engine :
QObject *Git::getCommitsBranch()
{
CommitsBranch* files = new CommitsBranch(repo.data());
QQmlEngine::setObjectOwnership(files, QQmlEngine::CppOwnership)
return files;
}
Note that you will have a memory leak as your CommitsBranch object will never be deleted. But at least your QML items should not disappear anymore !
EDIT: As suggested you can do something like this to avoid the memory leak :
// CommitsBranch Constructor
CommitsBranch::CommitsBranch(Repository *repo, QObject *parent) :
QAbstractListModel(parent) { /*stuff*/ }
QObject *Git::getCommitsBranch()
{
// Setting ownership is not necessary if you pass the parent to the QAbstractListModel
CommitsBranch* commits = new CommitsBranch(repo.data(), this);
return files;
}