Cache a state's parent state

QAbstractState::parentState() is called heavily by the state machine
algorithm. The parent state is obtained by qobject_cast'ing
QObject::parent(). qobject_cast() is expensive. This commit introduces
caching of the result in order to improve performance.

We expect that the cache won't be invalidated much since the parent-child
relationship of states usually doesn't change after the state machine is
started.

Reviewed-by: Eskil Abrahamsen Blomfeldt

2 files changed, 8 insertions, 2 deletions

diff --git a/src/corelib/statemachine/qabstractstate.cpp b/src/corelib/statemachine/qabstractstate.cpp
index 72d640bce9..cf67cdd103 100644
--- a/src/corelib/statemachine/qabstractstate.cpp
+++ b/src/corelib/statemachine/qabstractstate.cpp
@@ -78,7 +78,8 @@ QT_BEGIN_NAMESPACE
   function to perform custom processing when the state is exited.
 */
 
-QAbstractStatePrivate::QAbstractStatePrivate()    
+QAbstractStatePrivate::QAbstractStatePrivate()
+    : parentState(0)
 {
 }
 
@@ -150,7 +151,10 @@ QAbstractState::~QAbstractState()
 */
 QState *QAbstractState::parentState() const
 {
-    return qobject_cast<QState*>(parent());
+    Q_D(const QAbstractState);
+    if (d->parentState != parent())
+        d->parentState = qobject_cast<QState*>(parent());
+    return d->parentState;
 }
 
 /*!
diff --git a/src/corelib/statemachine/qabstractstate_p.h b/src/corelib/statemachine/qabstractstate_p.h
index 4b1306d712..cd57815b06 100644
--- a/src/corelib/statemachine/qabstractstate_p.h
+++ b/src/corelib/statemachine/qabstractstate_p.h
@@ -76,6 +76,8 @@ public:
 
     void emitEntered();
     void emitExited();
+
+    mutable QState *parentState;
 };
 
 QT_END_NAMESPACE