[Qt5-feedback] Adding better data access method to QAbstractItemModel?

[Stephen Kelly]

Hi,

Currently in the modelview API there are a lot of individual calls to the 
data() method. These can get expensive when proxy models are added to the 
mix.

I'm thinking of adding a method like this to QAbstractItemModel:

virtual QVariantist rangeData(const QModelIndex &topLeft, const QModelIndex 
&bottomRight, const QList<int> &roles) const;

with a default implementation like 

{
  QVariantList list;
  for (int row = topLeft.row(); row <= bottomRight.row(); ++row)
    for (int col = topLeft.column(); col <= bottomRight.column() ++col)
      foreach (int role, roles)
        list << index(row, column, topLeft.parent()).data(role);
  return list;
}

A model implementation and a proxy implementation can both do better of 
course, which is the whole idea. This change is source incompatible, but the 
itemData method currently in QAIM should maybe be deprecated.

I also want to change the dataChanged signal to 

dataChanged(const QModelIndex &topLeft, const QModelIndex &bottomRight, 
QSet<int> &roles = QSet<int>());

so that it actaully tells you what roles have changed. The extra roles 
argument can be safely ignored, and a signal emission with an empty roles 
set has the same meaning as dataChanged has today.

That means this is source compatible for the emitter but not the receiver, 
ie,

emit dataChanged(foo, bar);

still works, but

connect(model, SIGNAL(dataChanged(QMI,QMI)), SLOT(onDataChanged(QMI,QMI));

Needs to be changed to at least:

connect(model, SIGNAL(dataChanged(QMI,QMI,QSet<int>)), 
SLOT(onDataChanged(QMI,QMI));

The (source compatible) alternative would be to add a roleDataChanged signal 
with the signature I proposed instead of changing dataChanged (and possibly 
deprecating dataChanged).

What are your thoughts? Are small source incompatibilities like that 
acceptible?

Thanks,

Steve.