The Percy snapshot is a powerful command that allows you to capture visual snapshots of web pages, lists, sitemaps, or static directories.
On this page, we’ll explore how to use Percy snapshot method in your test script and what are the supported parameters. You have the option to either capture a full screenshot or take a screenshot of a single element.
Percy snapshot method
Incorporate the Percy snapshot method into your test script as shown below. You must specify a snapshots name that you intend to capture. The options can be passed as an optional parameter.
All Percy SDKs that support @percy/cli can accept snapshot options as the final argument of the SDK’s percySnapshot method.
These snapshot options will override or be merged with their equivalent Percy config file options.
Per-snapshot configuration
Option
Description
widths
An array of numbers, in pixels, representing the widths you’d like to capture for each snapshot; overrides global snapshot widths.
min-height
A number specifying the minimum height in pixels each snapshot should be.
percy-css
A string containing Percy specific CSS; appended to any global percy-css.
enable-javascript
A boolean that enables JavaScript for all snapshots in the build. This may cause side-effects like animations or redirects making your snapshots less reliable.
scope
A string of a selector to scope the given screenshot to.
discovery
A subset of discovery options to use for this snapshot’s asset discovery. - allowed-hostnames: An array of additional hostnames to save asset contents from; merged with any existing allowed-hostnames. - disallowed-hostnames: An array of hostnames to immediately abort requests from. - request-headers: An object containing HTTP headers to be sent for each request made during asset discovery; merged with any existing request-headers. - authorization: A username/password combo to authenticate requests for Percy. - disable-cache: Disables asset discovery caches. - user-agent: Custom user-agent string used when requesting assets.
The underlying CLI API accepts the following options in camelCase, PascalCase, snake_case, or kebab-case!
Exec command to execute script
Percy CLI commands for running a local snapshot server.
Commands
percy exec
percy exec:start
percy exec:stop
percy exec:ping
percy exec
Start and stop Percy around a supplied command
```
Usage:
$ percy exec [options] -- <command>
Subcommands:
exec:start [options] Starts a local Percy snapshot server
exec:stop [options] Stops a local running Percy snapshot server
exec:ping [options] Pings a local running Percy snapshot server
help [command] Display command help
Options:
-P, --port [number] Local CLI server port (default: 5338)
--parallel Marks the build as one of many parallel builds
--partial Marks the build as a partial build
Percy options:
-c, --config <file> Config file path
-d, --dry-run Print snapshot names only
-h, --allowed-hostname <hostname> Allowed hostnames to capture in asset discovery
-t, --network-idle-timeout <ms> Asset discovery network idle timeout
--disable-cache Disable asset discovery caches
--debug Debug asset discovery and do not upload snapshots
Global options:
-v, --verbose Log everything
-q, --quiet Log errors only
-s, --silent Log nothing
--help Display command help
Examples:
$ percy exec -- echo "percy is running around this echo command"
$ percy exec -- yarn test
```
Sometimes capturing a full-page screenshot isn’t necessary. For example, if there are dynamic parts of the page that you don’t need to test or are only interested in a very specific region to test.
For these cases, you can pass a scope snapshot option and Percy will only capture the scoped element on the given widths. This can be passed as a per-snapshot option.
The scope selector accepts any valid selector you would be able to pass to document querySelector.
If there are multiple matching selectors on the page, Percy will select the first matching element.
If you would like to scope a screenshot to a specific element that has the same matching selector as other elements on the page you’ll have to get more specific with your selector. This can be done by either adding another unique selector to that element or by using standard CSS selectors to get more specific. This is the same way you would write CSS – Percy doesn’t add anything to this process.
For example, given the below DOM:
```html
<html>
<head>
<title>Example</title>
</head>
<body>
<h1 class="underline">My example</h1>
<p class="underline">Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas tristique convallis sem, vitae sodales risus accumsan in</p>
<p class="underline">Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas tristique convallis sem, vitae sodales risus accumsan in</p>
</body>
</html>
```
Instead of using just .underline to select the element, you would want to either specify the element type (h1 / p) or by using CSS tree-structural pseudo-classes like :last-of-type or :nth-child.
p.underline:last-of-type scope selects last paragraph
Selector for elements serialized by Percy
It may happen that your single-element screenshots are not working correctly with canvas/video elements.
During DOM serialization <canvas>, <video> elements are converted to <img>. Check canvas elements in Percy’s SDK Workflow. This can cause the selector to not match, and an incorrect screenshot.
To work around this, we can use CSS tree structural pseudo-classes as suggested in the previous section.
```html HTML
<!-- Skipping most of the stuff for brevity -->
<!-- Original DOM -->
<div class="selector">
<canvas> </canvas>
<canvas> </canvas>
<video> </video>
</div>
<!-- Post DOM Serialization -->
<div class="selector">
<img> </img>
<img> </img>
<img> </img>
</div>
```
.selector img:nth-of-type(2) scope selects the ‘second’ canvas
.selector img:last-of-type scope selects the ‘last’ video
Usage when having Selenium's WebElement
When dealing with Web elements, you may use the below utility function that returns CSS selector i.e scope in our case, that could be easily passed to the percySnapshot function.
Make sure script eval is allowed in the test browser, Please check ref on how to verify.
```java
public String percyScope(WebDriver driver, WebElement webElement) {
// util for getting css selector using the DOM element
String script = String.join(System.getProperty("line.separator"),
"if(typeof UTILS === 'undefined') {",
"return fetch('https://gist.githubusercontent.com/itsjwala/a7ccb4d0ae4ceb5a9fc33de2823f0bf1/raw/9c5a1044794b5f8aa9dfba17884e01bf71019d67/dom_css_path.js')",
".then(res => res.text())",
".then(eval)",
".then( _ => UTILS.cssPath(arguments[0]));",
"} else {",
"return UTILS.cssPath(arguments[0]);",
"}"
);
JavascriptExecutor jse = (JavascriptExecutor) driver;
return (String) jse.executeScript(script, webElement);
}
```
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