Data for use by the parent render object.
The parent data is used by the render object that lays out this object (typically this object's parent in the render tree) to store information relevant to itself and to any other nodes who happen to know exactly what the data means. The parent data is opaque to the child.
- The parent data field must not be directly set, except by calling setupParentData on the parent node.
- The parent data can be set before the child is added to the parent, by calling setupParentData on the future parent node.
- The conventions for using the parent data depend on the layout protocol used between the parent and child. For example, in box layout, the parent data is completely opaque but in sector layout the child is permitted to read some fields of the parent data.