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[SOLVED] QPersistentModelIndex usage

  • Hi.

    I have own directory model based on QAbstractItemModel (I just copied TreeModel example and modified to my needs). This is my structure:

    @class HCDirItem
    explicit HCDirItem(HCDirItem *parent = 0);

    void appendChild(HCDirItem *child);
    void removeChild(HCDirItem *child);
    HCDirItem *child(int row);
    int childCount() const;
    int row() const;
    HCDirItem *parent();
    void clear();

    QList<HCDirItem*> childItems;
    HCDirItem *parentItem;

    class HCDirModel : public QAbstractItemModel

    explicit HCDirModel(QObject *parent = 0);
    HCDirItem *rootItem;

    QVariant data(const QModelIndex &index, int role) const;
    Qt::ItemFlags flags(const QModelIndex &index) const;
    QVariant headerData(int section, Qt::Orientation orientation,
                        int role = Qt::DisplayRole) const;
    QModelIndex index(int row, int column,
                      const QModelIndex &parent = QModelIndex()) const;
    QModelIndex parent(const QModelIndex &index) const;
    int rowCount(const QModelIndex &parent = QModelIndex()) const;
    int columnCount(const QModelIndex &parent = QModelIndex()) const;
    void loadTree(const QString &rootdir);
    void clear();
    HCDirItem *at(const QModelIndex &index);
    const QHash<QString, HCDirItem*> *items();
    HCDirItem *find(const QString &path);
    HCDirItem *addFolder(const QString &path);
    bool deleteFolder(const QString &path);

    QHash<QString, HCDirItem*> mItems;

    My application getting notifications that some file changed. I need to find it in my tree and update (or delete). As you can see I have QHash<QString, HCDirItem*> mItems so I can do quick lookup. So I know how to find HCDirItem but don't know how to get its index for emit dataChanged(QModelIndex...). I found that I should use QPersistentModelIndex but can't find any example how and where to create it

  • Moderators

    just pass your QModelIndex to the constructor of QPersistentModelIndex and your good to go.

    You then can save the QPersistentModelIndex. And don't forget to check for validity of the persistent index every time you use it.

  • Yes but I don't know how to create QModelIndex for current changed item. For example, lets say that I have this tree:

    @-A (0,0)
    |-D (0,0)
    |-E (1,0)
    -B (1,0)
    -C (2,0)@

    Now I want to add new item between D and E.

    1. I know that the parent HCDirItem is A
    2. I'm creating new instance F of HCDirItem and appending it to the parent by calling A.appendChild(F)
    3. Now I need to create QModelIndex for QPersistentModelIndex. index() function need 3 arguments, I know which row (1) and column (0) it is. But third arguments is parent QModelIndex and I don't know what to set here

  • Moderators

    the parent is the QModelIndex of A.
    And if it's a top level item, then the parent is a invalid QModelIndex()

  • I still don't understand :P . Ok more complicated example:

    |- B
    |- C
    |- D@

    Now if I want to create QModelIndex for D as third example I need QModelindex of C but to create index for C I need parent of C which is B etc. I need enumerate all parents to get index for D?

  • Moderators

    yes ... or you use the parent() of a already known index

  • So can I just create public QModelIndex index variable in HCDirItem object and setup it when item is created? Because I don't see difference between QModelIndex and QPersistentModelIndex

  • Moderators

    the difference is that the QPersistentIndex gets automatically invalid when the QModelIndex gets invalid (when they do not point to valid data anymore in the model). QModelIndexes don't.
    Think of it like QPointer for ordinary pointers.

  • Thanks! Now it is working perfect! I'm just wondering how it is working. For example. Each item is holding QPersistentModelIndex of its self. Now if I have this list:


    QList is holding these items. Now when I add new item


    ... it has QPersistentModelIndex (3,0). Same for QModelIndex. If I sort QList using qSort then list looks like:


    abc still has QPersistentModelIndex (3,0) but QTreeView.currentIndex is (1,0).

  • Last question. Can I use QPersistenModelIndex to build normal index? For example: I know that data in known QPersistenModelIndex changed. I need to emit dataChanged. My QPersistenModelIndex always point to first (0) column but I want to update whole row, so I need to create second index as "to" and point to the last column. Can I use QPersistenModelIndex.row() for "to" in QModelIndex? I'm confused because QPersistenModelIndex.row() point to old row after sorting.

  • Moderators

    how do you sort? Using a QSortFilterProxyModel? Or in a custom model?

