I've spent several weeks of evenings this year learning the ins and outs of California residency for expats. In fact I spent another few weeks teaching it to my Megacorp-hired US tax accountants, after reading the tax code, 20 years of appeals (including Joe Morgan the baseball player's), textbooks, and Franchise Tax Board training manuals.
Yes, California is aggressive in claiming residency, but not to a crazy extent. Basically it comes down to this. If you are a California resident, you can break your residency by: 1)establishing a domicile in another state or country...a domicile is your real physical home, with your doctors, church, clubs, friends...not just a POBox, drivers license, and a voter registration; or 2)leaving the state for "other than temporary or transitory purposes." There is now a safe harbor definition of "other than temporary or transitory"...i.e. being gone for 1.5 years without returning for more than 45 days. The appeal process has set a precedent for another definition... intending to be away from the state for work for an indefinite period expected to be more than 2 years while not maintaining a home in readiness to reoccupy (i.e...you'd better rent your house out while you're overseas). So in fact you can maintain a California domicile and not be considered a resident while you're gone.
For those who are considered residents, income tax is owed on worldwide income. For those not considered residents, tax is only owed on income generated in the state...for example your rental income.