Refactor theme plugin loading.

In the old implementation, the QPlatformIntegration was asked
for the theme first, so there was no way of overriding that
by a custom plugin. Also, there was a memory leak in case
the platform theme was actually created by a plugin.

QGuiApplication now asks the QPlatformIntegration for a list
of potential theme names first, tries to load them using
the plugin loader and finally invokes a factory method of
QPlatformIntegration in case that fails. The theme is now
owned by QGuiApplication.

In the XCB plugin, the environment variable DESKTOP_SESSION
is queried and appended to the list of themes, making it possible
to load plugins for other session types.

Change-Id: I1a4b4e061815bca16c65b23e591bb7563a3e44e2
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

3 files changed, 24 insertions, 6 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 77b69af3eb..ca1ecbef2b 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -538,13 +538,22 @@ static void init_platform(const QString &pluginArgument, const QString &platform
     }
 
     // Create the platform theme:
-    // 1) Ask the platform integration to create a platform theme
-    QGuiApplicationPrivate::platform_theme = QGuiApplicationPrivate::platform_integration->platformTheme();
+    // 1) Ask the platform integration for a list of names.
+    const QStringList themeNames = QGuiApplicationPrivate::platform_integration->themeNames();
+    foreach (const QString &themeName, themeNames) {
+        QGuiApplicationPrivate::platform_theme = QPlatformThemeFactory::create(themeName, platformPluginPath);
+        if (QGuiApplicationPrivate::platform_theme)
+            break;
+    }
 
     // 2) If none found, look for a theme plugin. Theme plugins are located in the
     // same directory as platform plugins.
     if (!QGuiApplicationPrivate::platform_theme) {
-        QGuiApplicationPrivate::platform_theme = QPlatformThemeFactory::create(name, platformPluginPath);
+        foreach (const QString &themeName, themeNames) {
+            QGuiApplicationPrivate::platform_theme = QGuiApplicationPrivate::platform_integration->createPlatformTheme(themeName);
+            if (QGuiApplicationPrivate::platform_theme)
+                break;
+        }
         // No error message; not having a theme plugin is allowed.
     }
 
@@ -748,6 +757,7 @@ QGuiApplicationPrivate::~QGuiApplicationPrivate()
 
     qt_cleanupFontDatabase();
 
+    delete  platform_theme;
     delete platform_integration;
     platform_integration = 0;
 }
diff --git a/src/gui/kernel/qplatformintegration_qpa.cpp b/src/gui/kernel/qplatformintegration_qpa.cpp
index e8721edce7..c9ccc27ad0 100644
--- a/src/gui/kernel/qplatformintegration_qpa.cpp
+++ b/src/gui/kernel/qplatformintegration_qpa.cpp
@@ -44,6 +44,7 @@
 #include <QtGui/QPlatformFontDatabase>
 #include <QtGui/QPlatformClipboard>
 #include <QtGui/QPlatformAccessibility>
+#include <QtGui/QPlatformTheme>
 #include <QtGui/private/qguiapplication_p.h>
 #include <QtGui/private/qpixmap_raster_p.h>
 #include <QtGui/private/qplatformscreen_qpa_p.h>
@@ -315,9 +316,15 @@ void QPlatformIntegration::screenAdded(QPlatformScreen *ps)
     emit qGuiApp->screenAdded(screen);
 }
 
-class QPlatformTheme *QPlatformIntegration::platformTheme() const
+QStringList QPlatformIntegration::themeNames() const
 {
-    return 0;
+    return QStringList();
+}
+
+class QPlatformTheme *QPlatformIntegration::createPlatformTheme(const QString &name) const
+{
+    Q_UNUSED(name)
+    return new QPlatformTheme;
 }
 
 QT_END_NAMESPACE
diff --git a/src/gui/kernel/qplatformintegration_qpa.h b/src/gui/kernel/qplatformintegration_qpa.h
index 3f9de9df5e..d6f5b6658a 100644
--- a/src/gui/kernel/qplatformintegration_qpa.h
+++ b/src/gui/kernel/qplatformintegration_qpa.h
@@ -126,7 +126,8 @@ public:
 
     virtual Qt::KeyboardModifiers queryKeyboardModifiers() const;
 
-    virtual QPlatformTheme *platformTheme() const;
+    virtual QStringList themeNames() const;
+    virtual QPlatformTheme *createPlatformTheme(const QString &name) const;
 
 protected:
     void screenAdded(QPlatformScreen *screen);