What is the test for an anticipatory breach?

17 To similar effect is this passage from Frost v. Knight (1872), L.R. 7 Ex. 111 at 112 (per Cockburn C.J.) about the situation of an anticipatory breach that the innocent party refuses to accept: . . . [i]n that case he keeps the contract alive for the benefit of the other party as well as his own; he remains subject to all his own obligations and liabilities under it, and enables the other party not only to complete the contract, if so advised, notwithstanding his previous repudiation of it, but also to take advantage of any supervening circumstances which would justify him in declining to complete it. [emphasis added]