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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 take 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.
Visual parent comes up most often in the following three 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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