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