subsurface/stats/statshelper.h

// SPDX-License-Identifier: GPL-2.0
// Helper functions to render the stats. Includes
// QSGNode template jugglery to overcome API flaws.
#ifndef STATSHELPER_H
#define STATSHELPER_H

#include <memory>
#include <QPointF>
#include <QSGNode>

// Round positions to integer values to avoid ugly artifacts
QPointF roundPos(const QPointF &p);

// A stupid pointer class that initializes to null and can be copy
// assigned. This is for historical reasons: unique_ptrs to ChartItems
// were replaced by plain pointers. Instead of nulling the plain pointers
// in the constructors, use this. Ultimately, we might think about making
// this thing smarter, once removal of individual ChartItems is implemented.
template <typename T>
class ChartItemPtr {
	friend class StatsView; // Only the stats view can create these pointers
	T *ptr;
	ChartItemPtr(T *ptr) : ptr(ptr)
	{
	}
public:
	ChartItemPtr() : ptr(nullptr)
	{
	}
	ChartItemPtr(const ChartItemPtr &p) : ptr(p.ptr)
	{
	}
	void reset()
	{
		ptr = nullptr;
	}
	ChartItemPtr &operator=(const ChartItemPtr &p)
	{
		ptr = p.ptr;
		return *this;
	}
	operator bool() const
	{
		return !!ptr;
	}
	bool operator!() const
	{
		return !ptr;
	}
	T &operator*() const
	{
		return *ptr;
	}
	T *operator->() const
	{
		return ptr;
	}
};

// In general, we want chart items to be hideable. For example to show/hide
// labels on demand. Very sadly, the QSG API is absolutely terrible with
// respect to temporarily disabling. Instead of simply having a flag,
// a QSGNode is queried using the "isSubtreeBlocked()" virtual function(!).
//
// Not only is this a slow operation performed on every single node, it
// also is often not possible to override this function: For improved
// performance, the documentation recommends to create QSG nodes via
// QQuickWindow. This provides nodes optimized for the actual hardware.
// However, this obviously means that these nodes cannot be derived from!
//
// In that case, there are two possibilities: Add a proxy node with an
// overridden "isSubtreeBlocked()" function or remove the node from the
// scene. The former was chosen here, because it is less complex.
//
// The following slightly cryptic templates are used to unify the two
// cases: The QSGNode is generated by our own code or the QSGNode is
// obtained from QQuickWindow.
//
// The "HideableQSGNode<Node>" template augments the QSGNode "Node"
// by a "setVisible()" function and overrides "isSubtreeBlocked()"
//
// The "QSGProxyNode<Node>" template is a QSGNode with a single
// child of type "Node".
//
// Thus, if the node can be created, use:
//	HideableQSGNode<NodeTypeThatCanBeCreated> node
// and if the node can only be obtained from QQuickWindow, use:
//	HideableQSGNode<QSGProxyNode<NodeThatCantBeCreated>> node
// The latter should obviously be typedef-ed.
//
// Yes, that's all horrible, but if nothing else it teaches us about
// composition.
template <typename Node>
class HideableQSGNode : public Node {
	bool hidden;
	bool isSubtreeBlocked() const override final;
public:
	template<class... Args>
	HideableQSGNode(bool visible, Args&&... args);
	void setVisible(bool visible);
};

template <typename Node>
class QSGProxyNode : public QSGNode {
public:
	std::unique_ptr<Node> node;
	QSGProxyNode(Node *node);
};

// Implementation detail of templates - move to serparate header file
template <typename Node>
QSGProxyNode<Node>::QSGProxyNode(Node *node) : node(node)
{
	appendChildNode(node);
}

template <typename Node>
bool HideableQSGNode<Node>::isSubtreeBlocked() const
{
	return hidden;
}

template <typename Node>
template<class... Args>
HideableQSGNode<Node>::HideableQSGNode(bool visible, Args&&... args) :
	Node(std::forward<Args>(args)...),
	hidden(!visible)
{
}

template <typename Node>
void HideableQSGNode<Node>::setVisible(bool visible)
{
	hidden = !visible;
	Node::markDirty(QSGNode::DirtySubtreeBlocked);
}

#endif