anchor-layout.qdoc

/****************************************************************************
**
** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** 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 qml-anchor-layout.html
\inqmlmodule QtQuick 2
\target anchor-layout
\contentspage QML Features
\previouspage {Using QML Positioner and Repeater Items}{Component Layouts}
\nextpage {QML Mouse Events}{Mouse Events}
\title Anchor-based Layout in QML

In addition to the more traditional \l Grid, \l Row, and \l Column,
QML also provides a way to layout items using the concept of \i anchors.
Each item can be thought of as having a set of 7 invisible "anchor lines":
\l {Item::anchors.left}{left}, \l {Item::anchors.horizontalCenter}{horizontalCenter},
\l {Item::anchors.right}{right}, \l {Item::anchors.top}{top},
\l {Item::anchors.verticalCenter}{verticalCenter}, \l {Item::anchors.baseline}{baseline},
and \l {Item::anchors.bottom}{bottom}.

\image edges_qml.png

The baseline (not pictured above) corresponds to the imaginary line on which
text would sit. For items with no text it is the same as \i top.

The QML anchoring system allows you to define relationships between the anchor lines of different items. For example, you can write:

\code
Rectangle { id: rect1; ... }
Rectangle { id: rect2; anchors.left: rect1.right; ... }
\endcode

In this case, the left edge of \i rect2 is bound to the right edge of \i rect1, producing the following:

\image edge1.png


You can specify multiple anchors. For example:

\code
Rectangle { id: rect1; ... }
Rectangle { id: rect2; anchors.left: rect1.right; anchors.top: rect1.bottom; ... }
\endcode

\image edge3.png

By specifying multiple horizontal or vertical anchors you can control the size of an item. Below,
\i rect2 is anchored to the right of \i rect1 and the left of \i rect3. If either of the blue
rectangles are moved, \i rect2 will stretch and shrink as necessary:

\code
Rectangle { id: rect1; x: 0; ... }
Rectangle { id: rect2; anchors.left: rect1.right; anchors.right: rect3.left; ... }
Rectangle { id: rect3; x: 150; ... }
\endcode

\image edge4.png

There are also some convenience anchors. anchors.fill is a convenience that is the same as setting the left,right,top and bottom anchors
to the left,right,top and bottom of the target item. anchors.centerIn is another convenience anchor, and is the same as setting the verticalCenter
and horizontalCenter anchors to the verticalCenter and horizontalCenter of the target item.

\section1 Anchor Margins and Offsets

The anchoring system also allows \i margins and \i offsets to be specified for an item's anchors.
Margins specify the amount of empty space to leave to the outside of an item's anchor, while
offsets allow positioning to be manipulated using the center anchor lines.  An item can
specify its anchor margins individually through \l {Item::anchors.leftMargin}{leftMargin},
\l {Item::anchors.rightMargin}{rightMargin}, \l {Item::anchors.topMargin}{topMargin} and
\l {Item::anchors.bottomMargin}{bottomMargin}, or use \l {Item::}{anchors.margins} to
specify the same margin value for all four edges. Anchor offsets are specified using
\l {Item::anchors.horizontalCenterOffset}{horizontalCenterOffset},
\l {Item::anchors.verticalCenterOffset}{verticalCenterOffset} and
\l {Item::anchors.baselineOffset}{baselineOffset}.

\image margins_qml.png

The following example specifies a left margin:

\code
Rectangle { id: rect1; ... }
Rectangle { id: rect2; anchors.left: rect1.right; anchors.leftMargin: 5; ... }
\endcode

In this case, a margin of 5 pixels is reserved to the left of \i rect2, producing the following:

\image edge2.png

\note Anchor margins only apply to anchors; they are \i not a generic means of applying margins to an \l Item.
If an anchor margin is specified for an edge but the item is not anchored to any item on that
edge, the margin is not applied.


\section1 Restrictions

For performance reasons, you can only anchor an item to its siblings and direct parent. For example,
the following anchor is invalid and would produce a warning:

\badcode
Item {
    id: group1
    Rectangle { id: rect1; ... }
}
Item {
    id: group2
    Rectangle { id: rect2; anchors.left: rect1.right; ... }    // invalid anchor!
}
\endcode

Also, anchor-based layouts cannot be mixed with absolute positioning. If an item specifies its
\l {Item::}{x} position and also sets \l {Item::}{anchors.left},
or anchors its left and right edges but additionally sets a \l {Item::}{width}, the
result is undefined, as it would not be clear whether the item should use anchoring or absolute
positioning. The same can be said for setting an item's \l {Item::}{y} and \l {Item::}{height}
with \l {Item::}{anchors.top} and \l {Item::}{anchors.bottom}, or setting \l {Item::}{anchors.fill}
as well as \l {Item::}{width} or \l {Item::}{height}. The same applies when using positioners
such as Row and Grid, which may set the item's \l {Item::}{x} and \l {Item::}{y} properties.
If you wish to change from using
anchor-based to absolute positioning, you can clear an anchor value by setting it to \c undefined.

*/