QStateMachine: add defaultTransition in QHistoryState

The history state had the limitation that it was hard (or impossible) to
use when more than one default state had to be entered. For example,
using it in a parallel state was impossible without ending up in an
infinite loop.

This patch changes the QHistoryState to only have an initial transition,
and the state selection algorithm is changed accordingly. It also brings
QStateMachine closer to the SCXML standard.

The existing defaultState is implemented on top of the
defaultTransition: when used, a new transition, with the default state as
its target, is set as the defaultTransition.

Task-number: QTBUG-46703
Change-Id: Ifbb44e4f0f26b72e365af4c94753e4483f9850e7
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>

1 files changed, 17 insertions, 8 deletions

diff --git a/src/corelib/statemachine/qstatemachine.cpp b/src/corelib/statemachine/qstatemachine.cpp
index 335610dac6..237b19bc0e 100644
--- a/src/corelib/statemachine/qstatemachine.cpp
+++ b/src/corelib/statemachine/qstatemachine.cpp
@@ -365,9 +365,9 @@ static QList<QAbstractState *> getEffectiveTargetStates(QAbstractTransition *tra
             if (!historyConfiguration.isEmpty()) {
                 // There is a saved history, so apply that.
                 targets.unite(historyConfiguration.toSet());
-            } else if (QAbstractState *defaultState = historyState->defaultState()) {
-                // Qt does not support initial transitions, but uses the default state of the history state for this.
-                targets.insert(defaultState);
+            } else if (QAbstractTransition *defaultTransition = historyState->defaultTransition()) {
+                // No saved history, take all default transition targets.
+                targets.unite(defaultTransition->targetStates().toSet());
             } else {
                 // Woops, we found a history state without a default state. That's not valid!
                 QStateMachinePrivate *m = QStateMachinePrivate::get(historyState->machine());
@@ -978,9 +978,16 @@ void QStateMachinePrivate::enterStates(QEvent *event, const QList<QAbstractState
 
         QAbstractStatePrivate::get(s)->callOnEntry(event);
         QAbstractStatePrivate::get(s)->emitEntered();
-        if (statesForDefaultEntry.contains(s)) {
-            // ### executeContent(s.initial.transition.children())
-        }
+
+        // FIXME:
+        // See the "initial transitions" comment in addDescendantStatesToEnter first, then implement:
+//        if (statesForDefaultEntry.contains(s)) {
+//            // ### executeContent(s.initial.transition.children())
+//        }
+        Q_UNUSED(statesForDefaultEntry);
+
+        if (QHistoryState *h = toHistoryState(s))
+            QAbstractTransitionPrivate::get(h->defaultTransition())->callOnTransition(event);
 
         // Emit propertiesAssigned signal if the state has no animated properties.
         {
@@ -1091,8 +1098,8 @@ void QStateMachinePrivate::addDescendantStatesToEnter(QAbstractState *state,
 #endif
         } else {
             QList<QAbstractState*> defaultHistoryContent;
-            if (QHistoryStatePrivate::get(h)->defaultState)
-                defaultHistoryContent.append(QHistoryStatePrivate::get(h)->defaultState);
+            if (QAbstractTransition *t = QHistoryStatePrivate::get(h)->defaultTransition)
+                defaultHistoryContent = t->targetStates();
 
             if (defaultHistoryContent.isEmpty()) {
                 setError(QStateMachine::NoDefaultStateInHistoryStateError, h);
@@ -1118,8 +1125,10 @@ void QStateMachinePrivate::addDescendantStatesToEnter(QAbstractState *state,
             if (QAbstractState *initial = toStandardState(state)->initialState()) {
                 Q_ASSERT(initial->machine() == q_func());
 
+                // FIXME:
                 // Qt does not support initial transitions (which is a problem for parallel states).
                 // The way it simulates this for other states, is by having a single initial state.
+                // See also the FIXME in enterStates.
                 statesForDefaultEntry.insert(initial);
 
                 addDescendantStatesToEnter(initial, statesToEnter, statesForDefaultEntry);