QQmlListModel: Optimize model updates

Only update changed indices in the list model on insertion, deletion or
move. This can greatly improves performance when inserting or removing
near the end of the list, or when moving elements in a large model.

On a work in progress version of qmlbench's modelperf_listmodel_addremove.qml,
this appears to improve performance quite a bit (~40%):

Before: Average: 363.4  frames; using  samples; MedianAll=363; StdDev=5.31977, CoV=0.0146389
 After: Average: 518.2  frames; using  samples; MedianAll=518; StdDev=20.5475, CoV=0.0396517

Change-Id: I8ce7bea4ab8b1c7c3d0fbccc40b9a12ca3eadf59
Reviewed-by: Robin Burchell <robin.burchell@crimson.no>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

1 files changed, 10 insertions, 5 deletions

diff --git a/src/qml/types/qqmllistmodel.cpp b/src/qml/types/qqmllistmodel.cpp
index 06caa77bb4..eebf0f74d8 100644
--- a/src/qml/types/qqmllistmodel.cpp
+++ b/src/qml/types/qqmllistmodel.cpp
@@ -359,7 +359,7 @@ int ListModel::appendElement()
 void ListModel::insertElement(int index)
 {
     newElement(index);
-    updateCacheIndices();
+    updateCacheIndices(index);
 }
 
 void ListModel::move(int from, int to, int n)
@@ -381,7 +381,7 @@ void ListModel::move(int from, int to, int n)
     for (int i=0 ; i < store.count() ; ++i)
         elements[from+i] = store[i];
 
-    updateCacheIndices();
+    updateCacheIndices(from, to + n);
 }
 
 void ListModel::newElement(int index)
@@ -390,9 +390,14 @@ void ListModel::newElement(int index)
     elements.insert(index, e);
 }
 
-void ListModel::updateCacheIndices()
+void ListModel::updateCacheIndices(int start, int end)
 {
-    for (int i=0 ; i < elements.count() ; ++i) {
+    int count = elements.count();
+
+    if (end < 0 || end > count)
+        end = count;
+
+    for (int i = start; i < end; ++i) {
         ListElement *e = elements.at(i);
         if (ModelNodeMetaObject *mo = e->objectCache())
             mo->m_elementIndex = i;
@@ -574,7 +579,7 @@ void ListModel::remove(int index, int count)
         delete elements[index+i];
     }
     elements.remove(index, count);
-    updateCacheIndices();
+    updateCacheIndices(index);
 }
 
 void ListModel::insert(int elementIndex, QV4::Object *object)