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29 \page qml-anchor-layout.html
30 \inqmlmodule QtQuick 2
32 \contentspage QML Features
33 \previouspage {Using QML Positioner and Repeater Items}{Component Layouts}
34 \nextpage {QML Mouse Events}{Mouse Events}
35 \title Anchor-based Layout in QML
37 In addition to the more traditional \l Grid, \l Row, and \l Column,
38 QML also provides a way to layout items using the concept of \i anchors.
39 Each item can be thought of as having a set of 7 invisible "anchor lines":
40 \l {Item::anchors.left}{left}, \l {Item::anchors.horizontalCenter}{horizontalCenter},
41 \l {Item::anchors.right}{right}, \l {Item::anchors.top}{top},
42 \l {Item::anchors.verticalCenter}{verticalCenter}, \l {Item::anchors.baseline}{baseline},
43 and \l {Item::anchors.bottom}{bottom}.
47 The baseline (not pictured above) corresponds to the imaginary line on which
48 text would sit. For items with no text it is the same as \i top.
50 The QML anchoring system allows you to define relationships between the anchor lines of different items. For example, you can write:
53 Rectangle { id: rect1; ... }
54 Rectangle { id: rect2; anchors.left: rect1.right; ... }
57 In this case, the left edge of \i rect2 is bound to the right edge of \i rect1, producing the following:
62 You can specify multiple anchors. For example:
65 Rectangle { id: rect1; ... }
66 Rectangle { id: rect2; anchors.left: rect1.right; anchors.top: rect1.bottom; ... }
71 By specifying multiple horizontal or vertical anchors you can control the size of an item. Below,
72 \i rect2 is anchored to the right of \i rect1 and the left of \i rect3. If either of the blue
73 rectangles are moved, \i rect2 will stretch and shrink as necessary:
76 Rectangle { id: rect1; x: 0; ... }
77 Rectangle { id: rect2; anchors.left: rect1.right; anchors.right: rect3.left; ... }
78 Rectangle { id: rect3; x: 150; ... }
83 There are also some convenience anchors. anchors.fill is a convenience that is the same as setting the left,right,top and bottom anchors
84 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
85 and horizontalCenter anchors to the verticalCenter and horizontalCenter of the target item.
87 \section1 Anchor Margins and Offsets
89 The anchoring system also allows \i margins and \i offsets to be specified for an item's anchors.
90 Margins specify the amount of empty space to leave to the outside of an item's anchor, while
91 offsets allow positioning to be manipulated using the center anchor lines. An item can
92 specify its anchor margins individually through \l {Item::anchors.leftMargin}{leftMargin},
93 \l {Item::anchors.rightMargin}{rightMargin}, \l {Item::anchors.topMargin}{topMargin} and
94 \l {Item::anchors.bottomMargin}{bottomMargin}, or use \l {Item::}{anchors.margins} to
95 specify the same margin value for all four edges. Anchor offsets are specified using
96 \l {Item::anchors.horizontalCenterOffset}{horizontalCenterOffset},
97 \l {Item::anchors.verticalCenterOffset}{verticalCenterOffset} and
98 \l {Item::anchors.baselineOffset}{baselineOffset}.
100 \image margins_qml.png
102 The following example specifies a left margin:
105 Rectangle { id: rect1; ... }
106 Rectangle { id: rect2; anchors.left: rect1.right; anchors.leftMargin: 5; ... }
109 In this case, a margin of 5 pixels is reserved to the left of \i rect2, producing the following:
113 \note Anchor margins only apply to anchors; they are \i not a generic means of applying margins to an \l Item.
114 If an anchor margin is specified for an edge but the item is not anchored to any item on that
115 edge, the margin is not applied.
118 \section1 Restrictions
120 For performance reasons, you can only anchor an item to its siblings and direct parent. For example,
121 the following anchor is invalid and would produce a warning:
126 Rectangle { id: rect1; ... }
130 Rectangle { id: rect2; anchors.left: rect1.right; ... } // invalid anchor!
134 Also, anchor-based layouts cannot be mixed with absolute positioning. If an item specifies its
135 \l {Item::}{x} position and also sets \l {Item::}{anchors.left},
136 or anchors its left and right edges but additionally sets a \l {Item::}{width}, the
137 result is undefined, as it would not be clear whether the item should use anchoring or absolute
138 positioning. The same can be said for setting an item's \l {Item::}{y} and \l {Item::}{height}
139 with \l {Item::}{anchors.top} and \l {Item::}{anchors.bottom}, or setting \l {Item::}{anchors.fill}
140 as well as \l {Item::}{width} or \l {Item::}{height}. The same applies when using positioners
141 such as Row and Grid, which may set the item's \l {Item::}{x} and \l {Item::}{y} properties.
142 If you wish to change from using
143 anchor-based to absolute positioning, you can clear an anchor value by setting it to \c undefined.