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

3 files changed, 17 insertions, 2 deletions

diff --git a/src/corelib/statemachine/qabstractstate.cpp b/src/corelib/statemachine/qabstractstate.cpp
index 72d640b..cf67cdd 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 4b1306d..cd57815 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
diff --git a/tests/auto/qstatemachine/tst_qstatemachine.cpp b/tests/auto/qstatemachine/tst_qstatemachine.cpp
index 97057c6..975b301 100644
--- a/tests/auto/qstatemachine/tst_qstatemachine.cpp
+++ b/tests/auto/qstatemachine/tst_qstatemachine.cpp
@@ -117,6 +117,7 @@ private slots:
     void cleanup();
 
     void rootState();
+    void machineWithParent();
     void addAndRemoveState();
     void stateEntryAndExit();
     void assignProperty();
@@ -1045,6 +1046,14 @@ void tst_QStateMachine::rootState()
     QCOMPARE(s2->parentState(), static_cast<QState*>(&machine));
 }
 
+void tst_QStateMachine::machineWithParent()
+{
+    QObject object;
+    QStateMachine *machine = new QStateMachine(&object);
+    QCOMPARE(machine->parent(), &object);
+    QCOMPARE(machine->parentState(), (QObject*)0);
+}
+
 void tst_QStateMachine::addAndRemoveState()
 {
 #ifdef QT_BUILD_INTERNAL