Compose render-to-texture widgets through QRhi

QPlatformTextureList holds a QRhiTexture instead of GLuint. A
QPlatformBackingStore now optionally can own a QRhi and a
QRhiSwapChain for the associated window.  Non-GL rendering must use
this QRhi everywhere, whereas GL (QOpenGLWidget) can choose to still
rely on resource sharing between contexts. A widget tells that it
wants QRhi and the desired configuration in a new virtual function in
QWidgetPrivate returning a QPlatformBackingStoreRhiConfig. This is
evaluated (among a top-level's all children) upon create() before
creating the repaint manager and the QWidgetWindow.

In QOpenGLWidget what do request is obvious: it will request an
OpenGL-based QRhi. QQuickWidget (or a potential future QRhiWidget)
will be more interesting: it needs to honor the standard Qt Quick
env.vars. and QQuickWindow APIs (or, in whatever way the user
configured the QRhiWidget), and so will set up the config struct
accordingly.

In addition, the rhiconfig and surface type is (re)evaluated when
(re)parenting a widget to a new tlw. If needed, this will now trigger
a destroy - create on the tlw. This should be be safe to do in
setParent. When multiple child widgets report an enabled rhiconfig,
the first one (the first child encountered) wins. So e.g. attempting
to have a QOpenGLWidget and a Vulkan-based QQuickWidget in the same
top-level window will fail one of the widgets (it likely won't
render).

RasterGLSurface is no longer used by widgets. Rather, the appropriate
surface type is chosen.

The rhi support in the backingstore is usable without widgets as well.
To make rhiFlush() functional, one needs to call setRhiConfig() after
creating the QBackingStore. (like QWidget does to top-level windows)

Most of the QT_NO_OPENGL ifdefs are eliminated all over the place.
Everything with QRhi is unconditional code at compile time, except the
actual initialization.

Having to plumb the widget tlw's shareContext (or, now, the QRhi)
through QWindowPrivate is no longer needed.  The old approach does not
scale: to implement composeAndFlush (now rhiFlush) we need more than
just a QRhi object, and this way we no longer pollute everything
starting from the widget level (QWidget's topextra -> QWidgetWindow ->
QWindowPrivate) just to send data around.

The BackingStoreOpenGLSupport interface and the QtGui - QtOpenGL split
is all gone. Instead, there is a QBackingStoreDefaultCompositor in
QtGui which is what the default implementations of composeAndFlush and
toTexture call. (overriding composeAndFlush and co. f.ex. in eglfs
should continue working mostly as-is, apart from adapting to the
texture list changes and getting the native OpenGL texture id out of
the QRhiTexture)

As QQuickWidget is way too complicated to just port as-is, an rhi
manual test (rhiwidget) is introduced as a first step, in ordewr to
exercise a simple, custom render-to-texture widget that does something
using a (not necessarily OpenGL-backed) QRhi and acts as fully
functional QWidget (modeled after QOpenGLWidget). This can also form
the foundation of a potential future QRhiWidget.

It is also possible to force the QRhi-based flushing always,
regardless of the presence of render-to-texture widgets. To exercise
this, set the env.var. QT_WIDGETS_RHI=1. This picks a
platform-specific default, and can be overridden with
QT_WIDGETS_RHI_BACKEND. (in sync with Qt Quick) This can eventually be
extended to query the platform plugin as well to check if the platform
plugin prefers to always do flushes with a 3D API.

QOpenGLWidget should work like before from the user's perspective, while
internally it has to do some things differently to play nice and prevent
regressions with the new rendering architecture. To exercise this
better, the qopenglwidget example gets a new tab-based view (that could
perhaps replace the example's main window later on?). The openglwidget
manual test is made compatible with Qt 6, and gets a counterpart in form
of the dockedopenglwidget manual test, which is a modified version of
the cube example that features dock widgets. This is relevant in
particular because render-to-texture widgets within a QDockWidget has
its own specific quirks, with logic taking this into account, hence
testing is essential.

For existing applications there are two important consequences with
this patch in place:

- Once the rhi-based composition is enabled, it stays active for the
lifetime of the top-level window.

