source: trunk/doc/src/declarative/anchor-layout.qdoc@ 846

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/*!
\page qml-anchor-layout.html
\target anchor-layout
\title Anchor-based Layout in QML

\section1 Overview

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 \e 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 \e 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 \e rect2 is bound to the right edge of \e 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,
\e rect2 is anchored to the right of \e rect1 and the left of \e rect3. If either of the blue
rectangles are moved, \e 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


\section1 Anchor Margins and Offsets

The anchoring system also allows \e margins and \e 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 \e rect2, producing the following:

\image edge2.png

\note Anchor margins only apply to anchors; they are \e 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}. If you wish to change from using
anchor-based to absolute positioning, you can clear an anchor value by setting it to \c undefined.

*/