I am attempting to create a proxy model that dynamically maps items from source model.
Following the implementation of QIdentityProxyModel with the intention to go from there I have discovered that it is in fact impossible to replicate it by examining the 4 core functions:
mapFromSource()
mapToSource()
index()
parent()
Consider this based on QIdentityProxyModel:
mapFromSource()
QModelIndex ProxyModel::mapFromSource(const QModelIndex &sourceIndex) const
{
if(sourceIndex.isValid())
return createIndex(sourceIndex.row(), sourceIndex.column(), sourceIndex.internalPointer());
else
return QModelIndex();
}
mapToSource()
QModelIndex ProxyModel::mapToSource(const QModelIndex &proxyIndex) const
{
if(proxyIndex.isValid())
return sourceModel()->createIndex(proxyIndex.row(), proxyIndex.column(), proxyIndex.internalPointer());
else
return QModelIndex();
}
index()
QModelIndex ProxyModel::index(int row, int column, const QModelIndex &parent) const
{
const QModelIndex sourceParent = mapToSource(parent);
const QModelIndex sourceIndex = sourceModel()->index(row, column, sourceParent);
return mapFromSource(sourceIndex);
}
parent()
QModelIndex ProxyModel::parent(const QModelIndex &index) const
{
const QModelIndex sourceIndex = mapToSource(index);
const QModelIndex sourceParent = sourceIndex.parent();
return mapFromSource(sourceParent);
}
THE ISSUE
Problem lies in mapToSource() line
return sourceModel()->createIndex(proxyIndex.row(), proxyIndex.column(), proxyIndex.internalPointer());
The QAbstractItemModel::createIndex is protected function and cannot be used unless the caller is declared friend. That is obviously not an option without modifying the QAbstractItemModel class directly. The only alternative is to use regular QAbstractItemModel::index but that requires parent in form of QModelIndex as one of the arguments. However doing this:
return sourceModel()->index(proxyIndex.row(), proxyIndex.column(), proxyIndex.parent());
causes an infinite loop due to the fact that parent() function relies on mapToSource() and vice versa now.
The alternative approach shown for example here relies on the stored map of QPersistentModelIndex objects that are queried in the above mentioned functions. This approach while viable has several disadvantages:
It requires a lots of extra memory to store the indexes
All persistent indexes need to be updated by the source model upon every structure change (as opposed to on demand index lookup with dynamic models)
Queries to the map may be slow when iterating over kyes of the map (this can be remedied by making two identical maps in reverse at the expense of yet more memory)
Hence my questions:
Is there another way to handle the hierarchical proxy model dynamically without relying on createIndex() and without running into infinite loop of function calls?
Is it in fact necessary to create and maintain the structure of the proxy model in a storage vis-à-vis the source structure or is there a way to create a dynamic proxy model?
Thanks!
Perhaps this is a late reply, but I've just faced the same problem. I solved it by casting sourceModel() to my model, meanwhile I declared my ProxyModel as a friend.
This is my mapToSource definition:
QModelIndex ProxyModel::mapToSource(const QModelIndex& proxyIndex) const
{
Model* pModel = qobject_cast<Model*>(sourceModel());
if (!pModel || !proxyIndex.isValid()) return QModelIndex();
...
return pModel->createIndex(row, col, proxyIndex.internalPointer());
}
and the friend declaration:
class Model : public QAbstractTableModel
{
Q_OBJECT
friend class ProxyModel;
...
};
I understand your concern about using only QAbstractItemModel interface (believe me, I had the same), but let's face it the ProxyModel can only be used with my Model and nothing else (at least according to my implementation). Therefore I don't see anything bad in "friendship" like this. As for UB there shouldn't be any issues here as well, because qobject_cast would return 0 if other model other than mine was provided.