- Dynamically creating and parenting the first render-to-texture
widget to an already created tlw will destroy and recreate the tlw
(and the underlying window). The visible effects of this depend on the
platform.  (e.g. the window may disappear and reappear on some,
whereas with other windowing systems it is not noticeable at all -
this is not really different from similar situtions with reparenting
or when moving windows between screens, so should be acceptable in
practice)

- On iOS raster windows are flushed with Metal (and rhi) from now on
(previously this was through OpenGL by making flush() call
composeAndFlush().

Change-Id: Id05bd0f7a26fa845f8b7ad8eedda3b0e78ab7a4e
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

6 files changed, 101 insertions, 22 deletions

diff --git a/examples/opengl/hellogl2/glwidget.cpp b/examples/opengl/hellogl2/glwidget.cpp
index 9988902071..23736cd752 100644
--- a/examples/opengl/hellogl2/glwidget.cpp
+++ b/examples/opengl/hellogl2/glwidget.cpp
@@ -131,6 +131,7 @@ void GLWidget::cleanup()
     delete m_program;
     m_program = nullptr;
     doneCurrent();
+    QObject::disconnect(context(), &QOpenGLContext::aboutToBeDestroyed, this, &GLWidget::cleanup);
 }
 
 static const char *vertexShaderSourceCore =
diff --git a/examples/opengl/qopenglwidget/glwidget.cpp b/examples/opengl/qopenglwidget/glwidget.cpp
index a480fe5726..da098319e4 100644
--- a/examples/opengl/qopenglwidget/glwidget.cpp
+++ b/examples/opengl/qopenglwidget/glwidget.cpp
@@ -66,9 +66,8 @@ const int bubbleNum = 8;
 #define GL_SRGB8_ALPHA8 0x8C43
 #endif
 
-GLWidget::GLWidget(MainWindow *mw, bool button, const QColor &background)
-    : m_mainWindow(mw),
-      m_hasButton(button),
+GLWidget::GLWidget(MainWindow *maybeMainWindow, const QColor &background)
+    : m_mainWindow(maybeMainWindow),
       m_background(background)
 {
     setMinimumSize(300, 250);
@@ -78,20 +77,41 @@ GLWidget::GLWidget(MainWindow *mw, bool button, const QColor &background)
 
 GLWidget::~GLWidget()
 {
+    reset();
+}
+
+void GLWidget::reset()
+{
     qDeleteAll(m_bubbles);
+    // Leave everything in a state suitable for a subsequent call to
+    // initialize(). This matters when coming from the context's
+    // aboutToBeDestroyed signal, would not matter when invoked from the
+    // destructor.
+    m_bubbles.clear();
 
     // And now release all OpenGL resources.
     makeCurrent();
     delete m_texture;
+    m_texture = nullptr;
     delete m_program1;
+    m_program1 = nullptr;
     delete m_program2;
+    m_program2 = nullptr;
     delete m_vshader1;
+    m_vshader1 = nullptr;
     delete m_fshader1;
+    m_fshader1 = nullptr;
     delete m_vshader2;
+    m_vshader2 = nullptr;
     delete m_fshader2;
+    m_fshader2 = nullptr;
     m_vbo1.destroy();
     m_vbo2.destroy();
     doneCurrent();
+
+    // We are done with the current QOpenGLContext, forget it. If there is a
+    // subsequent initialize(), that will then connect to the new context.
+    QObject::disconnect(m_contextWatchConnection);
 }
 
 void GLWidget::setScaling(int scale)
@@ -334,6 +354,13 @@ void GLWidget::initializeGL()
     m_vbo1.release();
 
     createBubbles(bubbleNum - m_bubbles.count());
+
+    // A well-behaved QOpenGLWidget releases OpenGL resources not only upon
+    // destruction, but also when the associated OpenGL context disappears. If
+    // the widget continues to exist, the context's destruction will be
+    // followed by a call to initialize(). This is not strictly mandatory in
+    // widgets that never change their parents.
+    m_contextWatchConnection = QObject::connect(context(), &QOpenGLContext::aboutToBeDestroyed, context(), [this] { reset(); });
 }
 
 void GLWidget::paintGL()
@@ -403,7 +430,7 @@ void GLWidget::paintGL()
 
     // When requested, follow the ideal way to animate: Rely on
     // blocking swap and just schedule updates continuously.
-    if (!m_mainWindow->timerEnabled())
+    if (!m_mainWindow || !m_mainWindow->timerEnabled())
         update();
 }
 
