styles example: Set default style up front, and react to style changes

Allows running the example with -style foo or QT_STYLE_OVERRIDE.

No changes to documentation needed.

Change-Id: Id7cef450f13faabd118badde51afb7273439c9fc
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

examples/widgets/widgets/styles/main.cpp

@@ -50,12 +50,15 @@
 
 #include <QApplication>
 
+#include "norwegianwoodstyle.h"
 #include "widgetgallery.h"
 
 int main(int argc, char *argv[])
 {
     Q_INIT_RESOURCE(styles);
 
+    QApplication::setStyle(new NorwegianWoodStyle);
+
     QApplication app(argc, argv);
     WidgetGallery gallery;
     gallery.show();

examples/widgets/widgets/styles/norwegianwoodstyle.cpp

@@ -58,6 +58,7 @@
 NorwegianWoodStyle::NorwegianWoodStyle() :
     QProxyStyle(QStyleFactory::create("windows"))
 {
+    setObjectName("NorwegianWood");
 }
 
 //! [0]

examples/widgets/widgets/styles/widgetgallery.cpp

@@ -143,6 +143,7 @@ WidgetGallery::WidgetGallery(QWidget *parent)
     setLayout(mainLayout);
 
     setWindowTitle(tr("Styles"));
+    styleChanged();
 }
 //! [4]
 
@@ -150,12 +151,10 @@ WidgetGallery::WidgetGallery(QWidget *parent)
 void WidgetGallery::changeStyle(const QString &styleName)
 //! [5] //! [6]
 {
-    if (styleName == "NorwegianWood") {
+    if (styleName == "NorwegianWood")
         QApplication::setStyle(new NorwegianWoodStyle);
-    } else {
+    else
         QApplication::setStyle(QStyleFactory::create(styleName));
-    }
-    changePalette();
 }
 //! [6]
 
@@ -170,6 +169,25 @@ void WidgetGallery::changePalette()
 }
 //! [8]
 
+void WidgetGallery::changeEvent(QEvent *event)
+{
+    if (event->type() == QEvent::StyleChange)
+        styleChanged();
+}
+
+void WidgetGallery::styleChanged()
+{
+    auto styleName = QApplication::style()->objectName();
+    for (int i = 0; i < styleComboBox->count(); ++i) {
+        if (QString::compare(styleComboBox->itemText(i), styleName, Qt::CaseInsensitive) == 0) {
+            styleComboBox->setCurrentIndex(i);
+            break;
+        }
+    }
+
+    changePalette();
+}
+
 //! [9]
 void WidgetGallery::advanceProgressBar()
 //! [9] //! [10]

examples/widgets/widgets/styles/widgetgallery.h

@@ -80,8 +80,12 @@ class WidgetGallery : public QDialog
 public:
     WidgetGallery(QWidget *parent = nullptr);
 
+protected:
+    void changeEvent(QEvent *) override;
+
 private slots:
     void changeStyle(const QString &styleName);
+    void styleChanged();
     void changePalette();
     void advanceProgressBar();