Constraints class abstract

An abstract set of layout constraints.

Concrete layout models (such as box) will create concrete subclasses to communicate layout constraints between parents and children.

Writing a Constraints subclass

When creating a new RenderObject subclass with a new layout protocol, one will usually need to create a new Constraints subclass to express the input to the layout algorithms.

A Constraints subclass should be immutable (all fields final). There are several members to implement, in addition to whatever fields, constructors, and helper methods one may find useful for a particular layout protocol:

• The isTight getter, which should return true if the object represents a case where the RenderObject class has no choice for how to lay itself out. For example, BoxConstraints returns true for isTight when both the minimum and maximum widths and the minimum and maximum heights are equal.

• The isNormalized getter, which should return true if the object represents its data in its canonical form. Sometimes, it is possible for fields to be redundant with each other, such that several different representations have the same implications. For example, a BoxConstraints instance with its minimum width greater than its maximum width is equivalent to one where the maximum width is set to that minimum width (2<w<1 is equivalent to 2<w<2, since minimum constraints have priority). This getter is used by the default implementation of debugAssertIsValid.

• The debugAssertIsValid method, which should assert if there's anything wrong with the constraints object. (We use this approach rather than asserting in constructors so that our constructors can be const and so that it is possible to create invalid constraints temporarily while building valid ones.) See the implementation of BoxConstraints.debugAssertIsValid for an example of the detailed checks that can be made.

• The == operator and the hashCode getter, so that constraints can be compared for equality. If a render object is given constraints that are equal, then the rendering library will avoid laying the object out again if it is not dirty.

• The toString method, which should describe the constraints so that they appear in a usefully readable form in the output of debugDumpRenderTree.