Improve performance when dynamically adding items to a layout

This was especially noticeable when a Repeater was populating the
children of a layout, and its model was dynamically changed: When the
model was changed, the repeater removed one item at a time until all
items were removed, then applied the new model and then added all the
new items for the new model.

The layout reacted to that by doing a full sync of the QML layout into
the internal gridlayout engine each time an item got removed or added.
For very large layouts (or layouts that have complex size hints to
calculate), this caused a major slowdown.

This patch fixes that by postponing the sync until we get a
updatePolish(), basically replacing most calls to updateLayoutItems()
(which does the sync) with a call to invalidate() (which schedules a
polish). It will also get rid of some binding loop warnings due to this
change.

This means that there is a small change in behavior:
impicitWidth, implicitHeight and Layout.{min,max}imum{Width,Height}
might in some cases be incorrect until the updatePolish() have been
done. (This is however consistent with how Row/Column/Grid behaves)

The creation test in qmlbench was quite simple, so it did not suffer
from the most severe performance issues, but we did not regress:

> compareresults:

auto/creation/layouts/delegates_rowlayout.qml: improvement by 3.91%
auto/creation/layouts/delegates_columnlayout.qml: improvement by 6.59%
auto/creation/layouts/delegates_gridlayout.qml: improvement by 1.83%
Overall average of differences: 4.11%

And for the gridlayout_large.qml (Repeater with 1000 dynamically
changing items):

> compareresults:

auto/layouts/gridlayout_large.qml: improvement by 66477.78%
Overall average of differences: 66477.78%

[ChangeLog][Qt Quick Layouts] Performance improvements to Qt Quick
Layouts. This has the small side-effect that size hints
(implicitWidth/implicithHeight etc) changes are not immediately emitted
after a layout has been modified (e.g item added)

Pick-to: 5.15
Fixes: QTBUG-71839
Fixes: QTBUG-65121
Fixes: QTBUG-66017
Change-Id: I6922efe449134246df66b177992e4442747bc8fb
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

0 files changed, 0 insertions, 0 deletions