Qt ApplicationManager的Compositor功能分析

• 根据Qt ApplicationManager官网介绍，它基于Wayland协议实现了Compositor功能。下述为官网介绍。实际上，QtApplicationManager是使用了QtWayland模块来实现Compositor的。Wayland是一套旨在替代XWindow的 Compositor标准，感兴趣的可自行了解。

To support multiple UI processes on an embedded Linux system, you need a central window compositor: a Wayland compositor is the state-of-the-art solution for this. Consequently, the application manager incorporates a compositor that is fully-compliant with the Wayland protocol, based on the QtWayland module.

• 关于Wayland Compositor，简单来说就是Client绘制Buffer，Compositor将这些Buffer融合，最终显示到屏幕上。大概就是下图这个样子（图片摘自QtWayland官网）。
  
• QtApplicationManager推荐使用QML方式进行开发，可以QML编写一套Compositor服务（进程）。

这里基于QML方式的Compositor Sample，分析一下QtApplicationManager是如何实现Compositor功能。

一个QML的Compositor例子

• 路径qtapplicationmanager\examples\applicationmanager\hello-world下提供了Hello World程序，其中system-ui.qml（启动入口）实现如下

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****************************************************************************/import QtQuick 2.4
import QtApplicationManager.SystemUI 2.0Item {// 省略// 重点关注这段代码// Show windowsColumn {anchors.right: parent.rightRepeater {model: WindowManagerWindowItem {width: 600height: 200window: model.window}}}
}

• 上面的代码中，将WindowManager对象内部的一系列window对象，绑定到 WindowItem上，并设置了宽和高。因为WaylandSurface与WindowManager中的 window对象是一对一的（后面会解释），所以这段代码，就是让Surface显示出来的本质。

• 创建WaylandSurface时WindowManager创建Window对象，具体实现如下。

// 绑定了QWaylandSurface::hasContentChanged信号，当有该信号时触发surfaceMapped信号。
// 该信号在Surface中有Buffer被Commit时触发
// 这块代码可以看出来 QtApplicationManager目前只支持 XDG和WlShell两种类型。void WaylandCompositor::createWlSurface(QWaylandSurface *surface, const QWaylandResource &resource)
{WindowSurface *windowSurface = static_cast<WindowSurface *>(surface);QWaylandWlShellSurface *ss = new QWaylandWlShellSurface(m_wlShell, windowSurface, resource);windowSurface->setShellSurface(ss);connect(windowSurface, &QWaylandSurface::hasContentChanged, this, [this, windowSurface]() {if (windowSurface->hasContent())emit this->surfaceMapped(static_cast<WindowSurface*>(windowSurface));});
}void WaylandCompositor::onXdgSurfaceCreated(QWaylandXdgSurface *xdgSurface)
{WindowSurface *windowSurface = static_cast<WindowSurface*>(xdgSurface->surface());Q_ASSERT(!windowSurface->m_wlSurface);windowSurface->m_xdgSurface = xdgSurface;connect(windowSurface, &QWaylandSurface::hasContentChanged, this, [this, windowSurface]() {if (windowSurface->hasContent())emit this->surfaceMapped(windowSurface);});emit windowSurface->xdgSurfaceChanged();
}

void WindowManager::registerCompositorView(QQuickWindow *view)
{// 省略#if defined(AM_MULTI_PROCESS)if (!ApplicationManager::instance()->isSingleProcess()) {if (!d->waylandCompositor) {d->waylandCompositor = new WaylandCompositor(view, d->waylandSocketName);for (const auto &extraSocket : d->extraWaylandSockets)d->waylandCompositor->addSocketDescriptor(extraSocket);connect(d->waylandCompositor, &QWaylandCompositor::surfaceCreated,this, &WindowManager::waylandSurfaceCreated);connect(d->waylandCompositor, &WaylandCompositor::surfaceMapped,this, &WindowManager::waylandSurfaceMapped);// 省略}
#elseif (!once)qCInfo(LogGraphics) << "WindowManager: running in single-process mode [forced at compile-time]";
#endif// 省略
}// 这里函数里面，会创建Window对象，并将Window对象与Surface对象绑定。
void WindowManager::waylandSurfaceMapped(WindowSurface *surface)
{qint64 processId = surface->processId();const auto apps = ApplicationManager::instance()->fromProcessId(processId);Application *app = nullptr;if (apps.size() == 1) {app = apps.constFirst();} else if (apps.size() > 1) {// if there is more than one app within the same process, check the XDG surface appIdconst QString xdgAppId = surface->applicationId();if (!xdgAppId.isEmpty()) {for (const auto &checkApp : apps) {if (checkApp->id() == xdgAppId) {app = checkApp;break;}}}}// 只有开启了非安全模式，才可以接收非App侧创建的Surfaceif (!app && ApplicationManager::instance()->securityChecksEnabled()) {qCCritical(LogGraphics) << "SECURITY ALERT: an unknown application with pid" << processId<< "tried to map a Wayland surface!";return;}Q_ASSERT(surface);qCDebug(LogGraphics) << "Mapping Wayland surface" << surface << "of" << d->applicationId(app, surface);// Only create a new Window if we don't have it already in the window list, as the user controls// whether windows are removed or notint index = d->findWindowByWaylandSurface(surface->surface());if (index == -1) {WaylandWindow *w = new WaylandWindow(app, surface);// 这里会将Window绑定到List上setupWindow(w);}
}

