Page: fix binding loop

When Dialog (which derives from Page) has only its title set,
there'd be a binding loop. A simplified version of the call stack:

- QQuickPopup::componentComplete
  - QQuickPopupPrivate::prepareEnterTransition
    - QQuickItem::setVisible
      - QQuickPagePrivate::itemVisibilityChanged
        - QQuickDialog::implicitHeaderWidthChanged
          - QQmlBinding::expressionChanged <== Dialog.qml's implicitWidth binding
            - QQmlPropertyData::readProperty
              - QQuickDialog::implicitHeaderWidth
                - QQuickPage::implicitHeaderWidth
                  - QQuickItem::implicitWidth <== QQuickPagePrivate::header, aka Label
                    - QQuickTextPrivate::getImplicitWidth
                      - QQuickTextPrivate::updateSize
                        - QQuickTextPrivate::setupTextLayout
                          - QQuickItem::setImplicitSize
                            - QQuickItemPrivate::implicitWidthChanged
                              - QQuickPagePrivate::itemImplicitWidthChanged
                                - QQuickDialog::implicitHeaderWidthChanged
                                  - QQmlBinding::expressionChanged
                                    - QQmlAbstractBinding::printBindingLoopError

Fix the issue by not emitting change signals if we're already in the
process of doing so.

Change-Id: I37d6fa35b1e70420e436c0e001f55035d7630542
Fixes: QTBUG-66494
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>

2 files changed, 13 insertions, 0 deletions

diff --git a/src/quicktemplates2/qquickpage.cpp b/src/quicktemplates2/qquickpage.cpp
index 56034297..cb90ac48 100644
--- a/src/quicktemplates2/qquickpage.cpp
+++ b/src/quicktemplates2/qquickpage.cpp
@@ -149,10 +149,12 @@ void QQuickPagePrivate::itemVisibilityChanged(QQuickItem *item)
     Q_Q(QQuickPage);
     QQuickPanePrivate::itemVisibilityChanged(item);
     if (item == header) {
+        QBoolBlocker signalGuard(emittingImplicitSizeChangedSignals);
         emit q->implicitHeaderWidthChanged();
         emit q->implicitHeaderHeightChanged();
         relayout();
     } else if (item == footer) {
+        QBoolBlocker signalGuard(emittingImplicitSizeChangedSignals);
         emit q->implicitFooterWidthChanged();
         emit q->implicitFooterHeightChanged();
         relayout();
@@ -163,6 +165,11 @@ void QQuickPagePrivate::itemImplicitWidthChanged(QQuickItem *item)
 {
     Q_Q(QQuickPage);
     QQuickPanePrivate::itemImplicitWidthChanged(item);
+
+    // Avoid binding loops by skipping signal emission if we're already doing it.
+    if (emittingImplicitSizeChangedSignals)
+        return;
+
     if (item == header)
         emit q->implicitHeaderWidthChanged();
     else if (item == footer)
@@ -173,6 +180,11 @@ void QQuickPagePrivate::itemImplicitHeightChanged(QQuickItem *item)
 {
     Q_Q(QQuickPage);
     QQuickPanePrivate::itemImplicitHeightChanged(item);
+
+    // Avoid binding loops by skipping signal emission if we're already doing it.
+    if (emittingImplicitSizeChangedSignals)
+        return;
+
     if (item == header)
         emit q->implicitHeaderHeightChanged();
     else if (item == footer)
diff --git a/src/quicktemplates2/qquickpage_p_p.h b/src/quicktemplates2/qquickpage_p_p.h
index b7d89ac4..6c8b0371 100644
--- a/src/quicktemplates2/qquickpage_p_p.h
+++ b/src/quicktemplates2/qquickpage_p_p.h
@@ -71,6 +71,7 @@ public:
     QString title;
     QQuickItem *header = nullptr;
     QQuickItem *footer = nullptr;
+    bool emittingImplicitSizeChangedSignals = false;
 };
 
 QT_END_NAMESPACE