iOS判断是否为空字符串的方法

- (BOOL) isBlankString:(NSString *)string {    if (string == nil || string == NULL) {        return YES;    }    if ([string isKindOfClass:[NSNull class]]) {        return YES;    }    if ([[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length]==0) {        return YES;    }    return NO;}