Apple GameCenter: how to check if playerId is valid (and online) from game server

I 开发者_JAVA百科post some data from my app to my game-server. This data always contains playedId from GKLocalPlayer.

How my game-server can check if this playerId is valid (online)? (for example by some get/post request to GameCenter server).

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For now GameCenter does not support any get/post requests.