使用WebDriverWait类处理等待（sleep）的问题

用selenium进行web UI的自动化开发时，经常遇到loading需要等待的时候，或者需要验证一个action之后某个dialog是否呈现或者消失。对于这类情况是不建议用sleep(xx)来死等，因为你无法准确判断要等多久，少了就会报error，多了就会浪费整个脚本跑的时间。聪明的做法就是灵活等待，WebDriverWait（org.openqa.selenium.support.ui）这个类是很好的选择。

/**
     * Wait until the element visible with customized timeout
     * @param driver
     * @param locator
     * @param errorMessage
     * @return
     */
    public WebElement waitForElementVisible(WebDriver driver,final By locator, long timeOutInSeconds, String errorMessage) {
        Function<WebDriver, WebElement> waitFn = new Function<WebDriver, WebElement>() {
            @Override
            public WebElement apply(WebDriver driver) {
                try {
                    WebElement el = driver.findElement(locator);
                    if (el.isDisplayed()) {
                        return el;
                    }
                } catch (Exception e) {
                }
                return null;
            }
        };

        WebDriverWait wait = new WebDriverWait(driver, timeOutInSeconds);
        if (errorMessage != "") {
            wait.withMessage(errorMessage);
        } else {
            wait.withMessage("Cannot find Element: " + locator.toString());
        }
        return wait.until(waitFn);
    } 

/**
     * Wait until the element present with customized timeout
     * @param driver
     * @param locator
     * @param errorMessage
     * @return
     */
    public Boolean waitForElementPresent(WebDriver driver,final By locator, long timeOutInSeconds, String errorMessage) {
        Function<WebDriver, Boolean> waitFn = new Function<WebDriver, Boolean>() {
            @Override
            public Boolean apply(WebDriver driver) {
                try {
                    driver.findElement(locator);
                    return true;
                } catch (Exception e) {
                    return false;
                }
            }
        };

        WebDriverWait wait = new WebDriverWait(driver, timeOutInSeconds);
        if (errorMessage != "") {
            wait.withMessage(errorMessage);
        } else {
            wait.withMessage("Cannot find Element: " + locator.toString());
        }
        return wait.until(waitFn);
    }