Building on the previous answers, I would like to add some alternate ways
First, the uglyest hack in the history of time itself is safe-ish on most compile:
class HackyDummy : public QAbstractItemModel
{
friend class ProxyModel;
};
QModelIndex ProxyModel::createSourceIndex(int r, int c, void *i) {
return ((HackyDummy*) sourceModel())->createIndex(r,c,i);
}
Now, of course, this is a terrible, terrible hack. It also wont compile in some compiler as the C++ spec isn't very clear (or at least as been mis-interpreted by some) on accessing parent class private and protected methods from sub-class friends. This lead us to a better plan. But first, a little detail: QIdentityProxyModel does it (almost). QIdentityModel own mapToSource/mapFromSource are public and re-implementable, so is its createIndex. So, it is possible do an internal/private QIdentityProxyModel with a method to create our little source indices from the row, column and internal pointers by accessing the hack contained inside of QIdentityProxyModel itself.
Edit: typo
I just inherited from QIdentityProxyModel and used it's mapToSource method in my mapToSourceMethod.
QModelIndex PropertyModel::mapToSource(const QModelIndex & proxyIndex) const
{
if(hasNoModel())
{
return QModelIndex();
}
QModelIndex remapped = createIndex( /* transform proxyIndex here */ );
return QIdentityProxyModel::mapToSource(remapped);
}
It means you don't end up with a cast that could crash your code should you ever change the source model type and forget to update your cast.
Related
I tried to implement a new model that sorts numerically as described here.
It looks like this:
#ifndef NUMERICMODEL_H
#define NUMERICMODEL_H
#include <QStandardItemModel>
class NumericModel : public QStandardItemModel
{
public:
enum Role {
SortRole=Qt::UserRole
};
NumericModel() {}
~NumericModel() {}
QVariant data(const QModelIndex & index, int role = Qt::DisplayRole) const {
switch ( role ) {
case Qt::DisplayRole:
return index.data().toString();
case SortRole:
return index.data().toUInt();
default:
return index.data().toString();
}
}
};
#endif // NUMERICMODEL_H
I am setting the sort role like this:
QSortFilterProxyModel * proxyModel = new QSortFilterProxyModel(this);
proxyModel->setSourceModel(&m_movesModel);
proxyModel->setSortRole(NumericModel::SortRole);
qDebug() << __LINE__;
ui->tableView_Moves->setModel(proxyModel);qDebug() << __LINE__;
ui->tableView_Moves->resizeColumnsToContents();qDebug() << __LINE__;
However my program crashes in the last line, when calling ui->tableView_Moves->resizeColumnsToContents().
why you are subleasing QStandardItemModel?
This is not needed since this model handles any kind of roles! Just set them.
Anyway your implementation lead to infinitive recursion!
The answer you provided is incomplete. You have to understand that QStandardItemModel can store any kind of data it can be QString, it can be int or double (and other types see doc). Default compare method for QSortFilterProxyModel handles all of this types so problem is of comparing strings with numbers, so problem is what type exactly QVariant stores. Linked answer tries enforce uniform type used for sorting and this solution is quite poor.
IMO it is safer to subclass QSortFilterProxyModel like that:
bool MyUIntSortFilterProxyModel::lessThan(const QModelIndex & left,
const QModelIndex & right ) const {
int cmpRole = sortRole();
return left.data(cmpRole).toUInt()<right.data(cmpRole).toUInt();
}
Thanks to the other answer I now know that my data function was incomplete. Instead of going through all possible cases manually, I just asked for my specific case and then called the data function of the base class.
QVariant data(const QModelIndex & index, int role = Qt::DisplayRole) const {
if (role == SortRole) {
return index.data().toUInt();
}
return QStandardItemModel::data(index, role);
}
I have a clicked()-signal which knows a selected index which is of the type QModelIndex.
void onListClicked(const QModelIndex & index) { /* ... */ }
No I want to access the data of the clicked item. I found out I can access the model using model():
void onListClicked(const QModelIndex & index)
{
QStandardItemModel * model {index.model()};
}
But this fails as the model() getter only allows me to return an QAbstractItemModel.
error: invalid conversion from 'const QAbstractItemModel*' to 'QStandardItemModel*' [-fpermissive]
How to access the QStandardItemModel or even better the selected QStandardItem? My unique identifier is stored in QStandardItem::data().
