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>

src/gui/image/qicon.cpp

@@ -918,7 +918,7 @@ void QIcon::paint(QPainter *painter, const QRect &rect, Qt::Alignment alignment,
 */
 bool QIcon::isNull() const
 {
-    return !d;
+    return !d || d->engine->isNull();
 }
 
 /*!\internal
@@ -933,7 +933,12 @@ bool QIcon::isDetached() const
 void QIcon::detach()
 {
     if (d) {
-        if (d->ref.load() != 1) {
+        if (d->engine->isNull()) {
+            if (!d->ref.deref())
+                delete d;
+            d = 0;
+            return;
+        } else if (d->ref.load() != 1) {
             QIconPrivate *x = new QIconPrivate;
             x->engine = d->engine->clone();
             if (!d->ref.deref())
@@ -957,11 +962,10 @@ void QIcon::addPixmap(const QPixmap &pixmap, Mode mode, State state)
 {
     if (pixmap.isNull())
         return;
+    detach();
     if (!d) {
         d = new QIconPrivate;
         d->engine = new QPixmapIconEngine;
-    } else {
-        detach();
     }
     d->engine->addPixmap(pixmap, mode, state);
 }
@@ -1001,6 +1005,7 @@ void QIcon::addFile(const QString &fileName, const QSize &size, Mode mode, State
 {
     if (fileName.isEmpty())
         return;
+    detach();
     if (!d) {
 #ifndef QT_NO_LIBRARY
         QFileInfo info(fileName);
@@ -1023,8 +1028,6 @@ void QIcon::addFile(const QString &fileName, const QSize &size, Mode mode, State
             d = new QIconPrivate;
             d->engine = new QPixmapIconEngine;
         }
-    } else {
-        detach();
     }
     d->engine->addFile(fileName, size, mode, state);
 

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

src/gui/image/qiconengine.h

@@ -57,7 +57,7 @@ public:
     virtual bool read(QDataStream &in);
     virtual bool write(QDataStream &out) const;
 
-    enum IconEngineHook { AvailableSizesHook = 1, IconNameHook };
+    enum IconEngineHook { AvailableSizesHook = 1, IconNameHook, IsNullHook };
 
     struct AvailableSizesArgument
     {
@@ -70,6 +70,7 @@ public:
                                     QIcon::State state = QIcon::Off) const;
 
     virtual QString iconName() const;
+    bool isNull() const; // ### Qt6 make virtual
 
     virtual void virtual_hook(int id, void *data);
 };

src/gui/image/qiconloader.cpp

@@ -587,6 +587,11 @@ void QIconLoaderEngine::virtual_hook(int id, void *data)
             name = m_info.iconName;
         }
         break;
+    case QIconEngine::IsNullHook:
+        {
+            *reinterpret_cast<bool*>(data) = m_info.entries.isEmpty();
+        }
+        break;
     default:
         QIconEngine::virtual_hook(id, data);
     }