Interface for the parent of a Builder that allows the builder to communicate invalidations back
to the parent for use when using nested builders.
public abstract void markDirty()
A builder becomes dirty whenever a field is modified -- including fields in nested builders
-- and becomes clean when build() is called. Thus, when a builder becomes dirty, all its
parents become dirty as well, and when it becomes clean, all its children become clean. The
dirtiness state is used to invalidate certain cached values.
To this end, a builder calls markDirty() on its parent whenever it transitions from clean
to dirty. The parent must propagate this call to its own parent, unless it was already dirty,
in which case the grandparent must necessarily already be dirty as well. The parent can only
transition back to "clean" after calling build() on all children.