Q(Untyped)Bindable: Print warnings when operations fail

Instead of silently failing, we now print an explanatory warning to aid
with debugging.

Task-number: QTBUG-89512
Change-Id: I36dd2ce452af12d0523c19286919095e366bd390
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

2 files changed, 75 insertions, 6 deletions

diff --git a/src/corelib/kernel/qproperty.cpp b/src/corelib/kernel/qproperty.cpp
index 8f95b5fd5e..607048d94b 100644
--- a/src/corelib/kernel/qproperty.cpp
+++ b/src/corelib/kernel/qproperty.cpp
@@ -42,9 +42,12 @@
 
 #include <qscopedvaluerollback.h>
 #include <QScopeGuard>
+#include <QtCore/qloggingcategory.h>
 
 QT_BEGIN_NAMESPACE
 
+Q_LOGGING_CATEGORY(lcQPropertyBinding, "qt.qproperty.binding");
+
 using namespace QtPrivate;
 
 void QPropertyBindingPrivatePtr::destroyAndFreeMemory()
@@ -2131,6 +2134,36 @@ bool isPropertyInBindingWrapper(const QUntypedPropertyData *property)
            bindingStatus.currentCompatProperty->property == property;
 }
 
+namespace BindableWarnings {
+
+void printUnsuitableBindableWarning(QAnyStringView prefix, BindableWarnings::Reason reason)
+{
+    switch (reason) {
+    case QtPrivate::BindableWarnings::NonBindableInterface:
+        qCWarning(lcQPropertyBinding).noquote() << prefix.toString()
+                                                << "The QBindable does not allow interaction with the binding.";
+        break;
+    case QtPrivate::BindableWarnings::ReadOnlyInterface:
+        qCWarning(lcQPropertyBinding).noquote() << prefix.toString()
+                                                << "The QBindable is read-only.";
+        break;
+    default:
+    case QtPrivate::BindableWarnings::InvalidInterface:
+        qCWarning(lcQPropertyBinding).noquote() << prefix.toString()
+                                                << "The QBindable is invalid.";
+        break;
+    }
+}
+
+void printMetaTypeMismatch(QMetaType actual, QMetaType expected)
+{
+    qCWarning(lcQPropertyBinding) << "setBinding: Could not set binding as the property expects it to be of type"
+                                  << actual.name()
+                                  << "but got" << expected.name() << "instead.";
+}
+
+} // namespace BindableWarnings end
+
 } // namespace QtPrivate end
 
 QT_END_NAMESPACE
diff --git a/src/corelib/kernel/qproperty.h b/src/corelib/kernel/qproperty.h
index 9f15a2de7d..9be10c0a31 100644
--- a/src/corelib/kernel/qproperty.h
+++ b/src/corelib/kernel/qproperty.h
@@ -43,9 +43,7 @@
 #include <QtCore/qglobal.h>
 #include <QtCore/qshareddata.h>
 #include <QtCore/qstring.h>
-#include <functional>
 #include <type_traits>
-#include <variant>
 
 #include <QtCore/qpropertyprivate.h>
 
@@ -555,6 +553,15 @@ public:
 
 }
 
+namespace QtPrivate {
+// used in Q(Untyped)Bindable to print warnings about various binding errors
+namespace BindableWarnings {
+enum Reason { InvalidInterface, NonBindableInterface, ReadOnlyInterface };
+Q_CORE_EXPORT void printUnsuitableBindableWarning(QAnyStringView prefix, Reason reason);
+Q_CORE_EXPORT void printMetaTypeMismatch(QMetaType actual, QMetaType expected);
+}
+}
+
 class QUntypedBindable
 {
 protected:
@@ -600,6 +607,11 @@ public:
     {
         if (iface)
             iface->setObserver(data, observer);
+#ifndef QT_NO_DEBUG
+        else
+            QtPrivate::BindableWarnings::printUnsuitableBindableWarning("observe:",
+                                                                        QtPrivate::BindableWarnings::InvalidInterface);
+#endif
     }
 
     template<typename Functor>
@@ -619,16 +631,31 @@ public:
 
     QUntypedPropertyBinding binding() const
     {
-        if (!iface->getBinding)
+        if (!isBindable()) {
+#ifndef QT_NO_DEBUG
+            QtPrivate::BindableWarnings::printUnsuitableBindableWarning("binding: ",
+                                                                        QtPrivate::BindableWarnings::NonBindableInterface);
+#endif
             return QUntypedPropertyBinding();
+        }
         return iface->getBinding(data);
     }
     bool setBinding(const QUntypedPropertyBinding &binding)
     {
-        if (!iface->setBinding)
+        if (isReadOnly()) {
+#ifndef QT_NO_DEBUG
+            const auto errorType = iface ? QtPrivate::BindableWarnings::ReadOnlyInterface :
+                                           QtPrivate::BindableWarnings::InvalidInterface;
+            QtPrivate::BindableWarnings::printUnsuitableBindableWarning("setBinding: Could not set binding via bindable interface.", errorType);
+#endif
             return false;
-        if (!binding.isNull() && binding.valueMetaType() != iface->metaType())
+        }
+        if (!binding.isNull() && binding.valueMetaType() != iface->metaType()) {
+#ifndef QT_NO_DEBUG
+            QtPrivate::BindableWarnings::printMetaTypeMismatch(iface->metaType(), binding.valueMetaType());
+#endif
             return false;
+        }
         iface->setBinding(data, binding);
         return true;
     }
@@ -675,7 +702,16 @@ public:
     QPropertyBinding<T> setBinding(const QPropertyBinding<T> &binding)
     {
         Q_ASSERT(!iface || binding.isNull() || binding.valueMetaType() == iface->metaType());
-        return (iface && iface->setBinding) ? static_cast<QPropertyBinding<T> &&>(iface->setBinding(data, binding)) : QPropertyBinding<T>();
+
+        if (iface && iface->setBinding)
+            return static_cast<QPropertyBinding<T> &&>(iface->setBinding(data, binding));
+#ifndef QT_NO_DEBUG
+        if (!iface)
+            QtPrivate::BindableWarnings::printUnsuitableBindableWarning("setBinding", QtPrivate::BindableWarnings::InvalidInterface);
+        else
+            QtPrivate::BindableWarnings::printUnsuitableBindableWarning("setBinding: Could not set binding via bindable interface.", QtPrivate::BindableWarnings::ReadOnlyInterface);
+#endif
+        return QPropertyBinding<T>();
     }
 #ifndef Q_CLANG_QDOC
     template <typename Functor>