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/****************************************************************************
**
** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** GNU Free Documentation License
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file.
**
** Other Usage
** Alternatively, this file may be used in accordance with the terms
** and conditions contained in a signed written agreement between you
** and Nokia.
**
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page qtquick-visualcanvas-visualparent.html
\title Concepts - Visual Parent in Qt Quick
\brief Description of the concept of visual parent in Qt Quick
\section1 Visual Parent
The concept of visual parent in Qt Quick is separate but related to the concept of the QObject parent hierarchy. In QtQuick, the
default property of Item is \l Item::data which can be any QObject derived type.
Any object that is assigned to an item's data property becomes a child of the item within its QObject hierarchy, for
memory management purposes. Additionally, if an object added to the data property is of the Item type, it is also
assigned to the \l Item::children property and becomes a child of the item within the visual scene hierarchy.
(Most Qt Quick hierarchy crawling algorithms, especially the rendering algorithms, only consider the visual parent
hierarchy.)
When an item becomes the child of another item:
\list
\li The child's \l{Item::parent}{parent} refers to its parent item
\li The parent's \l{Item::children}{children} and \l{Item::childrenRect}{childrenRect} properties takes that
child into account
\endlist
An item's visual parent can be changed at any time by setting its \l {Item::}{parent} property.
Declaring an item as a child of another does not automatically mean that the child item will be appropriately
positioned or sized to fit within its parent. Some QML types may have in-built behaviors that affect the positioning
of child items — for example, a \l Row object automatically re-positions its children into a horizontal formation —
but these are behaviors enforced by the types' own specific implementations. Additionally, a parent item will not
automatically clip its children to visually contain them within the parent's visual bounds, unless its \l{Item::}{clip}
property is set to true.
The visual parent of an item may come under consideration in the following circumstances:
\section2 Item Coordinates
As item coordinates are relative to the visual parent, they can be affected by changes to the visual hierarchy. See
the \l{Visual Coordinates}{qtquick-visualcanvas-visualcoordinates.html} concept page for more detail.
\section2 Stacking Order
Qt Quick items use a recursive drawing algorithm for determining which items are drawn on top in case of collisions.
In general items are drawn on top of their parent items, in the order they were created (or specified in the QML file).
So in the following example, the blue rectangle will be drawn on top of the green rectangle:
\snippet qml/visualparent.qml 0
\image visual-parent-example.png
Because the algorithm recurses through the visual item hierarchy, any children of the green rectangle will also be drawn beneath
the blue rectangle and beneath any of the blue rectangle's children.
Stacking order can be influenced with the \Item::z property. Z values below 0 will stack below the parent, and if z
values are assigned then siblings will stack in z-order (with creation order used to break ties). Z values only affect
stacking compared to siblings and the parent item. If you have an item who is obscured by a subtree rooted above its
parent item, no z value on that item will increase its stacking order to stack above that subtree. To stack that item
above the other subtree you'll have to alter z values farther up in the hierarchy, or re-arrange the visual item
hierarchy.
\snippet qml/visualparent2.qml 0
\image visual-parent-example2.png
In the above example, the red rectangle has a high z value, but is still stacked below the blue rectangle. This is
because it is a child of the green rectangle, and the green rectangle is a sibling of the blue rectangle. The z value
of the green rectangle is lower than that of the blue rectangle, so the green rectangle and all children will be
stacked beneath the blue rectangle.
\section2 Canvas Ownership
The definition of what is rendered in a Qt Quick scene is the visual item tree rooted at QQuickWindow::contentItem.
Therefore to add an Item to a specific Qt Quick scene for rendering it needs to become a visual hierarchy child of an
Item already in the visual item hierarchy, such as QQuickWindow::contentItem.
*/
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