statistics: draw legend as a QSGNode

In order not to waste CPU by constantly rerendering the chart,
we must use these weird OpenGL QSGNode things. The interface
is appallingly low-level and unfriendly.

As a first test, try to convert the legend. Create a wrapper
class that represents a rectangular item with a texture
and that will certainly need some (lots of) optimization.

Make sure that all low-level QSG-objects are only accessed
in the rendering thread. This means that the wrapper has
to maintain a notion of "dirtiness" of the state. I.e.
which part of the QSG-objects have to be modified.

From the low-level wrapper derive a class that draws a rounded
rectangle for every resize. The child class of that must then
paint on the rectangle after every resize.

That looks all not very fortunate, but it displays a
legend and will make it possible to move the legend
without and drawing operations, only shifting around
an OpenGL surface.

The render thread goes through all chart-items and
rerenders them if dirty. Currently, on deletion
of these items, this list is not reset. I.e. currently
it is not supported to remove individual items.
Only the full scene can be cleared!

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

1 files changed, 64 insertions, 23 deletions

diff --git a/stats/statsview.cpp b/stats/statsview.cpp
index e5e6b0b0d..61510d60c 100644
--- a/stats/statsview.cpp
+++ b/stats/statsview.cpp
@@ -25,13 +25,48 @@
 #include <QSGImageNode>
 #include <QSGTexture>
 
+// Constants that control the graph layouts
+static const QColor quartileMarkerColor(Qt::red);
+static const double quartileMarkerSize = 15.0;
+static const double sceneBorder = 5.0;			// Border between scene edges and statitistics view
+static const double titleBorder = 2.0;			// Border between title and chart
+
+StatsView::StatsView(QQuickItem *parent) : QQuickItem(parent),
+	highlightedSeries(nullptr),
+	xAxis(nullptr),
+	yAxis(nullptr),
+	rootNode(nullptr)
+{
+	setFlag(ItemHasContents, true);
+
+	connect(&diveListNotifier, &DiveListNotifier::numShownChanged, this, &StatsView::replotIfVisible);
+
+	setAcceptHoverEvents(true);
+
+	QFont font;
+	titleFont = QFont(font.family(), font.pointSize(), QFont::Light);	// Make configurable
+}
+
+StatsView::StatsView() : StatsView(nullptr)
+{
+}
+
+StatsView::~StatsView()
+{
+}
+
 QSGNode *StatsView::updatePaintNode(QSGNode *oldNode, QQuickItem::UpdatePaintNodeData *)
 {
 	// The QtQuick drawing interface is utterly bizzare with a distinct 1980ies-style memory management.
 	// This is just a copy of what is found in Qt's documentation.
 	QSGImageNode *n = static_cast<QSGImageNode *>(oldNode);
 	if (!n)
-		n = window()->createImageNode();
+		n = rootNode = window()->createImageNode();
+
+	for (ChartItem *item: items) {
+		if (item->dirty)
+			item->render();
+	}
 
 	QRectF rect = boundingRect();
 	if (plotRect != rect) {
@@ -47,33 +82,30 @@ QSGNode *StatsView::updatePaintNode(QSGNode *oldNode, QQuickItem::UpdatePaintNod
 	return n;
 }
 
-// Constants that control the graph layouts
-static const QColor quartileMarkerColor(Qt::red);
-static const double quartileMarkerSize = 15.0;
-static const double sceneBorder = 5.0;			// Border between scene edges and statitistics view
-static const double titleBorder = 2.0;			// Border between title and chart
-
-StatsView::StatsView(QQuickItem *parent) : QQuickItem(parent),
-	highlightedSeries(nullptr),
-	xAxis(nullptr),
-	yAxis(nullptr)
+void StatsView::addQSGNode(QSGNode *node, int)
 {
-	setFlag(ItemHasContents, true);
-
-	connect(&diveListNotifier, &DiveListNotifier::numShownChanged, this, &StatsView::replotIfVisible);
-
-	setAcceptHoverEvents(true);
+	rootNode->appendChildNode(node);
+}
 
-	QFont font;
-	titleFont = QFont(font.family(), font.pointSize(), QFont::Light);	// Make configurable
+// Currently this does an inefficient linear search in the chart-item vector.
+// The reason is that removing individual chart items is very rare: for now,
+// it is only done when hiding an InfoBox. In the future, this might have to
+// be improved.
+void StatsView::unregisterChartItem(const ChartItem *item)
+{
+	auto it = std::find(items.begin(), items.end(), item);
+	if (it != items.end())
+		items.erase(it);
 }
 
-StatsView::StatsView() : StatsView(nullptr)
+QQuickWindow *StatsView::w() const
 {
+	return window();
 }
 
-StatsView::~StatsView()
+QSizeF StatsView::size() const
 {
+	return boundingRect().size();
 }
 
 void StatsView::plotAreaChanged(const QSizeF &s)
@@ -202,6 +234,14 @@ T *StatsView::createAxis(const QString &title, Args&&... args)
 	return res;
 }
 
+template <typename T, class... Args>
+std::unique_ptr<T> StatsView::createChartItem(Args&&... args)
+{
+	std::unique_ptr<T> res(new T(*this, std::forward<Args>(args)...));
+	items.push_back(res.get());
+	return res;
+}
+
 void StatsView::setAxes(StatsAxis *x, StatsAxis *y)
 {
 	xAxis = x;
@@ -214,6 +254,7 @@ void StatsView::reset()
 {
 	highlightedSeries = nullptr;
 	xAxis = yAxis = nullptr;
+	items.clear(); // non-owning pointers
 	legend.reset();
 	series.clear();
 	quartileMarkers.clear();
@@ -406,7 +447,7 @@ void StatsView::plotBarChart(const std::vector<dive *> &dives,
 
 	// Paint legend first, because the bin-names will be moved away from.
 	if (showLegend)
-		legend = createItemPtr<Legend>(&scene, data.vbinNames);
+		legend = createChartItem<Legend>(data.vbinNames);
 
 	std::vector<BarSeries::MultiItem> items;
 	items.reserve(data.hbin_counts.size());
@@ -623,7 +664,7 @@ void StatsView::plotPieChart(const std::vector<dive *> &dives,
 	PieSeries *series = createSeries<PieSeries>(categoryVariable->name(), data, keepOrder, labels);
 
 	if (showLegend)
-		legend = createItemPtr<Legend>(&scene, series->binNames());
+		legend = createChartItem<Legend>(series->binNames());
 }
 
 void StatsView::plotDiscreteBoxChart(const std::vector<dive *> &dives,
@@ -959,7 +1000,7 @@ void StatsView::plotHistogramStackedChart(const std::vector<dive *> &dives,
 
 	BarPlotData data(categoryBins, *valueBinner);
 	if (showLegend)
-		legend = createItemPtr<Legend>(&scene, data.vbinNames);
+		legend = createChartItem<Legend>(data.vbinNames);
 
 	CountAxis *valAxis = createCountAxis(data.maxCategoryCount, isHorizontal);