What I need is something like that:
void onListClicked(const QModelIndex & index)
{
QStandardItemModel * model {index.model()};
QStandardItem * item {model->itemFromIndex(index)};
qDebug() << item->data().toString();
}
But that does not work. Why is that so difficult. What do I miss here?
I think you can get the data directly from the model index:
void onListClicked(const QModelIndex & index) {
index.data(Qt::UserRole + 1);
// ...
}
You can use any other role to retrieve different kind of data.
Just cast it:
QStandardItemModel *model { static_cast<QStandardItemModel *>(model()); }
I had the same problem, as I need to retrieve my special model:
auto myModel=const_cast<MySpecialModel*>(dynamic_cast<const MySpecialModel*>(modelIndex.model()));
This was a perfectly working solution for me.
But, there seems to be an important issue doing so:
http://doc.qt.io/qt-5/qmodelindex.html
It says there:
A const pointer to the model is returned because calls to non-const functions of the model might invalidate the model index and possibly crash your application.
Unfortunately, the docs didn't say, why there might be a possible crash and what one shouldn't do.
I currently have a QtAbstractListModel subcliass (shortened for clarity):
class HolidayTask;
class HolidayTaskModel: public QAbstractListModel
{
Q_OBJECT
public:
explicit HolidayTaskModel(QObject *parent = 0);
~HolidayTaskModel();
int rowCount(const QModelIndex& parent = QModelIndex()) const;
QVariant data(const QModelIndex& index, int role = Qt::DisplayRole) const;
Qt::ItemFlags flags(const QModelIndex& index) const;
bool insertRows(int row, int count, const QModelIndex& parent = QModelIndex());
bool removeRows(int row, int count, const QModelIndex& parent = QModelIndex());
bool setData(const QModelIndex& index, const QVariant& value,
int role = Qt::EditRole);
private:
QVector<HolidayTask*> m_items;
};
Where HolidayTask is the items I'm containing. I'm trying to figure out what needs to be reimplemented for this use case:
The model is modifiable, but not with in place editing in the view: another widget (not a *View subclass) will do the editing, outside from the view this model will be shown in;
It needs to implement not only appending (that would be easy) but also inserting and reordering.
In the case of appending, it would be very easy to make an appendTask function that calls beginInsertRows and endInsertRows, however at least inserting and/or removing is not as trivial.
Most of the code examples I've found around deal with in-place editing (createEditor, etc.), which, as I've wrote above, is not what I'm needing. What should I be implementing to modify this model to accomplish this task? Alternatively, are there any code examples that show this modus operandi?
If you don't need to edit your model data using views, you don't need to implement insertRows, removeRows and setData. Instead, you should create your own modifying functions, e.g. add_task(HolidayTask* task), set_task(int row, HolidayTask* task) and remove_task(int row). In these functions you need to change m_items value to reflect data changes. (Besides, your should switch from QVector to QList if you need insertions and deletions from the middle of the list to be fast). Additionally, you should notify views about the changes:
Call beginRemoveRows before removing a row and endRemoveRows after that.
Call beginInsertRows before inserting a row and endInsertRows after that.
Call emit dataChanged(...) after changing data.
Inserting is like appending, just with a row != m_items.size():
void HolidayTaskModel::addTask(HolidayTask* task)
{
const int row = ...find position...
beginInsertRows(QModelIndex(), row, row);
m_items.insert(row, task);
endInsertRows();
}
Remove a task:
void HolidayTaskModel::removeTask(HolidayTask* task)
{
const int row = m_items.indexOf(task);
if (row == -1)
return;
beginRemoveRows(QModelIndex(), row, row);
delete m_items[row]; //only if the item is owned by the model
m_items.remove(row);
endRemoveRows();
}
For reordering, it depends on your exact use case, for moving single items use beginMoveRows()/endMoveRows(), for sorting reimplement sort() or, often easier, leave the items in the base model unsorted and let a QSortFilterProxyModel proxy do the sorting.
