BoxConstraints class

Immutable layout constraints for RenderBox layout.

A Size respects a BoxConstraints if, and only if, all of the following relations hold:

• minWidth <= Size.width <= maxWidth
• minHeight <= Size.height <= maxHeight

The constraints themselves must satisfy these relations:

• 0.0 <= minWidth <= maxWidth <= double.infinity
• 0.0 <= minHeight <= maxHeight <= double.infinity

double.infinity is a legal value for each constraint.

The box layout model

Render objects in the Flutter framework are laid out by a one-pass layout model which walks down the render tree passing constraints, then walks back up the render tree passing concrete geometry.

For boxes, the constraints are BoxConstraints, which, as described herein, consist of four numbers: a minimum width minWidth, a maximum width maxWidth, a minimum height minHeight, and a maximum height maxHeight.

The geometry for boxes consists of a Size, which must satisfy the constraints described above.

Each RenderBox (the objects that provide the layout models for box widgets) receives BoxConstraints from its parent, then lays out each of its children, then picks a Size that satisfies the BoxConstraints.

Render objects position their children independently of laying them out. Frequently, the parent will use the children's sizes to determine their position. A child does not know its position and will not necessarily be laid out again, or repainted, if its position changes.

Terminology

When the minimum constraints and the maximum constraint in an axis are the same, that axis is tightly constrained. See: BoxConstraints.tightFor, BoxConstraints.tightForFinite, tighten, hasTightWidth, hasTightHeight, isTight.

An axis with a minimum constraint of 0.0 is loose (regardless of the maximum constraint; if it is also 0.0, then the axis is simultaneously tight and loose!). See: BoxConstraints.loose, loosen.

An axis whose maximum constraint is not infinite is bounded. See: hasBoundedWidth, hasBoundedHeight.

An axis whose maximum constraint is infinite is unbounded. An axis is expanding if it is tightly infinite (its minimum and maximum constraints are both infinite). See: BoxConstraints.expand.

An axis whose minimum constraint is infinite is just said to be infinite (since by definition the maximum constraint must also be infinite in that case). See: hasInfiniteWidth, hasInfiniteHeight.

A size is constrained when it satisfies a BoxConstraints description. See: constrain, constrainWidth, constrainHeight, constrainDimensions, constrainSizeAndAttemptToPreserveAspectRatio, isSatisfiedBy.