What is the test for retroactivity under the ex post facto doctrine?

"The purpose of the ex post facto doctrine is to ensure fair notice of the conduct that constitutes a crime and of the punishment that may be imposed for a crime. [Citation.]" ( In re Vicks (2013) 56 Cal.4th 274, 287, 153 Cal.Rptr.3d 471, 295 P.3d 863, fn. omitted.) Hence, " the operative event for retroactivity purposes, and the necessary reference point for any ex post facto analysis, is criminal conduct committed before the disputed law took effect. [Citation.]" ( People v. Trujeque (2015) 61 Cal.4th 227, 256, 188 Cal.Rptr.3d 1, 349 P.3d 103, italics added.)