A layout that places its children in a rectangular grid.
The grid is composed of a set of infinitely thin lines that separate the
viewing area into cells. Throughout the API, grid lines are referenced
by grid indices. A grid with
N + 1 grid indices that run from
N inclusive. Regardless of how GridLayout is
configured, grid index
0 is fixed to the leading edge of the
container and grid index
N is fixed to its trailing edge
(after padding is taken into account).
Row and Column SpecsChildren occupy one or more contiguous cells, as defined by their
columnSpeclayout parameters. Each spec defines the set of rows or columns that are to be occupied; and how children should be aligned within the resulting group of cells. Although cells do not normally overlap in a GridLayout, GridLayout does not prevent children being defined to occupy the same cell or group of cells. In this case however, there is no guarantee that children will not themselves overlap after the layout operation completes.
Default Cell AssignmentIf a child does not specify the row and column indices of the cell it wishes to occupy, GridLayout assigns cell locations automatically using its:
SpaceSpace between children may be specified either by using instances of the dedicated
Spaceview or by setting the
bottomMarginlayout parameters. When the
useDefaultMarginsproperty is set, default margins around children are automatically allocated based on the prevailing UI style guide for the platform. Each of the margins so defined may be independently overridden by an assignment to the appropriate layout parameter. Default values will generally produce a reasonable spacing between components but values may change between different releases of the platform.
Excess Space DistributionGridLayout's distribution of excess space accommodates the principle of weight. In the event that no weights are specified, columns and rows are taken as flexible if their views specify some form of alignment within their groups.
The flexibility of a view is therefore influenced by its alignment which is,
in turn, typically defined by setting the
gravity property of the child's layout parameters.
If either a weight or alignment were defined along a given axis then the component
is taken as flexible in that direction. If no weight or alignment was set,
the component is instead assumed to be inflexible.
Multiple components in the same row or column group are considered to act in parallel. Such a group is flexible only if all of the components within it are flexible. Row and column groups that sit either side of a common boundary are instead considered to act in series. The composite group made of these two elements is flexible if one of its elements is flexible.
To make a column stretch, make sure all of the components inside it define a weight or a gravity. To prevent a column from stretching, ensure that one of the components in the column does not define a weight or a gravity.
When the principle of flexibility does not provide complete disambiguation, GridLayout's algorithms favour rows and columns that are closer to its right and bottom edges. To be more precise, GridLayout treats each of its layout parameters as a constraint in the a set of variables that define the grid-lines along a given axis. During layout, GridLayout solves the constraints so as to return the unique solution to those constraints for which all variables are less-than-or-equal-to the corresponding value in any other valid solution.
Interpretation of GONEFor layout purposes, GridLayout treats views whose visibility status is
GONE, as having zero width and height. This is subtly different from the policy of ignoring views that are marked as GONE outright. If, for example, a gone-marked view was alone in a column, that column would itself collapse to zero width if and only if no gravity was defined on the view. If gravity was defined, then the gone-marked view has no effect on the layout and the container should be laid out as if the view had never been added to it. GONE views are taken to have zero weight during excess space distribution.
These statements apply equally to rows as well as columns, and to groups of rows or columns.