Make preview loading faster when scrolling

KFileItemListView notifies KFileItemModelRolesUpdater of changes of the
visible index range and the icon size with a delay, to prevent that
expensive operations are triggered repeatedly, and that scrolling feels
sluggish because the GUI thread is blocked by icon loading.

This patch ensures that the "long" delay of 300 ms is only used when
the zoom level is changed, and the "short" delay if only the visible
index range has changed.

Thanks to Christoph Feck for helping to analyze this problem!

BUG: 322093
FIXED-IN: 4.11.2
REVIEW: 112580

1 files changed, 25 insertions, 40 deletions

diff --git a/src/kitemviews/kfileitemlistview.cpp b/src/kitemviews/kfileitemlistview.cpp
index 2da238d7a..1ffd5019b 100644
--- a/src/kitemviews/kfileitemlistview.cpp
+++ b/src/kitemviews/kfileitemlistview.cpp
@@ -37,7 +37,15 @@
 // #define KFILEITEMLISTVIEW_DEBUG
 
 namespace {
+    // If the visible index range changes, KFileItemModelRolesUpdater is not
+    // informed immediatetly, but with a short delay. This ensures that scrolling
+    // always feels smooth and is not interrupted by icon loading (which can be
+    // quite expensive if a disk access is required to determine the final icon).
     const int ShortInterval = 50;
+
+    // If the icon size changes, a longer delay is used. This prevents that
+    // the expensive re-generation of all previews is triggered repeatedly when
+    // chaning the zoom level.
     const int LongInterval = 300;
 }
 
@@ -58,7 +66,7 @@ KFileItemListView::KFileItemListView(QGraphicsWidget* parent) :
 
     m_updateIconSizeTimer = new QTimer(this);
     m_updateIconSizeTimer->setSingleShot(true);
-    m_updateIconSizeTimer->setInterval(ShortInterval);
+    m_updateIconSizeTimer->setInterval(LongInterval);
     connect(m_updateIconSizeTimer, SIGNAL(timeout()), this, SLOT(updateIconSize()));
 
     setVisibleRoles(QList<QByteArray>() << "text");
@@ -309,7 +317,6 @@ void KFileItemListView::resizeEvent(QGraphicsSceneResizeEvent* event)
 void KFileItemListView::slotItemsRemoved(const KItemRangeList& itemRanges)
 {
     KStandardItemListView::slotItemsRemoved(itemRanges);
-    updateTimersInterval();
 }
 
 void KFileItemListView::slotSortRoleChanged(const QByteArray& current, const QByteArray& previous)
@@ -328,7 +335,12 @@ void KFileItemListView::triggerVisibleIndexRangeUpdate()
         return;
     }
     m_modelRolesUpdater->setPaused(true);
-    m_updateVisibleIndexRangeTimer->start();
+
+    // If the icon size has been changed recently, wait until
+    // m_updateIconSizeTimer expires.
+    if (!m_updateIconSizeTimer->isActive()) {
+        m_updateVisibleIndexRangeTimer->start();
+    }
 }
 
 void KFileItemListView::updateVisibleIndexRange()
@@ -341,17 +353,7 @@ void KFileItemListView::updateVisibleIndexRange()
     const int count = lastVisibleIndex() - index + 1;
     m_modelRolesUpdater->setMaximumVisibleItems(maximumVisibleItems());
     m_modelRolesUpdater->setVisibleIndexRange(index, count);
-
-    if (m_updateIconSizeTimer->isActive()) {
-        // If the icon-size update is pending do an immediate update
-        // of the icon-size before unpausing m_modelRolesUpdater. This prevents
-        // an unnecessary expensive recreation of all previews afterwards.
-        m_updateIconSizeTimer->stop();
-        m_modelRolesUpdater->setIconSize(availableIconSize());
-    }
-
     m_modelRolesUpdater->setPaused(isTransactionActive());
-    updateTimersInterval();
 }
 
 void KFileItemListView::triggerIconSizeUpdate()
@@ -361,6 +363,11 @@ void KFileItemListView::triggerIconSizeUpdate()
     }
     m_modelRolesUpdater->setPaused(true);
     m_updateIconSizeTimer->start();
+
+    // The visible index range will be updated when m_updateIconSizeTimer expires.
+    // Stop m_updateVisibleIndexRangeTimer to prevent an expensive re-generation
+    // of all previews (note that the user might change the icon size again soon).
+    m_updateVisibleIndexRangeTimer->stop();
 }
 
 void KFileItemListView::updateIconSize()
@@ -371,35 +378,13 @@ void KFileItemListView::updateIconSize()
 
     m_modelRolesUpdater->setIconSize(availableIconSize());
 
-    if (m_updateVisibleIndexRangeTimer->isActive()) {
-        // If the visibility-index-range update is pending do an immediate update
-        // of the range before unpausing m_modelRolesUpdater. This prevents
-        // an unnecessary expensive recreation of all previews afterwards.
-        m_updateVisibleIndexRangeTimer->stop();
-        const int index = firstVisibleIndex();
-        const int count = lastVisibleIndex() - index + 1;
-        m_modelRolesUpdater->setVisibleIndexRange(index, count);
-    }
+    // Update the visible index range (which has most likely changed after the
+    // icon size change) before unpausing m_modelRolesUpdater.
+    const int index = firstVisibleIndex();
+    const int count = lastVisibleIndex() - index + 1;
+    m_modelRolesUpdater->setVisibleIndexRange(index, count);
 
     m_modelRolesUpdater->setPaused(isTransactionActive());
-    updateTimersInterval();
-}
-
-void KFileItemListView::updateTimersInterval()
-{
-    if (!model()) {
-        return;
-    }
-
-    // The ShortInterval is used for cases like switching the directory: If the
-    // model is empty and filled later the creation of the previews should be done
-    // as soon as possible. The LongInterval is used when the model already contains
-    // items and assures that operations like zooming don't result in too many temporary
-    // recreations of the previews.
-
-    const int interval = (model()->count() <= 0) ? ShortInterval : LongInterval;
-    m_updateVisibleIndexRangeTimer->setInterval(interval);
-    m_updateIconSizeTimer->setInterval(interval);
 }
 
 void KFileItemListView::applyRolesToModel()