QML engine: Deprecate DefaultMethod

Assigning objects to signal handlers can be convenient, as seen in the
examples which use it together with ListView.onRemove.
However, that convenience makes it hard to reason about what actually
happens. Moreover, the only user of that functionality are the Animation
classes, and the usage of DefaultMethod is not documented anywhere.

[ChangeLog][QtQml] Assigning an object to a signal handler is deprecated.
Instead, create the object, give it an id, and call the desired slot
from the signal handler. For instance, instead of of
ListView.onRemove: SequentialAnimation {...}
use
SequentialAnimation {id: removeAnimation; ...}
ListView.onRemove: removeAnimation.start()
A warning will be printed whenever an assignment of an object to a
signal handler occurs. The warning can be controlled via the
qt.qml.defaultmethod logging category.

Change-Id: I001ddf4d7933871977f84a5e012d020fb043cc64
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

5 files changed, 22 insertions, 6 deletions

diff --git a/examples/quick/views/listview/dynamiclist.qml b/examples/quick/views/listview/dynamiclist.qml
index f37aab98e2..973d3ce389 100644
--- a/examples/quick/views/listview/dynamiclist.qml
+++ b/examples/quick/views/listview/dynamiclist.qml
@@ -200,18 +200,22 @@ Rectangle {
 
             // Animate adding and removing of items:
 //! [1]
-            ListView.onAdd: SequentialAnimation {
+            SequentialAnimation {
+                id: addAnimation
                 PropertyAction { target: delegateItem; property: "height"; value: 0 }
                 NumberAnimation { target: delegateItem; property: "height"; to: 80; duration: 250; easing.type: Easing.InOutQuad }
             }
+            ListView.onAdd: addAnimation.start()
 
-            ListView.onRemove: SequentialAnimation {
+            SequentialAnimation {
+                id: removeAnimation
                 PropertyAction { target: delegateItem; property: "ListView.delayRemove"; value: true }
                 NumberAnimation { target: delegateItem; property: "height"; to: 0; duration: 250; easing.type: Easing.InOutQuad }
 
                 // Make sure delayRemove is set back to false so that the item can be destroyed
                 PropertyAction { target: delegateItem; property: "ListView.delayRemove"; value: false }
             }
+            ListView.onRemove: removeAnimation.start()
         }
 //! [1]
     }
diff --git a/src/qml/qml/qqmlobjectcreator.cpp b/src/qml/qml/qqmlobjectcreator.cpp
index 8bf16fec96..76d1bd20bd 100644
--- a/src/qml/qml/qqmlobjectcreator.cpp
+++ b/src/qml/qml/qqmlobjectcreator.cpp
@@ -66,6 +66,9 @@
 
 #include <qtqml_tracepoints_p.h>
 #include <QScopedValueRollback>
+#include <QLoggingCategory>
+
+Q_LOGGING_CATEGORY(lcQmlDefaultMethod, "qt.qml.defaultmethod")
 
 QT_USE_NAMESPACE
 
@@ -1036,7 +1039,7 @@ bool QQmlObjectCreator::setPropertyBinding(const QQmlPropertyData *bindingProper
             return false;
         }
 
-        // Assigning object to signal property?
+        // Assigning object to signal property? ### Qt 7: Remove that functionality
         if (binding->flags & QV4::CompiledData::Binding::IsSignalHandlerObject) {
             if (!bindingProperty->isFunction()) {
                 recordError(binding->valueLocation, tr("Cannot assign an object to signal property %1").arg(bindingProperty->name(_qobject)));
@@ -1047,6 +1050,9 @@ bool QQmlObjectCreator::setPropertyBinding(const QQmlPropertyData *bindingProper
                 recordError(binding->valueLocation, tr("Cannot assign object type %1 with no default method").arg(QString::fromLatin1(createdSubObject->metaObject()->className())));
                 return false;
             }
+            qCWarning(lcQmlDefaultMethod) << "Assigning an object to a signal handler is deprecated."
+                                             "Instead, create the object, give it an id, and call the desired slot from the signal handler."
+                                             ;
 
             QMetaMethod signalMethod = _qobject->metaObject()->method(bindingProperty->coreIndex());
             if (!QMetaObject::checkConnectArgs(signalMethod, method)) {
diff --git a/src/quick/doc/snippets/qml/listview/listview.qml b/src/quick/doc/snippets/qml/listview/listview.qml
index 759344b574..b7b4b508c3 100644
--- a/src/quick/doc/snippets/qml/listview/listview.qml
+++ b/src/quick/doc/snippets/qml/listview/listview.qml
@@ -94,11 +94,13 @@ Rectangle {
 Component {
     id: delegate
     Item {
-        ListView.onRemove: SequentialAnimation {
+        SequentialAnimation {
+            id: removeAnimation
             PropertyAction { target: wrapper; property: "ListView.delayRemove"; value: true }
             NumberAnimation { target: wrapper; property: "scale"; to: 0; duration: 250; easing.type: Easing.InOutQuad }
             PropertyAction { target: wrapper; property: "ListView.delayRemove"; value: false }
         }
+        ListView.onRemove: removeAnimation.start()
     }
 }
 //![delayRemove]
diff --git a/tests/auto/quick/qquicklistview/data/listviewtest.qml b/tests/auto/quick/qquicklistview/data/listviewtest.qml
index b27c8e45e0..2bcba82035 100644
--- a/tests/auto/quick/qquicklistview/data/listviewtest.qml
+++ b/tests/auto/quick/qquicklistview/data/listviewtest.qml
@@ -95,12 +95,14 @@ Rectangle {
                     text: wrapper.y
                 }
                 color: ListView.isCurrentItem ? "lightsteelblue" : "white"
-                ListView.onRemove: SequentialAnimation {
+                SequentialAnimation {
+                    id: removeAnimation
                     PropertyAction { target: wrapper; property: "ListView.delayRemove"; value: true }
                     NumberAnimation { target: wrapper; property: "scale"; to: 0; duration: 250; easing.type: "InOutQuad" }
                     PropertyAction { target: wrapper; property: "ListView.delayRemove"; value: false }
 
                 }
+                ListView.onRemove: removeAnimation.start()
             }
         },
         Component {
diff --git a/tests/testapplications/listview/onRemove.qml b/tests/testapplications/listview/onRemove.qml
index c5fffb0e55..e8abbbed0e 100644
--- a/tests/testapplications/listview/onRemove.qml
+++ b/tests/testapplications/listview/onRemove.qml
@@ -87,13 +87,15 @@ Item {
                 lifeSpan: 2000
             }
 
-            ListView.onRemove: SequentialAnimation {
+            SequentialAnimation {
+                id: removeAnimation
                 PropertyAction { target: item; property: "ListView.delayRemove"; value: true }
                 PropertyAction { target: item; property: "opacity"; value: 0 }
                 ScriptAction { script: item.boom() }
                 PauseAnimation { duration: 1000 }
                 PropertyAction { target: item; property: "ListView.delayRemove"; value: false }
             }
+            ListView.onRemove: removeAnimation.start()
         }
     }