QIcon: add a hook in the engine so a non null QIconEngine can still be a null icon

Implement it in the QIconLoader

We have to change detach() because some code does:
icon = QIcon::fromTheme("foobar"); if (icon.isNull()) icon.addPixmap(...);
so addPixmap and addFile have to work on a null QIcon by resetting
the iconEngine.

Change-Id: I07719bef93930cf4692384a8c64e21a97dcce25c
Reviewed-by: David Faure <david.faure@kdab.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/gui/image/qiconengine.cpp b/src/gui/image/qiconengine.cpp
index a25b216432..49a3bead53 100644
--- a/src/gui/image/qiconengine.cpp
+++ b/src/gui/image/qiconengine.cpp
@@ -143,6 +143,11 @@ void QIconEngine::addFile(const QString &/*fileName*/, const QSize &/*size*/, QI
     icon, for example when instantiating an icon using
     QIcon::fromTheme().
 
+    \value IsNullHook Allow to query if this engine represents a null
+    icon. The \a data argument of the virtual_hook() is a pointer to a
+    bool that can be set to true if the icon is null. This enum value
+    was added in Qt 5.7.
+
     \sa virtual_hook()
  */
 
@@ -276,4 +281,16 @@ QString QIconEngine::iconName() const
     return name;
 }
 
+/*!
+    \since 5.7
+
+    Returns true if this icon engine represent a null QIcon.
+ */
+bool QIconEngine::isNull() const
+{
+    bool isNull = false;
+    const_cast<QIconEngine *>(this)->virtual_hook(QIconEngine::IsNullHook, &isNull);
+    return isNull;
+}
+
 QT_END_NAMESPACE