如何在Core Text中使用CTTypesetterSuggestClusterBreak

I am having trouble in using CTTypesetterSuggestClusterBreak function of CTTypeSetterRef class. I want to use this method to get closest word boundry near an index. I am having difficult in the implementationof this method, i.e how and where this method must be used. I have been banging my head over this but with no success yet. If anyone can help me in using this method I would be very greatful.I am having trouble in using CTTypesetterSugges