问题
I have a QGraphicsScene
which contains custom objects (with QGraphicsItem
as base class). I can retrieve these objects with:
foreach (QGraphicsItem* item, items()) {
if (item->type() == CustomItem::Type)
qgraphicsitem_cast<CustomItem*>(item).doStuff(...);
}
Is it possible to write a custom iterator
for just this type of objects? I want to be able to use range-based for-loops and algorithms from the STL.
for(CustomItem& a : custom_items())
a.doStuff(...);
I want to avoid storing pointers to these objects manually (for example in a vector) since this involves additional book-keeping when objects are deleted, which is error prone. How would such an iterator look like. How would I implement the custom_items()
method?
回答1:
The layout iterator adapter from this answer can be easily coerced to work on scene items:
// https://github.com/KubaO/stackoverflown/tree/master/questions/scene-iterator-adapter-32567814
#include <QGraphicsScene>
#include <QList>
#include <QDebug>
#include <utility>
template<class> class IterableSceneAdapter;
template<typename IT>
class ItemListIterator {
const QList<QGraphicsItem*> * m_items;
int m_index;
friend class IterableSceneAdapter<IT>;
ItemListIterator(const QList<QGraphicsItem*> * items, int dir) :
m_items(items), m_index(dir>0 ? -1 : items->count()) {
if (dir > 0) ++*this;
}
friend QDebug operator<<(QDebug dbg, const ItemListIterator & it) {
return dbg << (it.m_items->isEmpty() ? nullptr : it.m_items->first()->scene())
<< it.m_index;
}
friend void swap(ItemListIterator& a, ItemListIterator& b) {
using std::swap;
swap(a.m_items, b.m_items);
swap(a.m_index, b.m_index);
}
template <typename T> T item_cast(QGraphicsItem* item) {
// use qgraphicsitem_cast if you don't have RTTI enabled
return dynamic_cast<T>(item);
}
public:
ItemListIterator() : m_index(0) {}
ItemListIterator(const ItemListIterator & o) :
m_items(o.m_items), m_index(o.m_index) {}
ItemListIterator(ItemListIterator && o) { swap(*this, o); }
ItemListIterator & operator=(ItemListIterator o) {
swap(*this, o);
return *this;
}
IT * operator*() const { return static_cast<IT*>(m_items->at(m_index)); }
const ItemListIterator & operator++() {
while (++m_index < m_items->count() && !item_cast<IT*>(m_items->at(m_index)));
return *this;
}
ItemListIterator operator++(int) {
ItemListIterator temp{*this};
++*this;
return temp;
}
const ItemListIterator & operator--() {
while (!item_cast<IT*>(m_items->at(--m_index)) && m_index > 0);
return *this;
}
ItemListIterator operator--(int) {
ItemListIterator temp(*this);
--*this;
return temp;
}
bool operator==(const ItemListIterator & o) const { return m_index == o.m_index; }
bool operator!=(const ItemListIterator & o) const { return m_index != o.m_index; }
};
template <class IT = QGraphicsItem>
class IterableSceneAdapter {
const QList<QGraphicsItem*> m_items;
public:
typedef ItemListIterator<IT> iterator;
typedef iterator const_iterator;
IterableSceneAdapter(QGraphicsScene * scene) : m_items(scene->items()) {}
const_iterator begin() const { return const_iterator(&m_items, 1); }
const_iterator end() const { return const_iterator(&m_items, -1); }
const_iterator cbegin() const { return const_iterator(&m_items, 1); }
const_iterator cend() const { return const_iterator(&m_items, -1); }
};
template <class IT = QGraphicsItem>
class ConstIterableSceneAdapter : public IterableSceneAdapter<const IT> {
public:
ConstIterableSceneAdapter(const QGraphicsScene * scene) :
IterableSceneAdapter<const IT>(const_cast<QGraphicsScene*>(scene)) {}
};
It is used as follows:
int main(int argc, char ** argv) {
QApplication app(argc, argv);
tests();
QGraphicsScene s;
QGraphicsLineItem b1, b3;
QGraphicsEllipseItem b2;
s.addItem(&b1);
s.addItem(&b2);
s.addItem(&b3);
// Iterate all item types as constant items
qDebug() << "all, range-for";
for (auto item : ConstIterableSceneAdapter<>(&s)) qDebug() << item;
qDebug() << "all, Q_FOREACH";
Q_FOREACH (const QGraphicsItem * item, ConstIterableSceneAdapter<>(&s)) qDebug() << item;
// Iterate ellipses only
qDebug() << "ellipses, range-for";
for (auto item : IterableSceneAdapter<QGraphicsEllipseItem>(&s)) qDebug() << item;
qDebug() << "ellipses, Q_FOREACH";
Q_FOREACH (QGraphicsEllipseItem * item, IterableSceneAdapter<QGraphicsEllipseItem>(&s)) qDebug() << item;
}
Some rudimentary tests:
#include <QtWidgets>
void tests() {
QGraphicsScene s;
QGraphicsLineItem b1, b3;
QGraphicsEllipseItem b2;
IterableSceneAdapter<> s0{&s};
auto i0 = s0.begin();
qDebug() << i0; Q_ASSERT(i0 == s0.begin() && i0 == s0.end());
s.addItem(&b1);
s.addItem(&b2);
s.addItem(&b3);
IterableSceneAdapter<> s1{&s};
auto i1 = s1.begin();
qDebug() << i1; Q_ASSERT(i1 == s1.begin() && i1 != s1.end());
++i1; qDebug() << i1; Q_ASSERT(i1 != s1.begin() && i1 != s1.end());
++i1; qDebug() << i1; Q_ASSERT(i1 != s1.begin() && i1 != s1.end());
++i1; qDebug() << i1; Q_ASSERT(i1 != s1.begin() && i1 == s1.end());
--i1; qDebug() << i1; Q_ASSERT(i1 != s1.begin() && i1 != s1.end());
--i1; qDebug() << i1; Q_ASSERT(i1 != s1.begin() && i1 != s1.end());
--i1; qDebug() << i1; Q_ASSERT(i1 == s1.begin() && i1 != s1.end());
IterableSceneAdapter<QGraphicsEllipseItem> s2{&s};
auto i2 = s2.begin();
qDebug() << i2; Q_ASSERT(i2 == s2.begin() && i2 != s2.end());
++i2; qDebug() << i2; Q_ASSERT(i2 != s2.begin() && i2 == s2.end());
--i2; qDebug() << i2; Q_ASSERT(i2 == s2.begin() && i2 != s2.end());
}
来源:https://stackoverflow.com/questions/32567814/iterator-for-elements-in-qgraphicsscene