[libcalamaresui] Create the slideshow on activation

- Load QML on startup, compile async
 - Create QML component when the page is reached.
 - On leave, stop the slideshow (otherwise, e.g. timers will keep running)

This should move some of the delay from loading a large
slideshow forward as the engine is already initialized when
we reach the install / slideshow page.

src/libcalamaresui/ExecutionViewStep.cpp

@@ -36,6 +36,7 @@
 #include <QDir>
 #include <QLabel>
 #include <QProgressBar>
+#include <QQmlComponent>
 #include <QQmlEngine>
 #include <QQuickWidget>
 #include <QVBoxLayout>
@@ -49,7 +50,8 @@ ExecutionViewStep::ExecutionViewStep( QObject* parent )
     , m_progressBar( new QProgressBar )
     , m_label( new QLabel )
     , m_qmlShow( new QQuickWidget )
-    , m_qmlShowLoaded( false )
+    , m_qmlComponent( nullptr )
+    , m_qmlObject( nullptr )
 {
     QVBoxLayout* layout = new QVBoxLayout( m_widget );
     QVBoxLayout* innerLayout = new QVBoxLayout;
@@ -72,11 +74,6 @@ ExecutionViewStep::ExecutionViewStep( QObject* parent )
     loadQml();
 
     connect( JobQueue::instance(), &JobQueue::progress, this, &ExecutionViewStep::updateFromJobQueue );
-
-    CALAMARES_RETRANSLATE_WIDGET( m_widget,
-        if ( m_qmlShowLoaded )
-            m_qmlShow->setSource( QUrl::fromLocalFile( Calamares::Branding::instance()->slideshowPath() ) );
-    )
 }
 
 
@@ -138,10 +135,12 @@ ExecutionViewStep::isAtEnd() const
 void
 ExecutionViewStep::loadQml()
 {
-    if ( !m_qmlShowLoaded && !Calamares::Branding::instance()->slideshowPath().isEmpty() )
+    if ( !m_qmlComponent && !Calamares::Branding::instance()->slideshowPath().isEmpty() )
     {
-        m_qmlShow->setSource( QUrl::fromLocalFile( Calamares::Branding::instance()->slideshowPath() ) );
+        m_qmlComponent = new QQmlComponent( m_qmlShow->engine(),
-        m_qmlShowLoaded = true;
+                                            QUrl::fromLocalFile( Calamares::Branding::instance()->slideshowPath() ),
+                                            QQmlComponent::CompilationMode::Asynchronous
+                                          );
     }
 }
 
@@ -149,6 +148,12 @@ void
 ExecutionViewStep::onActivate()
 {
     loadQml();
+    if ( m_qmlComponent )
+    {
+        m_qmlObject = m_qmlComponent->create();
+        cDebug() << "Created QML object" << (void *)m_qmlObject << m_qmlObject->objectName();
+        cDebug() << "Show root" << m_qmlShow->rootObject() << "context" << m_qmlShow->rootContext();
+    }
 
     JobQueue* queue = JobQueue::instance();
     foreach ( const QString& instanceKey, m_jobInstanceKeys )
@@ -191,4 +196,11 @@ ExecutionViewStep::updateFromJobQueue( qreal percent, const QString& message )
     m_label->setText( message );
 }
 
+void
+ExecutionViewStep::onLeave()
+{
+    delete m_qmlObject;
+    m_qmlObject = nullptr;
+}
+
 } // namespace

src/libcalamaresui/ExecutionViewStep.h

@@ -25,8 +25,10 @@
 #include <QStringList>
 
 class QLabel;
+class QObject;
 class QProgressBar;
 class QQuickWidget;
+class QQmlComponent;
 
 namespace Calamares
 {
@@ -51,6 +53,7 @@ public:
     bool isAtEnd() const override;
 
     void onActivate() override;
+    void onLeave() override;
 
     JobList jobs() const override;
 
@@ -61,7 +64,8 @@ private:
     QProgressBar* m_progressBar;
     QLabel* m_label;
     QQuickWidget* m_qmlShow;
-    bool m_qmlShowLoaded;
+    QQmlComponent* m_qmlComponent;
+    QObject* m_qmlObject;  //< The actual show
 
     QStringList m_jobInstanceKeys;