Add small documentation tidbits for screen grabbers

Some explanation for the abstract class and various implementations that
should help other people in the future.

Author: redstrate

plugins/quickinspector/quickscreengrabber.cpp

@@ -797,9 +797,7 @@ UnsupportedScreenGrabber::UnsupportedScreenGrabber(QQuickWindow *window)
 {
 }
 
-UnsupportedScreenGrabber::~UnsupportedScreenGrabber()
-{
-}
+UnsupportedScreenGrabber::~UnsupportedScreenGrabber() = default;
 
 void UnsupportedScreenGrabber::requestGrabWindow(const QRectF & /*userViewport*/)
 {
@@ -832,8 +830,10 @@ void UnsupportedScreenGrabber::requestGrabWindow(const QRectF & /*userViewport*/
 
 void UnsupportedScreenGrabber::drawDecorations()
 {
+    // Intentionally dummied out because this grabber doesn't do anything useful
 }
 
 void UnsupportedScreenGrabber::updateOverlay()
 {
+    // Intentionally dummied out because this grabber doesn't do anything useful
 }

plugins/quickinspector/quickscreengrabber.h

@@ -94,6 +94,9 @@ class GrabbedFrame
     QVector<QuickItemGeometry> itemsGeometry;
 };
 
+/**
+ * @brief Base for all screen grabbers that decorate and return a pixmap for the window.
+ */
 class AbstractScreenGrabber : public QObject
 {
     Q_OBJECT
@@ -127,7 +130,13 @@ class AbstractScreenGrabber : public QObject
     explicit AbstractScreenGrabber(QQuickWindow *window);
     ~AbstractScreenGrabber() override;
 
+    /**
+     * @return The graphics API used to render @p window.
+     */
     static RenderInfo::GraphicsApi graphicsApiFor(QQuickWindow *window);
+    /**
+     * @return Screen grabber compatible with the given @p window.
+     */
     static std::unique_ptr<AbstractScreenGrabber> get(QQuickWindow *window);
 
     QQuickWindow *window() const;
@@ -156,6 +165,9 @@ class AbstractScreenGrabber : public QObject
     void doDrawDecorations(QPainter &painter);
     void gatherRenderInfo();
 
+    /**
+     * @brief Draw decorations on the target window.
+     */
     virtual void drawDecorations() = 0;
 
     virtual void updateOverlay();
@@ -201,6 +213,9 @@ class OpenGLScreenGrabber : public AbstractScreenGrabber
 };
 #endif
 
+/**
+ * @brief Screen grabber meant for software rendering.
+ */
 class SoftwareScreenGrabber : public AbstractScreenGrabber
 {
     Q_OBJECT
@@ -222,6 +237,9 @@ class SoftwareScreenGrabber : public AbstractScreenGrabber
     QPointF m_lastItemPosition;
 };
 
+/**
+ * @brief Screen grabber used for unsupported rendering backends. Doesn't do anything useful otherwise.
+ */
 class UnsupportedScreenGrabber : public AbstractScreenGrabber
 {
     Q_OBJECT