排序sort

[self.recentSessionssortUsingComparator:^NSComparisonResult(idobj1,idobj2) {

NIMRecentSession*item1 = obj1;

NIMRecentSession*item2 = obj2;

if(item1.lastMessage.timestamp< item2.lastMessage.timestamp) {

returnNSOrderedDescending;

}

if(item1.lastMessage.timestamp> item2.lastMessage.timestamp) {

returnNSOrderedAscending;

}

returnNSOrderedSame;

}];