I have a QTreeView in my application with a data model. I'm capturing when items are double clicked with the following slot:
void MainWindow::on_treeView_doubleClicked(const QModelIndex &index)
{
if (index.parent().isValid()) {
QSharedPointer<GMResource> resource;
resource = index.data(Qt::UserRole).value<QSharedPointer<GMResource> >();
Workspace::GetSingleton()->OpenResourceEditor(resource);
}
}
I expected the QModelIndex::data() method to (execute and) return the underlying QStandardItem::data() for the item referenced by that index, however its not returning anything. I set a breakpoint in my QStandardItem::data() method, and it's not even being called, so I might have incorrectly assumed what QModelIndex::data() actually returns.
How can I access the item data referenced by the QModelIndex (eg. Access to the original QStandardItem I added to the model).
Here is my data() method for my QStandardItem derived class:
virtual QVariant data( int role) const {
if (role==Qt::UserRole) {
return QVariant(resource);
}
return QStandardItem::data(role);
}
Any help would be much appreciated
I found the solution to the problem.
I replaced this code:
return QVariant(resource);
With this code:
QVariant r;
r.setValue<QSharedPointer<GMResource> >(resource);
return r;
Seems to be working as expected. I guess the data() method was being executed, but the breakpoints weren't being triggered for some reason.
You should add Q_DECLARE_METATYPE(QSharedPointer<GMResource>) to be abe to wrap QSharedPointer<GMResource> type in QVariant
Suppose I want to implement a model/view architecture using the QTableView and QAbstractTableModel classes. So I subclass the QAbstractTableModel to create class MyModel and implement the QAbstractTableModel interface. Then connect the instance of this model to a QTableView instance using the setModel method.
#include <QtGui/QApplication>
#include <QtGui/QTableView>
#include "mymodel.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QTableView tableView;
MyModel myModel(0);
tableView.setModel( &myModel );
tableView.show();
return a.exec();
}
But how can I make the model read only? I cannot declare
const MyModel myModel(0);
because setModel takes a non constant argument. I reimplemented only constant methods of QAbstractTableModel.
What do you mean by const in this case? What do you want to achieve?
Do you want your underlying data to be immutable - so that edition from QTableView would be impossible? Then just disallow editing the model - e.g. by not implementing setData.
Also note that the standard implementation of
Qt::ItemFlags QAbstractItemModel::flags ( const QModelIndex & index ) const
will not return Qt::ItemIsEditable which is sufficient.
You will have to take care not to modify the model outside of the UI (note that modifying it outside without sending appropriate signals can result in bad things). But as it is your code - this shouldn't be an issue.
You can't make the model constant, as there are things that views need to be able to do that affect the QAbstractItemModel object, such as creating persistent model indexes.
The best way to communicate the fact that your model is read-only to the view is by overriding QAbstractTableModel::flags to always unset ItemIsEditable:
Qt::ItemFlags MyModel::flags ( const QModelIndex & index ) const
{
return QAbstractTableModel::flags(index) & ~Qt::ItemIsEditable;
}
Well on the assumption you mean read only by the end-user, and not read-only by the programmer, this article explains that the model is only editable when you re-implement QAbstractItemModel::flags(), QAbstractItemModel::setData(), and less importantly QAbstractItemModel::setHeaderData().
Furthermore, the functions you need to reimplement for a read only model, are all const:
Qt::ItemFlags TreeModel::flags(QModelIndex const & index) const { }
QModelIndex TreeModel::index(int row, int column, QModelIndex const & parentIndex) const { }
QModelIndex TreeModel::parent(QModelIndex const & childIndex) const { }
QVariant TreeModel::data(QModelIndex const & index, int role) const { }
QVariant TreeModel::headerData(int section, Qt::Orientation orientation, int role) const { }
int TreeModel::rowCount(QModelIndex const & parent) const { }
int TreeModel::columnCount(QModelIndex const & parent) const { }