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Simple modern page models for Selenium


What is a page model?

A page model is a class that models how a person interacts with a specific page in the UI. The public methods of a page model represent the tasks that users perform on the page -- what they see and what they say to get things done and to understand the results. The private methods of a page model implement the specific interactions with page content needed to perform these tasks.

The page model pattern helps to keep UI tests more resilient to changes in the UI. Because tests use only the public interface to pages, they are isolated from details of page construction. Page layout and content can be revised and reorganized with requiring any changes to test scenarios or test assertions.

Page models are ultimately implemented using the Selenium WebDriver. For details, see the WebDriver API Javadoc.

Basics: Site

Use the Site class to model the overall context for testing a specific Web app. This context includes not only information about the app itself (for example, the base URI for app pages) but also information about the testing context (for example the WebDriver instance used to access the UI).

All Page objects must be created in the context of a specific Site. That's why Page is a generic type parameterized by the type of its associated Site.

A Site can be configured to reflect the latencies that occur when interacting with the app. For example, the maximum app wait time can be configured to reflect the maximum timeout to wait for this particular app to update its page elements. In addition, the driver latency factor can be configured to reflect additional latencies incurred when using a particular remote WebDriver.

The Site class is typically used as a base class for classes that model additional application-specific context. ResourceSite implements a special-purpose Site that provides Web pages defined by resource files.

The Site class defines two basic behaviors for tests.

Method What it does
enter Associates the Site with a WebDriver instance
exit Terminates the WebDriver associated with the Site

Basics: Page

The Page class is the foundation for all page models. This abstract base class offers many different forms for constructors that model the various ways that a new page can appear in the UI.

  • By direct navigation to specific URL
  • By a request from a prior page (called the "parent" page)
  • By a request to open a new browser window

A Page is always associated with a Site instance and inherits many of its properties. For example, Page.getDriver() returns the WebDriver used by the Site for this page. Similarly, by default, Page.getMaxAppWait() returns the maximum app wait time defined for its Site. (But note that you can replace this default with a different maximum wait time for any specific Page instance.)

The Page class defines the basic methods used by a page model to:

Basics: Finder

A Finder object defines how to search for a specific page element. Fundamentally, a page element must be identified by matching it to some By condition. (For example, By.cssSelector identifies elements using a CSS selector expression.) But a Finder contains many other optional parameters that may be needed to reliably guide the search.

The Finder class defines the following basic search methods.

Method What it does
findElement Returns a specific element that is assumed to exist
findElements Returns a list of all matching elements
findOptionalElement Returns an Optional<WebElement> that may or may not be present
findVisibleElement Equivalent to when( PageUtils.isVisible).findElement
awaitNoElements Returns successfully when no matching elements can be found

Finder also provides a convenient "fluent" interface for defining the search context, using sensible defaults for unspecified parameters. For example:

List<WebElement> hasDataDefined =
    startingAt( someDivElement)
    .waitingFor( 30, SECONDS)
    .checkingEvery( 200, MILLISECONDS)
    .when( PageUtils.hasAttribute( "data-defined"))
    .whenStableFor( 1, SECONDS)
    .findElements( By.cssSelector( ".someClass"));

Page models seldom create Finder instances directly. Instead, it's simpler to use the Page methods that provide the same interfaces for defining and executing an element search.

Design principles

  • Less is more. Crescent is a very minimal extension of the basic WebDriver API. It makes no rules about the internal structure of a page model. It does not try to model individual interaction techniques, like menus or various <input> types. (Although the PageUtils class offers many useful helper methods for that sort of thing.) Less framework means more freedom for you to choose the right way to model your UI.

  • BYO WebDriver. Crescent has no support for creating a WebDriver and connecting it to a browser. Many are the ways in which this can be done, depending on your test environment, your preferred tools for configuration, and so on. You are free to choose the best way to handle this part of your test setup.

  • When a page is (re)visited, create a new Page. Because this is a good practice for any WebDriver program, the Page class assumes that you will create a new Page instance when the corresponding page becomes available for interaction in the UI. Therefore, every page model constructor leads to the basic Page constructor, which calls initPage(). By default, this calls visit(), which performs the following steps.

    • Synchronizes the state of the Page with the state of the UI. Depending on the constructor arguments, this can entail actively navigating to a specific URL, passively querying the current location of the browser, or switching the WebDriver to a specified browser window.

    • Calls visited(). By default, this method does nothing. But you can override it to perform any page-specific actions needed after arriving at a new instance of this page.

  • Error states are exceptions. Users make mistakes. So good UIs help them recover by showing an "error state" -- for example, by displaying an error message. And good tests verify that these error states appear as expected. But how? Tests use the public page model interface, which models the user's tasks. But checking for errors is not the user's job! That job belongs to the page model implementation. When an error state is detected (expected or not!), the page model should throw an exception. Crescent provides several basic exception types that are handy for reporting the details of an error state.

Page components

For many Web apps, all individual pages share many common graphic design forms or interaction techniques. You can use the Component class to model common UI elements that are shared by multiple page models. A Component always belongs to a specific Page instance. That's why Component is a generic type parameterized by the type of its associated Page.

Page actions

Some page elements implement actions that produce a certain result in the UI. For example, clicking a button can cause a different page to appear. It's not unusual for a UI to present multiple elements that all implement the same action. The PageAction class offers a general way to model this kind of interaction.

PageAction is a generic type that is parameterized by the type of the source page and the type of the result produced by the action. There are two basic PageAction types, each of which are abstract base classes you can use to model app-specific actions.

  • ElementAction: An action triggered by an interaction with a single element

  • NewWindowAction: An action that produces content in a new browser window

The Page class defines the following methods for handling page actions.

Method What it does
getAction Returns a specific type of PageAction that is implemented by a specific optional page element
perform Given an optional PageAction expected to be shown, either performs the action or reports a failure.

Handling multiple windows

Although it's common for Web apps to display content in multiple browser windows, it's tricky to handle that using the basic WebDriver API. But the WindowProducer class makes things a bit simpler.

WindowProducer is an abstract base class for an action that opens a new window. The basic handles the bookkeeping needed to discover and return the WindowHandle for the expected new window. Together with a specific WindowProducer, you can use the NewWindowAction class to model not only the interaction that produces the new window, but also the type of content now available for interaction.

Given a specific WindowHandle argument for a new Page, the basic visit() method will automatically switch the focus of the WebDriver to the specified window. Afterwards, your test must be responsible for keeping track of which Page instances are modeling content for different windows. But, for any Page instance, when you call close(), it will automatically switch the WebDriver focus back to the window for the getParent() page.

Failure exceptions

Because error states should cause exceptions, Crescent provides several basic exception types that are handy for reporting the details of an error state.

Exception Purpose
PageException Base class for all error states reported by a Page
ElementMissingException Reports a failure when searching for a specific page element
InvalidFormException Reports a FieldFailure for each invalid value entered into a form
InvalidStateException Reports a general error state in a Page
RequestException Reports a failure in a system request made from a Page
WindowException Reports a failure to create a new browser window


Simple modern page models for Selenium





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