Handle function expressions as signal handlers

There are two ways to use function expressions on the right-hand side
of bindings:

  property var somethingPressed
  somethingPressed: function() { /* ..press something else.. */ }

  signal buttonPressed
  onButtonPressed: function() { /* ..handle buttonPress.. */ }

In the former case, it declares a property that holds a function. So on
initialization, the right-hand side of the binding returns a closure
that gets assigned to the property 'somethingPressed'.

In the latter case, the signal handler is explicitly marked as a
function for clarity. So, the handler should not be returning the
closure, but the handler should *be* the closure.

In general, it is not possible to detect if the left-hand side is a
property or a signal handler when generating QML cache files ahead of
time. So for this case, we mark the function as only returning a
closure. Then when instantiating the object, we check if it is a signal
handler, and if the handler is marked as only returning a closure. If
so, we set that closure to be the signal handler.

Task-number: QTBUG-57043
Task-number: QTBUG-50328
Change-Id: I3008ddd847e30b7d0adef07344a326f84d85f1ba
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/qml/qml/qqmlobjectcreator.cpp b/src/qml/qml/qqmlobjectcreator.cpp
index 7051fb51da..7c36f59035 100644
--- a/src/qml/qml/qqmlobjectcreator.cpp
+++ b/src/qml/qml/qqmlobjectcreator.cpp
@@ -879,6 +879,14 @@ bool QQmlObjectCreator::setPropertyBinding(const QQmlPropertyData *bindingProper
     if (binding->type == QV4::CompiledData::Binding::Type_Script || binding->containsTranslations()) {
         if (binding->flags & QV4::CompiledData::Binding::IsSignalHandlerExpression) {
             QV4::Function *runtimeFunction = compilationUnit->runtimeFunctions[binding->value.compiledScriptIndex];
+
+            // When a user writes the following:
+            //    onSignal: function() { doSomethingUsefull }
+            // then do not run the binding that returns the closure, but run the closure
+            // instead.
+            if (auto closure = runtimeFunction->nestedFunction())
+                runtimeFunction = closure;
+
             int signalIndex = _propertyCache->methodIndexToSignalIndex(bindingProperty->coreIndex());
             QQmlBoundSignal *bs = new QQmlBoundSignal(_bindingTarget, signalIndex, _scopeObject, engine);
             QQmlBoundSignalExpression *expr = new QQmlBoundSignalExpression(_bindingTarget, signalIndex,