  • No, I'm using qSort on QList<HCFileItem>

  • Moderators

    and are you sure you are triggering the right signals of the model to inform about the model change?

  • I think so:

    @emit layoutAboutToBeChanged();
    qSort(childItems.begin(), childItems.end(), compareFilenameA);
    emit layoutChanged();@

  • Moderators

    try "QAbstractItemModel::dataChanged()":http://qt-project.org/doc/qt-4.8/qabstractitemmodel.html#dataChanged
    If it doesn't make a difference you can also try "QAbstractItemModel::changePersistentIndex()":http://qt-project.org/doc/qt-4.8/qabstractitemmodel.html#changePersistentIndex

  • Same result. My model is sorted correctly but QPersistenModelIndex.row() has old value. Don't know if it's ok

  • Moderators

    before further investigating i would recommend that you try "ModelTest":http://qt-project.org/wiki/Model_Test to make sure your model is implemented correctly.

  • Ehh, can't adjust ModelTest to my model. There are really missing example of QPersistentModelIndex usage. Summarizing, my model is working ok, but don't know why :P . I have this structure:

    @struct HCFileItem {
    QPersistentModelIndex persistentIndex;

    class HCFileModel: public QAbstractItemModel
    explicit HCFileModel(QObject parent=0);
    > *mList=0;


    My model contain one item:


    Now I'm inserting (using beginInsertRows) item "aaaa" and my persistent indexes looks like:

    "bbbb" (0,0)
    "aaaa" (1,0)

    Now I'm sorting my model using qSort on mList and emitting signal dataChanged(firstIndex, secondIndex). After this my persistent indexes looks like:

    "aaaa" (1,0)
    "bbbb" (0,0)

    But it works, if I change data in "aaaa" and emit signal dataChanged with saved persistent index in HCFileItem then correct item is refreshed even if QPersistentModelIndex.row() point to wrong row. I'm confused

  • Moderators

    using ModelTest is as easy (nothing to adjust at all) as:
    HCFileModel* model = new HCFileModel(parent);
    new ModelTest(model, parent);
    If your model has an error in any state while you use it an assertion will trigger and give you info what went wrong.

  • Ok my model pass test. So I have no idea why QPersistentModelIndex row() has old value after sort. But everything work, IsValid return True. Must check on QStandardItemModel and QSortFilterProxyModel if persistent index will keep old row too

  • Maybe should I somehow refresh all persistent indexes after sorting?

  • I did some dirty test. Instead of qSort(), I'm moving new added item manually to destination row by:
    @beginMoveRows(parent->index, 4, 4, parent->index, 0);
    parent->childItems.move(4,0); // this is QList
    ... and now all persistent indexes are updated automatically. So I suppose that I need to do something with persistent indexes after qSort() call. Or just call modelreset (I don't know which items changed after sort anyway) and create new persistent indexes for all items. changePersistentIndex doesn't change anything
    And another question: where QPersistentModelIndex should be created? Now I'm creating it from "outside" model class (after childItems.append()) but maybe should I do this in QAbstractItemModel.createIndex() ?

  • Moderators

    [quote author="Kobid" date="1383942370"]
    And another question: where QPersistentModelIndex should be created? Now I'm creating it from "outside" model class (after childItems.append()) but maybe should I do this in QAbstractItemModel.createIndex() ?[/quote]
    Since QpersistenModelIndex are a bit of an overhead i would only create them when needed. Thus outside of the model is fine, thats what they are actually for. THe modle should only create QModelIndex objects.

  • Problem solved! I looked what QStandardItemModel do in sort method and I used similar algorithm. So here is complete solution if someone have same problem (note that my childs are based on QVectior, if you are using QList then you need to modify it a little):
    @void HCDirModel::sortChilds(HCDirItem *p)
    if (p->childCount()==0) return;

    QVector<QPair<HCDirItem*, int> > l(p->childItems.count());
    for (int i=0; i < p->childItems.count(); ++i) {
        l[i] = QPair<HCDirItem*,int>(p->childItems[i], i);
    qStableSort(l.begin(), l.end(), compareFilenameA);
    QModelIndexList mFrom, mTo;
    for (int i = 0; i < l.count(); ++i) {
        p->childItems[i] = l[i].first;
        QModelIndex from = createIndex(l[i].second, 0, l[i].first);
        if (persistentIndexList().contains(from)) {
            QModelIndex to = createIndex(i, 0, l[i].first);
    if (!mFrom.empty())


    And usage:
    @emit layoutAboutToBeChanged();
    emit layoutChanged();@

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