Migitate the performance regression caused by deferred execution

458eb65f7 introduced a performance regression. Before 458eb65f7, qmlbench
delegates_button.qml results were around 40 frames on a TX1. After 458eb65f,
it dropped to 32. This patch is unfortunately not able to bring it on the
original level, but at least improves the results to 37.

For QQuickText, it is extremely important that implicitWidth() gets called
before the component is completed, to avoid doing implicit size calculation
multiple times. Deferred execution caused a performance regression by creating
contentItem in "one go". We need to split the deferred execution to two parts
so that bindings get first setup, and later enabled upon completion. This way,
we utilize QQuickText's performance optimization for implicit size calculation.

Task-number: QTBUG-50992
Change-Id: I4bf00af71b6d7dbf1d4b58b00fe547c6c321f8ed
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

1 files changed, 19 insertions, 9 deletions

diff --git a/src/quicktemplates2/qquickabstractbutton.cpp b/src/quicktemplates2/qquickabstractbutton.cpp
index f87ee0ef..48d5fceb 100644
--- a/src/quicktemplates2/qquickabstractbutton.cpp
+++ b/src/quicktemplates2/qquickabstractbutton.cpp
@@ -133,8 +133,9 @@ QQuickAbstractButtonPrivate::QQuickAbstractButtonPrivate()
 
 QQuickItem *QQuickAbstractButtonPrivate::getContentItem()
 {
-    executeContentItem();
-    return QQuickControlPrivate::getContentItem();
+    if (!contentItem)
+        executeContentItem();
+    return contentItem;
 }
 
 void QQuickAbstractButtonPrivate::handlePress(const QPointF &point)
@@ -265,10 +266,18 @@ void QQuickAbstractButtonPrivate::toggle(bool value)
         emit q->toggled();
 }
 
-void QQuickAbstractButtonPrivate::executeIndicator()
+static inline QString indicatorName() { return QStringLiteral("indicator"); }
+
+void QQuickAbstractButtonPrivate::executeIndicator(bool complete)
 {
-    Q_Q(QQuickAbstractButton);
-    quickExecuteDeferred(q, QStringLiteral("indicator"), indicator);
+    Q_Q(QQuickControl);
+    if (indicator.wasExecuted())
+        return;
+
+    if (!indicator)
+        quickBeginDeferred(q, indicatorName(), indicator);
+    if (complete)
+        quickCompleteDeferred(q, indicatorName(), indicator);
 }
 
 QQuickAbstractButton *QQuickAbstractButtonPrivate::findCheckedButton() const
@@ -559,7 +568,8 @@ void QQuickAbstractButton::setAutoRepeat(bool repeat)
 QQuickItem *QQuickAbstractButton::indicator() const
 {
     QQuickAbstractButtonPrivate *d = const_cast<QQuickAbstractButtonPrivate *>(d_func());
-    d->executeIndicator();
+    if (!d->indicator)
+        d->executeIndicator();
     return d->indicator;
 }
 
@@ -594,9 +604,9 @@ void QQuickAbstractButton::toggle()
 void QQuickAbstractButton::componentComplete()
 {
     Q_D(QQuickAbstractButton);
-    d->executeIndicator();
-    d->executeBackground();
-    d->executeContentItem();
+    d->executeIndicator(true);
+    d->executeBackground(true);
+    d->executeContentItem(true);
     QQuickControl::componentComplete();
 }