NSRunLoop

NSInteger couter = 0;
while (self.aa == nil && couter < 10) {
     NSLog(@"aa为空，阻塞不去构造JSON字典");
     couter++;
　　　//每隔一秒检查一下aa是否为空,最多尝试10次.
     [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1]];
}