@@ -545,16 +572,16 @@ void GLWidget::setTransparent(bool transparent)
 
 void GLWidget::resizeGL(int, int)
 {
-    if (m_hasButton) {
+    if (m_mainWindow) {
         if (!m_btn) {
-            m_btn = new QPushButton("A widget on top.\nPress for more widgets.", this);
-            connect(m_btn, &QPushButton::clicked, this, &GLWidget::handleButtonPress);
+            m_btn = new QPushButton("\nAdd widget\n", this);
+            connect(m_btn, &QPushButton::clicked, this, [this] { m_mainWindow->addNew(); });
         }
         m_btn->move(20, 80);
+        if (!m_btn2) {
+            m_btn2 = new QPushButton("\nI prefer tabbed widgets\n", this);
+            connect(m_btn2, &QPushButton::clicked, this, [this] { m_mainWindow->showNewWindow(); });
+        }
+        m_btn2->move(20, 160);
     }
 }
-
-void GLWidget::handleButtonPress()
-{
-    m_mainWindow->addNew();
-}
diff --git a/examples/opengl/qopenglwidget/glwidget.h b/examples/opengl/qopenglwidget/glwidget.h
index 50f8aa9be5..bc664f474c 100644
--- a/examples/opengl/qopenglwidget/glwidget.h
+++ b/examples/opengl/qopenglwidget/glwidget.h
@@ -71,7 +71,7 @@ class GLWidget : public QOpenGLWidget, protected QOpenGLFunctions
 {
     Q_OBJECT
 public:
-    GLWidget(MainWindow *mw, bool button, const QColor &background);
+    GLWidget(MainWindow *maybeMainWindow, const QColor &background);
     ~GLWidget();
 
 public slots:
@@ -81,9 +81,6 @@ public slots:
     void setShowBubbles(bool);
     void setTransparent(bool transparent);
 
-private slots:
-    void handleButtonPress();
-
 protected:
     void resizeGL(int w, int h) override;
     void paintGL() override;
@@ -96,6 +93,7 @@ private:
     void createBubbles(int number);
     void quad(qreal x1, qreal y1, qreal x2, qreal y2, qreal x3, qreal y3, qreal x4, qreal y4);
     void extrude(qreal x1, qreal y1, qreal x2, qreal y2);
+    void reset();
 
     MainWindow *m_mainWindow;
     qreal m_fAngle = 0;
@@ -126,8 +124,9 @@ private:
     int m_textureUniform2 = 0;
     bool m_transparent = false;
     QPushButton *m_btn = nullptr;
-    bool m_hasButton;
+    QPushButton *m_btn2 = nullptr;
     QColor m_background;
+    QMetaObject::Connection m_contextWatchConnection;
 };
 
 #endif
diff --git a/examples/opengl/qopenglwidget/main.cpp b/examples/opengl/qopenglwidget/main.cpp
index 968b2a7258..08d1f2f14f 100644
--- a/examples/opengl/qopenglwidget/main.cpp
+++ b/examples/opengl/qopenglwidget/main.cpp
@@ -84,6 +84,7 @@ int main( int argc, char ** argv )
     QSurfaceFormat::setDefaultFormat(format);
 
     MainWindow mw;
-    mw.showMaximized();
+    mw.resize(1280, 720);
+    mw.show();
     return a.exec();
 }
diff --git a/examples/opengl/qopenglwidget/mainwindow.cpp b/examples/opengl/qopenglwidget/mainwindow.cpp
index eb34f87cb7..b3cbb4a69b 100644
--- a/examples/opengl/qopenglwidget/mainwindow.cpp
+++ b/examples/opengl/qopenglwidget/mainwindow.cpp
@@ -59,13 +59,16 @@
 #include <QRandomGenerator>
 #include <QSpinBox>
 #include <QScrollArea>
+#include <QTabWidget>
+#include <QTabBar>
+#include <QToolButton>
 
 #include "glwidget.h"
 