• 为什么将Window（对应Surface）绑定到WindowItem上可以显示出Surface的内容？将Window赋值给WindowItem，实际上调用的是下面的函数。所以实际上，是Surface设置给了QWaylandQuickItem。

void WindowItem::WaylandImpl::setup(Window *window)
{Q_ASSERT(!m_waylandWindow);Q_ASSERT(window && !window->isInProcess());m_waylandWindow = static_cast<WaylandWindow*>(window);if (!m_waylandItem)createWaylandItem();m_waylandItem->setBufferLocked(false);m_waylandItem->setSurface(m_waylandWindow->surface());
}void WindowItem::WaylandImpl::createWaylandItem()
{m_waylandItem = new QWaylandQuickItem(q);connect(m_waylandItem, &QWaylandQuickItem::surfaceDestroyed, q, [this]() {// keep the buffer there to allow us to animate the window destructionm_waylandItem->setBufferLocked(true);});
}

• QWaylandQuickItem继承QQuickItem，并实现了updatePaintNode函数。继承QQuickItem的类，可以通过实现该虚函数来绘制自己想要的内容。这段代码比较长，大体上就是从Surface（也就是View）中拿出Buffer，做成纹理节点并返回。QT底层的RenderThread会将该节点绘制到屏幕上。

QSGNode *QWaylandQuickItem::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *data)
{Q_D(QWaylandQuickItem);d->lastMatrix = data->transformNode->combinedMatrix();const bool bufferHasContent = d->view->currentBuffer().hasContent();if (d->view->isBufferLocked() && d->paintEnabled)return oldNode;if (!bufferHasContent || !d->paintEnabled || !surface()) {delete oldNode;return nullptr;}QWaylandBufferRef ref = d->view->currentBuffer();const bool invertY = ref.origin() == QWaylandSurface::OriginBottomLeft;const QRectF rect = invertY ? QRectF(0, height(), width(), -height()): QRectF(0, 0, width(), height());if (ref.isSharedMemory()
#if QT_CONFIG(opengl)|| bufferTypes[ref.bufferFormatEgl()].canProvideTexture
#endif) {
#if QT_CONFIG(opengl)if (oldNode && !d->paintByProvider) {// Need to re-create a nodedelete oldNode;oldNode = nullptr;}d->paintByProvider = true;
#endif// This case could covered by the more general path below, but this is more efficient (especially when using ShaderEffect items).QSGSimpleTextureNode *node = static_cast<QSGSimpleTextureNode *>(oldNode);if (!node) {node = new QSGSimpleTextureNode();if (smooth())node->setFiltering(QSGTexture::Linear);d->newTexture = true;}if (!d->provider)d->provider = new QWaylandSurfaceTextureProvider();if (d->newTexture) {d->newTexture = false;d->provider->setBufferRef(this, ref);node->setTexture(d->provider->texture());}d->provider->setSmooth(smooth());node->setRect(rect);qreal scale = surface()->bufferScale();QRectF source = surface()->sourceGeometry();node->setSourceRect(QRectF(source.topLeft() * scale, source.size() * scale));return node;}#if QT_CONFIG(opengl)Q_ASSERT(!d->provider);if (oldNode && d->paintByProvider) {// Need to re-create a nodedelete oldNode;oldNode = nullptr;}d->paintByProvider = false;QSGGeometryNode *node = static_cast<QSGGeometryNode *>(oldNode);if (!node) {node = new QSGGeometryNode;d->newTexture = true;}QSGGeometry *geometry = node->geometry();QWaylandBufferMaterial *material = static_cast<QWaylandBufferMaterial *>(node->material());if (!geometry)geometry = new QSGGeometry(QSGGeometry::defaultAttributes_TexturedPoint2D(), 4);if (!material)material = new QWaylandBufferMaterial(ref.bufferFormatEgl());if (d->newTexture) {d->newTexture = false;material->setBufferRef(this, ref);}const QSize surfaceSize = ref.size() / surface()->bufferScale();const QRectF sourceGeometry = surface()->sourceGeometry();const QRectF normalizedCoordinates =sourceGeometry.isValid()? QRectF(sourceGeometry.x() / surfaceSize.width(),sourceGeometry.y() / surfaceSize.height(),sourceGeometry.width() / surfaceSize.width(),sourceGeometry.height() / surfaceSize.height()): QRectF(0, 0, 1, 1);QSGGeometry::updateTexturedRectGeometry(geometry, rect, normalizedCoordinates);node->setGeometry(geometry);node->setFlag(QSGNode::OwnsGeometry, true);node->setMaterial(material);node->setFlag(QSGNode::OwnsMaterial, true);return node;
#elseqCWarning(qLcWaylandCompositor) << "Without OpenGL support only shared memory textures are supported";return nullptr;
#endif // QT_CONFIG(opengl)
}