NSString去除两边空格字符，like trim()

NSString *cleanString = [dirtyString stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];

还有就是去除多于的空格，如下：

NSString *theString = @"    Hello      this  is a   long       string!   ";

NSCharacterSet *whitespaces = [NSCharacterSet whitespaceCharacterSet];
NSPredicate *noEmptyStrings = [NSPredicate predicateWithFormat:@"SELF != ''"];

NSArray *parts = [theString componentsSeparatedByCharactersInSet:whitespaces];
NSArray *filteredArray = [parts filteredArrayUsingPredicate:noEmptyStrings];
theString = [filteredArray componentsJoinedByString:@" "];