QTreeView display of graph (multiple parents of item)

[wysota]

Unfortunately this is impossible to express using QAbstractItemModel. One can assume the "first" parent is the "real" parent but then again the structure becomes:

struct Item {
    Item *parent;
    QList<Item*> otherParents;
    QList<Item*> children;
};

struct Item {
    Item *parent;
    QList<Item*> otherParents;
    QList<Item*> children;
};

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The only way I see happening to represent a graph as a tree is to "snapshot" a subgraph that actually forms a tree and represent only this snapshot in the model. The data structure itself may contain other members (including pointers to items) however they are not relevant to the tree hierarchy of the model. You may look at it like on a genealogy tree. It is not really a tree but a graph but at a particular moment one is only looking only at its subgraph that is a regular tree.

[Santosh Reddy]

Unfortunately this is impossible to express using QAbstractItemModel.

Somehow I still believe that it is possible using QAbstractItemModel and QTreeView, but it not possible using the QModelIndex. As QTreeView uses QModelIndex to reference items in QAbstractItemModel, I would say it not possible using QModelIndex

If we manage to have a CustomModelIndex (enhances QModelIndex) with information required to reference items in the graph (both downlinks and uplinks) it should be possible. But Again the problem is how do we make QTreeView use CustomModelIndex

If somehow we are able to store two internalId / internalPointers in QModelIndex it should be possible. I would use the first internalPointer to store the item reference and the second internalPointer to store the item's parent (the parent under which the item is displayed in the QTreeView) reference.

So it boils down to one question. Is it possible to store two internalPointers in QModelIndex?

At the moment, I am thinking of sub-classing QModelIndex (ugh) to extend it with the full path to the object.

IMO OP tried to sub-class QModelIndex for same reason as above. Again here we have the same problem as above QTreeView is not able to understand sub-classed QModelIndex.

[wysota]

Somehow I still believe that it is possible using QAbstractItemModel and QTreeView, but it not possible using the QModelIndex.

Of course, if you reimplement the whole architecture to do something totally different then it will be possible. The same way as it will be possible to launch a rocket into space using QAbstractItemModel. However this will have little to do with the architecture of QAbstractItemModel.

As QTreeView uses QModelIndex to reference items in QAbstractItemModel, I would say it not possible using QModelIndex

It's not the fault of QTreeView. QAbstractItemModel has a parent() method that returns a SINGLE index. All views rely on that (if not for other things then at least for the root index of the view).

If we manage to have a CustomModelIndex (enhances QModelIndex) with information required to reference items in the graph (both downlinks and uplinks) it should be possible. But Again the problem is how do we make QTreeView use CustomModelIndex

You can put a custom pointer into the index but this will not teach QAbstractItemView to manipulate the graph.


A side note: it IS possible to express a graph in QAbstractItemModel because it is possible to express a graph using a plain two dimensional array. However you need a custom view for that and you lose almost all benefits of having a QAbstractItemModel. it will be much simpler to do without the burden of QAbstractItemModel.

[fleebness]

Thanks, everyone, for continuing the discussion.

I think wysota has it right, that it could be expressed, but it isn't practical. To do this, it seems you have to build a tree first, then model the tree. That negates the point of using a model, so I may as well use QTreeWidget instead of QTreeView, if I'm going to show this as a tree. The more I think about this, the more I see it as impossible, and probably impractical, to use the view, so I probably shouldn't bother.

Alternatively, I suppose, I could try creating a QGraphWidget and QGraphView (with associated QAbstractGraphModel, if I want to make this extensible, etc). That seems like an aggressive lot of code to write for someone new to Qt, though.

[Petro Svintsitskyi]

I know this thread is too old, but maybe my findings will be useful.

I also wanted to have multiple parents for my qtreeview entities (items) and came up with the following solution.

I have an IndexData structure that contains information about current item and it's parent:

struct IndexData{
        IndexData *parentIndex;
        Entity *parent;
        Entity *item;
        int row;
};

struct IndexData{
        IndexData *parentIndex;
        Entity *parent;
        Entity *item;
        int row;
};

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When QAbstractItemModel::createIndex() function is called, I pass a pointer to IndexData structure as the third parameter.

Full item model class is the following (Please note that it is a quick and dirty solution with memory leaks that was not well tested):

#include "entitymodel.h"
#include <QDebug>
 
EntityModel::EntityModel(Entity *root, QObject *parent) :
    QAbstractItemModel(parent)
{
    m_root = root;
    m_rootIndexData = new IndexData;
    m_rootIndexData->parent = 0;
    m_rootIndexData->parentIndex = 0;
    m_rootIndexData->row = 0;
    m_rootIndexData->item = root;
}
 
QModelIndex EntityModel::index(int row, int column, const QModelIndex &parent) const
{
    if (!hasIndex(row, column, parent)) {
        return QModelIndex();
    }
 
    IndexData *parentData = indexData(parent);
    Entity *child = parentData->item->child(row);
 
    if (child)
        return createIndex(row, column, createIndexData(parentData, child, row));
 
    return QModelIndex();
}
 
QModelIndex EntityModel::parent(const QModelIndex &child) const
{
    if (!child.isValid())
        return QModelIndex();
 
