California Code, Code of Civil Procedure - CCP § 844

(a) A court may require the parties in a comprehensive adjudication to submit written testimony of relevant witnesses in the forms of affidavits or declarations under penalty of perjury in lieu of presenting live testimony.  The required written testimony may include, but is not limited to, expert witness opinions and testimony that authenticates documentary evidence.  The court may order that the written testimony constitutes the entirety of the witness's direct testimony, require the written testimony to include any exhibits offered in support of the written testimony, and, in the case of written testimony of an expert witness, require a statement of the witness's qualifications.

(b) If the court requires the submission of written testimony pursuant to subdivision (a), a complete copy of the direct testimony shall be served at least 21 days before trial.  A complete copy of any rebuttal testimony shall be served no later than the first day of trial.

(c) If the contents of the written testimony would have been admissible if the witness testified orally, the written testimony shall be received by the court as a documentary exhibit if the witness whose written testimony is being offered is made available for cross-examination by all parties.