What is the test for awarding attorney fees to a party who has recovered or preserved a monetary fund for the benefit of others?

"California has long recognized, as an exception to the general American rule that parties bear the costs of their own attorneys, the propriety of awarding an attorney fee to a party who has recovered or preserved a monetary fund for the benefit of himself or herself and others. In awarding a fee from the fund or from the other benefited parties, the trial court acts within its equitable power to prevent the other parties' unjust enrichment.' " (Laffitte v. Robert Half Internat. Inc. (2016) 1 Cal.5th 480, 488-489 (Laffitte).)