UITextField判断输入值是否为空从而选择时候保存

#pragma mark- UITextFieldDelegate
//监听texefield里面是否有值
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string

{
    NSMutableString * changedString=[[NSMutableString alloc]initWithString:textField.text];
    [changedString replaceCharactersInRange:range withString:string];
    if (changedString.length!=0) {
        self.navigationItem.rightBarButtonItem.enabled=YES;
    }else{
        self.navigationItem.rightBarButtonItem.enabled=NO;
    }
    return YES;
}