Implement lazy tree traversing

The implementation is based on coroutines

src/qrangecoro_p.h

+#ifndef QRANGEFALLBACK_H
+#define QRANGEFALLBACK_H
+
+#include <range/v3/view_facade.hpp>
+
+#include <cppcoro/generator.hpp>
+#include <cppcoro/recursive_generator.hpp>
+
+#include <QtCore/QObject>
+
+namespace qt::detail
+{
+    cppcoro::recursive_generator<QObject*> takeChildRecursivelyImpl(
+        const QObjectList &children, Qt::FindChildOptions opts)
+    {
+        for (QObject *c : children) {
+            if (opts == Qt::FindChildrenRecursively) {
+                co_yield takeChildRecursivelyImpl(c->children(), opts);
+            }
+            co_yield c;
+        }
+    }
+
+    cppcoro::recursive_generator<QObject*> takeChildRecursively(
+        QObject *root, Qt::FindChildOptions opts = Qt::FindChildrenRecursively)
+    {
+        if (root) {
+            co_yield takeChildRecursivelyImpl(root->children(), opts);
+        }
+    }
+
+    template <class T = QObject>
+    class children_view : public ranges::view_facade<children_view<T>>
+    {
+        friend ranges::range_access;
+
+        struct Data {
+            T *obj;
+            cppcoro::recursive_generator<QObject*> gen;
+        };
+        std::shared_ptr<Data> m_data;
+
+        struct cursor;
+
+        cursor begin_cursor()
+        {
+            return cursor(m_data->gen.begin());
+        }
+
+    public:
+        children_view() = default;
+
+        explicit children_view(T *o, Qt::FindChildOptions opts = Qt::FindChildrenRecursively)
+            : m_data(std::make_shared<Data>(Data{o, takeChildRecursively(o, opts)}))
+        {}
+
+        explicit children_view(T &o, Qt::FindChildOptions opts = Qt::FindChildrenRecursively)
+             : children_view(&o, opts)
+        {}
+    };
+
+    template <class T>
+    struct children_view<T>::cursor
+    {
+        cppcoro::recursive_generator<QObject*>::iterator it;
+
+        decltype(auto) read() const
+        {
+            return *it;
+        }
+
+        void next()
+        {
+            ++it;
+        }
+
+        auto equal(ranges::default_sentinel_t) const
+        {
+            return it == cppcoro::recursive_generator<QObject*>::iterator(nullptr);
+        }
+
+        explicit cursor(cppcoro::recursive_generator<QObject*>::iterator it)
+                : it(it)
+        {}
+
+        cursor() = default;
+    };
+
+} // qt::detail
+
+#endif // QRANGEFALLBACK_H