使用PageFactory时Selenium Webdriver C#等待存在

using OpenQA.Selenium;
using OpenQA.Selenium.Support.PageObjects;
using OpenQA.Selenium.Support.UI;

public class LoginPage
{
    private IWebDriver driver;

    [FindsBy(How = How.Id, Using = "username")]
    private IWebElement usernameInput;

    [FindsBy(How = How.Id, Using = "password")]
    private IWebElement passwordInput;

    [FindsBy(How = How.Id, Using = "loginButton")]
    private IWebElement loginButton;

    public LoginPage(IWebDriver driver)
    {
        this.driver = driver;
        PageFactory.InitElements(driver, this);
    }

    public void Login(string username, string password)
    {
        WebDriverWait wait = new WebDriverWait(driver, TimeSpan.FromSeconds(10));
        wait.Until(ExpectedConditions.ElementExists(By.Id("username")));

        usernameInput.SendKeys(username);
        passwordInput.SendKeys(password);
        loginButton.Click();
    }
}