QProperty: Add private isAnyBindingEvaluating function

To optimize certain operations, it can be useful to know whether we are
currently evaluating a binding. For instance, we have properties whose
storage is only alloctaed on-demand when they are set. However, we would
also allocate them if they are used in a binding context, as we would
otherwise not properly track the dependency. Using
isAnyBindingEvaluating in the getter, we can detect this
situation, and avoid the allocation if it returns false.

This API is private for now, as it exposes some internals of the
property system and should be used with care. As it needs to access the
TLS variable, it also has a non-negligible cost.

Change-Id: I373aabee644fe7020b2ffba7d6a0ad9a1e1b4ec0
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/kernel/qproperty.cpp

@@ -1667,4 +1667,19 @@ void restoreBindingStatus(BindingEvaluationState *status)
     bindingStatus.currentlyEvaluatingBinding = status;
 }
 
+namespace QtPrivate {
+/*!
+    \internal
+    This function can be used to detect whether we are currently
+    evaluating a binding. This can e.g. be used to defer the allocation
+    of extra data for a QPropertyBindingStorage in a getter.
+    Note that this function accesses TLS storage, and is therefore soemwhat
+    costly to call.
+ */
+bool isAnyBindingEvaluating()
+{
+    return bindingStatus.currentlyEvaluatingBinding != nullptr;
+}
+} // namespace QtPrivate end
+
 QT_END_NAMESPACE

src/corelib/kernel/qproperty_p.h

@@ -59,6 +59,10 @@
 
 QT_BEGIN_NAMESPACE
 
+namespace QtPrivate {
+    Q_CORE_EXPORT bool isAnyBindingEvaluating();
+}
+
 // Keep all classes related to QProperty in one compilation unit. Performance of this code is crucial and
 // we need to allow the compiler to inline where it makes sense.
 

tests/auto/corelib/kernel/qproperty/tst_qproperty.cpp

@@ -1140,6 +1140,15 @@ void tst_QProperty::testNewStuff()
     QCOMPARE(object.bindableFoo().value(), 111);
     object.bindableFoo().setValue(24);
     QCOMPARE(object.foo(), 24);
+
+    auto isCurrentlyEvaluatingBinding = []() {
+        return QtPrivate::isAnyBindingEvaluating();
+    };
+    QVERIFY(!isCurrentlyEvaluatingBinding());
+    QProperty<bool> evaluationDetector {false};
+    evaluationDetector.setBinding(isCurrentlyEvaluatingBinding);
+    QVERIFY(evaluationDetector.value());
+    QVERIFY(!isCurrentlyEvaluatingBinding());
 }
 
 void tst_QProperty::qobjectObservers()