判断一个字符串是否为空

- (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;
}