    IndexData *data = indexData(child);
    if (data->parent == m_root) {
        return QModelIndex();
    }
 
    if (data->parentIndex == 0)
        return QModelIndex();
 
    return createIndex(data->parentIndex->row, 0, data->parentIndex);
}
 
int EntityModel::rowCount(const QModelIndex &parent) const
{
    IndexData *data = indexData(parent);
    return data->item->childrenCount();
}
 
int EntityModel::columnCount(const QModelIndex &parent) const
{
    return 2;
}
 
QVariant EntityModel::data(const QModelIndex &index, int role) const
{
    if (!index.isValid())
        return QVariant();
 
    if (role != Qt::DisplayRole)
        return QVariant();
 
    Entity *item = indexData(index)->item;
 
    if (index.column() == 0) {
        return item->name();
    } else if (index.column() == 1) {
        return "Entity";
    } else {
        qCritical() << "Invalid column";
        return "";
    }
}
 
EntityModel::IndexData *EntityModel::indexData(const QModelIndex &index) const
{
    if (index.isValid()) {
        IndexData *data = static_cast<IndexData*>(index.internalPointer());
        if (data)
            return data;
    }
 
    return m_rootIndexData;
}
 
EntityModel::IndexData *EntityModel::createIndexData(IndexData *parent, Entity *item, int row) const
{
    if (!m_indexData.contains(quintptr(parent->item))) {
        QHash<quintptr, IndexData*>* hash = new QHash<quintptr, IndexData*>();
        m_indexData.insert(quintptr(parent->item), hash);
    }
 
    if (!m_indexData[quintptr(parent->item)]->contains(quintptr(item))) {
        IndexData *data = new IndexData;
        data->parent = parent->item;
        data->item = item;
        data->parentIndex = parent;
        data->row = row;
        m_indexData[quintptr(parent->item)]->insert(quintptr(item), data);
    }
 
    return m_indexData[quintptr(parent->item)]->value(quintptr(item));
}

#include "entitymodel.h"
#include <QDebug>

EntityModel::EntityModel(Entity *root, QObject *parent) :
    QAbstractItemModel(parent)
{
    m_root = root;
    m_rootIndexData = new IndexData;
    m_rootIndexData->parent = 0;
    m_rootIndexData->parentIndex = 0;
    m_rootIndexData->row = 0;
    m_rootIndexData->item = root;
}

QModelIndex EntityModel::index(int row, int column, const QModelIndex &parent) const
{
    if (!hasIndex(row, column, parent)) {
        return QModelIndex();
    }

    IndexData *parentData = indexData(parent);
    Entity *child = parentData->item->child(row);

    if (child)
        return createIndex(row, column, createIndexData(parentData, child, row));

    return QModelIndex();
}

QModelIndex EntityModel::parent(const QModelIndex &child) const
{
    if (!child.isValid())
        return QModelIndex();

    IndexData *data = indexData(child);
    if (data->parent == m_root) {
        return QModelIndex();
    }

    if (data->parentIndex == 0)
        return QModelIndex();

    return createIndex(data->parentIndex->row, 0, data->parentIndex);
}

int EntityModel::rowCount(const QModelIndex &parent) const
{
    IndexData *data = indexData(parent);
    return data->item->childrenCount();
}

int EntityModel::columnCount(const QModelIndex &parent) const
{
    return 2;
}

QVariant EntityModel::data(const QModelIndex &index, int role) const
{
    if (!index.isValid())
        return QVariant();

    if (role != Qt::DisplayRole)
        return QVariant();

    Entity *item = indexData(index)->item;

    if (index.column() == 0) {
        return item->name();
    } else if (index.column() == 1) {
        return "Entity";
    } else {
        qCritical() << "Invalid column";
        return "";
    }
}

EntityModel::IndexData *EntityModel::indexData(const QModelIndex &index) const
{
    if (index.isValid()) {
        IndexData *data = static_cast<IndexData*>(index.internalPointer());
        if (data)
            return data;
    }

    return m_rootIndexData;
}

EntityModel::IndexData *EntityModel::createIndexData(IndexData *parent, Entity *item, int row) const
{
    if (!m_indexData.contains(quintptr(parent->item))) {
        QHash<quintptr, IndexData*>* hash = new QHash<quintptr, IndexData*>();
        m_indexData.insert(quintptr(parent->item), hash);
    }

    if (!m_indexData[quintptr(parent->item)]->contains(quintptr(item))) {
        IndexData *data = new IndexData;
        data->parent = parent->item;
        data->item = item;
        data->parentIndex = parent;
        data->row = row;
        m_indexData[quintptr(parent->item)]->insert(quintptr(item), data);
    }

    return m_indexData[quintptr(parent->item)]->value(quintptr(item));
}

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[d_stranz]

Thanks for this. Now I need to figure out how to implement a model that will handle an undirected cyclic graph which may contain disjoint subgraphs :-) I might have to save that for another lifetime.