Anchor-based Layout in QML

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

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 top.

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

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

Each Item has two properties for each anchor line: one to bind from and one to bind to. The properties to bind from are contained in the anchors attached property (seen as anchors.left above). The properties to bind to are normal properties (seen as rect1.right above). This way, each item can have several bindings to the same anchor line. Note that the properties to bind to are not visible in the documentation for Item. So in the example above, the left edge of rect2 is bound to the right edge of rect1, producing the following:

You can specify multiple anchors. For example:

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

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

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

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.

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

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