 MainWindow::MainWindow()
     : m_nextX(1), m_nextY(1)
 {
-    GLWidget *glwidget = new GLWidget(this, true, qRgb(20, 20, 50));
+    GLWidget *glwidget = new GLWidget(this, qRgb(20, 20, 50));
     m_glWidgets << glwidget;
     QLabel *label = new QLabel(this);
     m_timer = new QTimer(this);
@@ -127,6 +130,7 @@ MainWindow::MainWindow()
     QAction *showBubbles = showMenu->addAction("Show bubbles", glwidget, &GLWidget::setShowBubbles);
     showBubbles->setCheckable(true);
     showBubbles->setChecked(true);
+    showMenu->addAction("Open tab window", this, &MainWindow::showNewWindow);
     QMenu *helpMenu = menuBar()->addMenu("&Help");
     helpMenu->addAction("About Qt", qApp, &QApplication::aboutQt);
 
@@ -156,9 +160,9 @@ void MainWindow::addNew()
 {
     if (m_nextY == 4)
         return;
-    GLWidget *w = new GLWidget(this, false, qRgb(QRandomGenerator::global()->bounded(256),
-                                                 QRandomGenerator::global()->bounded(256),
-                                                 QRandomGenerator::global()->bounded(256)));
+    GLWidget *w = new GLWidget(nullptr, qRgb(QRandomGenerator::global()->bounded(256),
+                                             QRandomGenerator::global()->bounded(256),
+                                             QRandomGenerator::global()->bounded(256)));
     m_glWidgets << w;
     connect(m_timer, &QTimer::timeout, w, QOverload<>::of(&QWidget::update));
     m_layout->addWidget(w, m_nextY, m_nextX, 1, 1);
@@ -185,3 +189,47 @@ void MainWindow::resizeEvent(QResizeEvent *)
 {
      m_glWidgets[0]->setMinimumSize(size() + QSize(128, 128));
 }
+
+void MainWindow::showNewWindow()
+{
+    QTabWidget *tabs = new QTabWidget;
+    tabs->resize(800, 600);
+
+    QToolButton *tb = new QToolButton;
+    tb->setText(QLatin1String("+"));
+    tabs->addTab(new QLabel(QLatin1String("Add OpenGL widgets with +")), QString());
+    tabs->setTabEnabled(0, false);
+    tabs->tabBar()->setTabButton(0, QTabBar::RightSide, tb);
+    tabs->tabBar()->setTabsClosable(true);
+    QObject::connect(tabs->tabBar(), &QTabBar::tabCloseRequested, tabs, [tabs](int index) {
+        tabs->widget(index)->deleteLater();
+    });
+
+    const QString msgToTopLevel = QLatin1String("Break out to top-level window");
+    const QString msgFromTopLevel = QLatin1String("Move back under tab widget");
+
+    QObject::connect(tb, &QAbstractButton::clicked, tabs, [=] {
+        GLWidget *glwidget = new GLWidget(nullptr, Qt::blue);
+        glwidget->resize(tabs->size());
+        glwidget->setWindowTitle(QString::asprintf("QOpenGLWidget %p", glwidget));
+
+        QPushButton *btn = new QPushButton(msgToTopLevel, glwidget);
+        connect(btn, &QPushButton::clicked, glwidget, [=] {
+            if (glwidget->parent()) {
+                glwidget->setAttribute(Qt::WA_DeleteOnClose, true);
+                glwidget->setParent(nullptr);
+                glwidget->show();
+                btn->setText(msgFromTopLevel);
+            } else {
+                glwidget->setAttribute(Qt::WA_DeleteOnClose, false);
+                tabs->addTab(glwidget, glwidget->windowTitle());
+                btn->setText(msgToTopLevel);
+            }
+        });
+
+        tabs->setCurrentIndex(tabs->addTab(glwidget, glwidget->windowTitle()));
+    });
+
+    tabs->setAttribute(Qt::WA_DeleteOnClose);
+    tabs->show();
+}
diff --git a/examples/opengl/qopenglwidget/mainwindow.h b/examples/opengl/qopenglwidget/mainwindow.h
index 43cc55100e..69dcb34bb9 100644
--- a/examples/opengl/qopenglwidget/mainwindow.h
+++ b/examples/opengl/qopenglwidget/mainwindow.h
@@ -68,6 +68,9 @@ public:
 
     void resizeEvent(QResizeEvent *);
 
+public slots:
+    void showNewWindow();
+
 private slots:
     void updateIntervalChanged(int value);
     void timerUsageChanged(bool enabled);