滚动时Qml Listview项目消失

Question

我得到了以下带有列表视图的滚动视图：

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
            }
        }
    }
}

问题是当我滚动时一些项目消失了（每次都是随机的，有时我必须快速滚动但并非总是如此）。

当我点击没有消失的项目时，我得到 undefined 模型的所有属性。当触发 Mousearea 的 onclick 时，它会打印以下内容：

qml: Msg: undefined

qml: Hash: undefined

我从我的 git 自定义组件返回的方法 (QAbstractListModel) 获取模型信息。

这是我的 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;
}

而git只是一个继承自QObject的class，它有以下方法：

Q_INVOKABLE QObject* getCommitsBranch();
QObject *Git::getCommitsBranch()
{
    CommitsBranch* files = new CommitsBranch(repo.data());
    return files;
}

我在没有滚动视图的情况下得到了相同的行为。

编辑：如果我使用一个有很多提交的存储库（更多行到列表视图），即使增加 cacheBuffer 也无济于事，如果我滚动得有点快，所有项目都会消失。

Answer 1

这里的问题是，默认情况下，如果您 return 一个 QObject*，它会将所有权转移给 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.

您必须手动设置 returned QObject* 所有权，这样它就不会被 QML 引擎破坏：

QObject *Git::getCommitsBranch()
{
    CommitsBranch* files = new CommitsBranch(repo.data());

    QQmlEngine::setObjectOwnership(files, QQmlEngine::CppOwnership)

    return files;
}

请注意，您将遇到内存泄漏，因为您的 CommitsBranch 对象永远不会被删除。但至少你的 QML 项目不应该再消失了！

编辑：按照建议，您可以这样做以避免内存泄漏：

// 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;
}

滚动时Qml Listview项目消失

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c++

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