QQmlTableInstanceModel: factor out reuse pool

Factor out the reuse pool logic in TableInstanceModel into a
separate class, so that we can share it with the upcoming implementation
for recycling items in QQmlDelegateModel.

Change-Id: If8f700b7a0208bac7d1cb1de087792e2c3a9b512
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

1 files changed, 11 insertions, 82 deletions

diff --git a/src/qmlmodels/qqmltableinstancemodel.cpp b/src/qmlmodels/qqmltableinstancemodel.cpp
index 1af93e87d8..b4d1e61e31 100644
--- a/src/qmlmodels/qqmltableinstancemodel.cpp
+++ b/src/qmlmodels/qqmltableinstancemodel.cpp
@@ -141,7 +141,7 @@ QQmlDelegateModelItem *QQmlTableInstanceModel::resolveModelItem(int index)
         return nullptr;
 
     // Check if the pool contains an item that can be reused
-    modelItem = takeFromReusableItemsPool(delegate);
+    modelItem = m_reusableItemsPool.takeItem(delegate, index);
     if (modelItem) {
         reuseItem(modelItem, index);
         m_modelItems.insert(index, modelItem);
@@ -225,16 +225,21 @@ QQmlInstanceModel::ReleaseFlags QQmlTableInstanceModel::release(QObject *object,
     m_modelItems.remove(modelItem->index);
 
     if (reusable == Reusable) {
-        insertIntoReusableItemsPool(modelItem);
+        m_reusableItemsPool.insertItem(modelItem);
+        emit itemPooled(modelItem->index, modelItem->object);
         return QQmlInstanceModel::Pooled;
     }
 
     // The item is not reused or referenced by anyone, so just delete it
-    modelItem->destroyObject();
-    emit destroyingItem(object);
+    destroyModelItem(modelItem);
+    return QQmlInstanceModel::Destroyed;
+}
 
+void QQmlTableInstanceModel::destroyModelItem(QQmlDelegateModelItem *modelItem)
+{
+    emit destroyingItem(modelItem->object);
+    modelItem->destroyObject();
     delete modelItem;
-    return QQmlInstanceModel::Destroyed;
 }
 
 void QQmlTableInstanceModel::cancel(int index)
@@ -257,85 +262,9 @@ void QQmlTableInstanceModel::cancel(int index)
     delete modelItem;
 }
 
-void QQmlTableInstanceModel::insertIntoReusableItemsPool(QQmlDelegateModelItem *modelItem)
-{
-    // Currently, the only way for a view to reuse items is to call QQmlTableInstanceModel::release()
-    // with the second argument explicitly set to QQmlTableInstanceModel::Reusable. If the released
-    // item is no longer referenced, it will be added to the pool. Reusing of items can be specified
-    // per item, in case certain items cannot be recycled.
-    // A QQmlDelegateModelItem knows which delegate its object was created from. So when we are
-    // about to create a new item, we first check if the pool contains an item based on the same
-    // delegate from before. If so, we take it out of the pool (instead of creating a new item), and
-    // update all its context-, and attached properties.
-    // When a view is recycling items, it should call QQmlTableInstanceModel::drainReusableItemsPool()
-    // regularly. As there is currently no logic to 'hibernate' items in the pool, they are only
-    // meant to rest there for a short while, ideally only from the time e.g a row is unloaded
-    // on one side of the view, and until a new row is loaded on the opposite side. In-between
-    // this time, the application will see the item as fully functional and 'alive' (just not
-    // visible on screen). Since this time is supposed to be short, we don't take any action to
-    // notify the application about it, since we don't want to trigger any bindings that can
-    // disturb performance.
-    // A recommended time for calling drainReusableItemsPool() is each time a view has finished
-    // loading e.g a new row or column. If there are more items in the pool after that, it means
-    // that the view most likely doesn't need them anytime soon. Those items should be destroyed to
-    // not consume resources.
-    // Depending on if a view is a list or a table, it can sometimes be performant to keep
-    // items in the pool for a bit longer than one "row out/row in" cycle. E.g for a table, if the
-    // number of visible rows in a view is much larger than the number of visible columns.
-    // In that case, if you flick out a row, and then flick in a column, you would throw away a lot
-    // of items in the pool if completely draining it. The reason is that unloading a row places more
-    // items in the pool than what ends up being recycled when loading a new column. And then, when you
-    // next flick in a new row, you would need to load all those drained items again from scratch. For
-    // that reason, you can specify a maxPoolTime to the drainReusableItemsPool() that allows you to keep
-    // items in the pool for a bit longer, effectively keeping more items in circulation.
-    // A recommended maxPoolTime would be equal to the number of dimenstions in the view, which
-    // means 1 for a list view and 2 for a table view. If you specify 0, all items will be drained.
-    Q_ASSERT(!modelItem->incubationTask);
-    Q_ASSERT(!modelItem->isObjectReferenced());
-    Q_ASSERT(!modelItem->isReferenced());
-    Q_ASSERT(modelItem->object);
-
-    modelItem->poolTime = 0;
-    m_reusableItemsPool.append(modelItem);
-    emit itemPooled(modelItem->index, modelItem->object);
-}
-
-QQmlDelegateModelItem *QQmlTableInstanceModel::takeFromReusableItemsPool(const QQmlComponent *delegate)
-{
-    // Find the oldest item in the pool that was made from the same delegate as
-    // the given argument, remove it from the pool, and return it.
-    if (m_reusableItemsPool.isEmpty())
-        return nullptr;
-
-    for (auto it = m_reusableItemsPool.begin(); it != m_reusableItemsPool.end(); ++it) {
-        if ((*it)->delegate != delegate)
-            continue;
-        auto modelItem = *it;
-        m_reusableItemsPool.erase(it);
-        return modelItem;
-    }
-
-    return nullptr;
-}
-
 void QQmlTableInstanceModel::drainReusableItemsPool(int maxPoolTime)
 {
-    // Rather than releasing all pooled items upon a call to this function, each
-    // item has a poolTime. The poolTime specifies for how many loading cycles an item
-    // has been resting in the pool. And for each invocation of this function, poolTime
-    // will increase. If poolTime is equal to, or exceeds, maxPoolTime, it will be removed
-    // from the pool and released. This way, the view can tweak a bit for how long
-    // items should stay in "circulation", even if they are not recycled right away.
-    for (auto it = m_reusableItemsPool.begin(); it != m_reusableItemsPool.end();) {
-        auto modelItem = *it;
-        modelItem->poolTime++;
-        if (modelItem->poolTime <= maxPoolTime) {
-            ++it;
-        } else {
-            it = m_reusableItemsPool.erase(it);
-            release(modelItem->object, NotReusable);
-        }
-    }
+    m_reusableItemsPool.drain(maxPoolTime, [=](QQmlDelegateModelItem *modelItem){ destroyModelItem(modelItem); });
 }
 
 void QQmlTableInstanceModel::reuseItem(QQmlDelegateModelItem *item, int newModelIndex)