Method mount

modules/gui/gui/src/framework/layer/layer.cpp:8–44

lifecycle & tree ctor -> mount <-> unmount -> destroy

// lifecycle & tree
// ctor -> mount <-> unmount -> destroy
void Layer::mount(NotNull<Layer*> parent) SKR_NOEXCEPT
{
    // validate
    if (_parent != nullptr)
    {
        unmount();
        SKR_GUI_LOG_ERROR(u8"already mounted");
    }
    {
        Layer* node = parent;
        while (node->_parent)
        {
            node = node->_parent;
            if (node == this)
            {
                SKR_GUI_LOG_ERROR(u8"cycle in the tree");
                break;
            }
        }
    }

    // mount
    _parent = parent;
    if (parent->owner())
    {
        struct _RecursiveHelper {
            NotNull<BuildOwner*> owner;

            void operator()(NotNull<Layer*> obj) const SKR_NOEXCEPT
            {
                obj->attach(owner);
                obj->visit_children(_RecursiveHelper{ owner });
            }
        };
        _RecursiveHelper{ parent->owner() }(this);
    }
}
void Layer::unmount() SKR_NOEXCEPT
{
    // validate