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In addKey() procedure, it is mentioned that if next message is not null, then fire keymessage to mediakeysession then to server.I do not quiet understand what is the "next message"? and what information will be delivered through keymessage at what scenario?
"next message" is a variable, as indicated by the <var> markup and resulting italics font. If you read it as a variable, the algorithm describes exactly what it is and what to do with it.