QPA: improve plugin and theme documentation and logging output

This makes it easier to understand which plugin and theme are loaded.

Change-Id: I0eee395e1b7da5d5017d8f7839beefb547fdc7b0
Pick-to: 6.2 6.3
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 27 insertions, 5 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 9d383355e1..6e93ef9e7c 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -1195,10 +1195,16 @@ QString QGuiApplication::platformName()
 }
 
 Q_LOGGING_CATEGORY(lcQpaPluginLoading, "qt.qpa.plugin");
+Q_LOGGING_CATEGORY(lcQpaTheme, "qt.qpa.theme");
 Q_LOGGING_CATEGORY(lcPtrDispatch, "qt.pointer.dispatch");
 
 static void init_platform(const QString &pluginNamesWithArguments, const QString &platformPluginPath, const QString &platformThemeName, int &argc, char **argv)
 {
+    qCDebug(lcQpaPluginLoading) << "init_platform called with"
+        << "pluginNamesWithArguments" << pluginNamesWithArguments
+        << "platformPluginPath" << platformPluginPath
+        << "platformThemeName" << platformThemeName;
+
     QStringList plugins = pluginNamesWithArguments.split(QLatin1Char(';'), Qt::SkipEmptyParts);
     QStringList platformArguments;
     QStringList availablePlugins = QPlatformIntegrationFactory::keys(platformPluginPath);
@@ -1214,6 +1220,8 @@ static void init_platform(const QString &pluginNamesWithArguments, const QString
         argumentsKey[0] = argumentsKey.at(0).toUpper();
         arguments.append(QLibraryInfo::platformPluginArguments(argumentsKey));
 
+        qCDebug(lcQpaPluginLoading) << "Attempting to load Qt platform plugin" << name << "with arguments" << arguments;
+
         // Create the platform integration.
         QGuiApplicationPrivate::platform_integration = QPlatformIntegrationFactory::create(name, arguments, argc, argv, platformPluginPath);
         if (Q_UNLIKELY(!QGuiApplicationPrivate::platform_integration)) {
@@ -1227,6 +1235,7 @@ static void init_platform(const QString &pluginNamesWithArguments, const QString
                         << QDir::toNativeSeparators(platformPluginPath) << "\"";
             }
         } else {
+            qCDebug(lcQpaPluginLoading) << "Successfully loaded Qt platform plugin" << name;
             QGuiApplicationPrivate::platform_name = new QString(name);
             platformArguments = arguments;
             break;
@@ -1255,37 +1264,50 @@ static void init_platform(const QString &pluginNamesWithArguments, const QString
 
     // 1) Fetch the platform name from the environment if present.
     QStringList themeNames;
-    if (!platformThemeName.isEmpty())
+    if (!platformThemeName.isEmpty()) {
+        qCDebug(lcQpaTheme) << "Adding" << platformThemeName << "from environment to list of theme names";
         themeNames.append(platformThemeName);
+    }
 
     // 2) Special case - check whether it's a flatpak or snap app to use xdg-desktop-portal platform theme for portals support
     if (checkNeedPortalSupport()) {
+        qCDebug(lcQpaTheme) << "Adding xdgdesktopportal to list of theme names";
         themeNames.append(QStringLiteral("xdgdesktopportal"));
     }
 
     // 3) Ask the platform integration for a list of theme names
-    themeNames += QGuiApplicationPrivate::platform_integration->themeNames();
+    const auto platformIntegrationThemeNames = QGuiApplicationPrivate::platform_integration->themeNames();
+    qCDebug(lcQpaTheme) << "Adding platform integration's theme names to list of theme names:" << platformIntegrationThemeNames;
+    themeNames += platformIntegrationThemeNames;
     // 4) Look for a theme plugin.
     for (const QString &themeName : qAsConst(themeNames)) {
+        qCDebug(lcQpaTheme) << "Attempting to create platform theme" << themeName << "via QPlatformThemeFactory::create";
         QGuiApplicationPrivate::platform_theme = QPlatformThemeFactory::create(themeName, platformPluginPath);
-        if (QGuiApplicationPrivate::platform_theme)
+        if (QGuiApplicationPrivate::platform_theme) {
+            qCDebug(lcQpaTheme) << "Successfully created platform theme" << themeName;
             break;
+        }
     }
 
     // 5) If no theme plugin was found ask the platform integration to
     // create a theme
     if (!QGuiApplicationPrivate::platform_theme) {
         for (const QString &themeName : qAsConst(themeNames)) {
+            qCDebug(lcQpaTheme) << "Attempting to create platform theme" << themeName << "via createPlatformTheme";
             QGuiApplicationPrivate::platform_theme = QGuiApplicationPrivate::platform_integration->createPlatformTheme(themeName);
-            if (QGuiApplicationPrivate::platform_theme)
+            if (QGuiApplicationPrivate::platform_theme) {
+                qCDebug(lcQpaTheme) << "Successfully created platform theme" << themeName;
                 break;
+            }
         }
         // No error message; not having a theme plugin is allowed.
     }
 
     // 6) Fall back on the built-in "null" platform theme.
-    if (!QGuiApplicationPrivate::platform_theme)
+    if (!QGuiApplicationPrivate::platform_theme) {
+        qCDebug(lcQpaTheme) << "Failed to create platform theme; using \"null\" platform theme";
         QGuiApplicationPrivate::platform_theme = new QPlatformTheme;
+    }
 
     // Set arguments as dynamic properties on the native interface as
     // boolean 'foo' or strings: 'foo=bar'