[Qt5-feedback] A childrenLayoutChanged signal for QAbstractItemModel

[Stephen Kelly]

Hi,

One of the most expensive signals to process from a QAbstractItemModel is 
layoutAboutToBeChanged and layoutChanged. 

Processing them in a proxy model is more expensive than processing the 
reset() signals, because you have to recreate the mappings for all 
QPersistentModelIndexes as well as all of their ancestors.

The problem is that the signals contain no granularity regarding what part 
of the structure is being changed. A layout change can result in rows in a 
tree being added, removed and moved.

In theory it would be possible to take a QPersistentModelIndex of every 
element in the tree on layoutAboutToBeChanged and examine the changes to 
them on layoutChanged to figure out what happened, but taking a 
QPersistentModelIndex of every element in the tree is prohibitively 
expensive and not smart.

The layout change signals are used when sorting in QSFPM for example (even 
if only sorting a single level when data changes for example). They can also 
be used to move stuff around.

I intend to add more granular ways to signal a structural change.

signals:
  void childrenLayoutsAboutToBeChanged(QModelIndex, QModelIndex)
  void childrenLayoutsChanged(QModelIndex, QModelIndex)

The new signals allow signalling about a change localized in just two parent 
indexes. That is enough to signal a move between two parents, and can be 
used with the same index for both arguments in the case of sorting the 
children of a single parent index, similar to the way dataChanged() is used.

If you're paying close attention you might think that the existing move 
signal is enough, but it is not really because it is too exact for a QSFPM 
that does sorting:

If a contiguous range of rows is moved in the source model, the result in 
the sorting proxy model might be entirely different, or no change at all.

See also https://qt.gitorious.org/qt/qt/merge_requests/776 which is this 
feature applied to Qt4.

The 

+     if (ignoreNextLayoutAboutToBeChanged) {
+         ignoreNextLayoutAboutToBeChanged = false;
+         return;
+     }

code complained about is not needed in Qt5. 

The branch contains some commits so you should look at the commits in the 
branch if you're interested for clarity, and not at the diff in gitorious.

Thanks,

Steve.