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Percy

Storybook addons for visual testing with Percy

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@percy/storybook

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Visual Testing with Percy

Percy is an all-in-one visual testing and review platform. It captures screenshots, compares them against the baseline, and highlights visual changes. With increased visual coverage, teams can deploy code changes with confidence with every commit.

Storybook supports cross-browser visual testing natively using Percy. You can use Percy for visual testing of web applications on desktop and mobile browsers.

Sign up here to get started with Percy for free.

The benefits of utilizing Percy for visual testing include the following:

  • Consistency: Promotes consistent user experience by identifying visual discrepancies early in the development process.
  • Efficiency: Improves efficiency by reducing the time and effort required to manually spot visual regressions.
  • Integrations: Percy integrates with popular tools and services like GitHub, GitLab, Bitbucket, and more.
  • Collaboration: Improves collaboration between developers, designers, and QA teams by providing a visual representation of changes.
  • Prevent regression: Prevents you from experiencing unintended visual regressions.

How does Percy work?

Percy compares new snapshots against relevant baselines to detect visual changes. Percy manages baseline selection across branches so your tests are always relevant. If visual changes are detected, Percy highlights and groups the resulting diffs for you to review.

Run your first build with percy

Percy’s sample build tutorial for Storybook.

Installation

$ npm install --save-dev @percy/cli @percy/storybook

Usage

# Unix
$ export PERCY_TOKEN="<your-project-token>"

# Windows
$ set PERCY_TOKEN="<your-project-token>"

# Powershell
$ $Env:PERCY_TOKEN="<your-project-token>"

With a static Storybook build:

$ percy storybook ./storybook-build

With a local or live Storybook URL:

$ percy storybook http://localhost:9009
$ percy storybook https://storybook.foobar.com

Automatically run start-storybook:

$ percy storybook:start --port=9009 --static-dir=./public

Commands:

percy storybook

Snapshot static or hosted Storybook stories

Usage:
  $ percy storybook [options] <url|directory>

Arguments:
  url|directory                      Storybook url or build output directory

Subcommands:
  storybook:start [options]          Run start-storybook to snapshot stories
  help [command]                     Display command help

Options:
  -i, --include <pattern>            Pattern matching story names to include in snapshots
  -e, --exclude <pattern>            Pattern matching story names to exclude from snapshots
  --shard-count <number>             Number of shards to split snapshots into
  --shard-size <number>              Size of each shard to split snapshots into
  --shard-index <index>              Index of the shard to take snapshots of
  --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
  --disallowed-hostname <hostname>   Disallowed hostnames to abort 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 storybook ./build
  $ percy storybook http://localhost:9000/

percy storybook:start

Run start-storybook to snapshot stories

Usage:
  $ percy storybook:start [options]

Options:
  -i, --include <pattern>            Pattern matching story names to include in snapshots
  -e, --exclude <pattern>            Pattern matching story names to exclude from snapshots
  --shard-count <number>             Number of shards to split snapshots into
  --shard-size <number>              Size of each shard to split snapshots into
  --shard-index <index>              Index of the shard to take snapshots of
  --partial                          Marks the build as a partial build
  --port [number]                    Port to start Storybook (default: 9000)
  --host [hostname]                  Host to start Storybook (default: "localhost")

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
  --disallowed-hostname <hostname>   Disallowed hostnames to abort 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 storybook:start
  $ percy storybook:start --port 9000
  $ percy storybook:start --static-dir public

Configuration

Storybook parameters are a set of static, named metadata about a story, used to control the behavior of Storybook features and addons. The percy parameter can be provided to add per-snapshot configuration options to a story or set of stories.

// individual stories
MyStory.parameters = {
  percy: { ... }
};

// .storybook/preview.js
export const parameters = {
  percy: { ... }
};

The following percy Storybook parameters are accepted in addition to common per-snapshot options:

  • skip - Boolean indicating whether or not to skip this story.
  • name - Snapshot name. (default: ${story.kind}: ${story.name})
  • args - Story args to use when taking the snapshot.
  • globals - Story globals to use when taking the snapshot.
  • queryParams - Query parameters to use when snapshotting.
  • waitForTimeout - Wait for a timeout before taking the snapshot.
  • waitForSelector - Wait for a selector to exist before taking the snapshot.
  • include - An array of regex patterns matching story names to only include for snapshotting.
  • exclude - An array of regex patterns matching story names to always exclude from snapshotting.
  • additionalSnapshots - An array of additional snapshots to take of this story.
    • prefix - A prefix added to this additional snapshot's name.
    • suffix - A suffix added to this additional snapshot's name.
    • name - Snapshot name. Replaces the inherited name.
    • args - Additional story args for this additional snapshot.
    • globals - Additional story globals for this additional snapshot.
    • queryParams - Additional query parameters for this additional snapshot.
    • include - Only apply the additional snapshot to matching stories.
    • exclude - Do not apply the additional snapshot to matching stories.
MyStory.parameters = {
  percy: {
    name: 'My snapshot',
    additionalSnapshots: [
      { prefix: '[Dark mode] ', args: { colorScheme: 'dark' } },
      { suffix: ' with a search', queryParams: { search: 'foobar' } }
    ]
  }
};

With this example, 3 snapshots will be taken of this story with args and query params appended to the URL of each snapshot:

# --dry-run will log snapshots without creating a new build
# --verbose will show debug logs, including the snapshot url
$ percy storybook --dry-run --verbose ./example-storybook
# ...
[percy] Snapshot found: My snapshot
[percy] -> url: [...]?id=component--my-story
[percy] Snapshot found: [Dark mode] My snapshot
[percy] -> url: [...]?id=component--my-story&args=colorScheme:dark
[percy] Snapshot found: My snapshot with a search
[percy] -> url: [...]?id=component--my-story&search=foobar

Percy config file options

In addition to common Percy config file options, this SDK also adds the following Storybook specific options:

# .percy.yml
version: 2
storybook:
  args: {}
  globals: {}
  queryParams: {}
  waitForTimeout: 0
  waitForSelector: ''
  additionalSnapshots: []
  include: []
  exclude: []

See the configuration options above for details about each accepted config file option (note: the skip and name parameters are not accepted as Percy config file options).

Upgrading

Prior versions of the Storybook SDK were drastically different than the current version. The command, it's arguments, and how the SDK works internally have changed completely. Using the old command with new versions will now result in an error message. The new command is now integrated into @percy/cli as a plugin.

To use new versions of this SDK, you will have to also install the CLI with the SDK:

$ npm install --save-dev @percy/cli @percy/storybook

Since both the command and arguments have changed, you'll need to replace your existing usage with the new usage described above. For some projects, this may require setting additional configuration options. See the list of breaking changes below for details.

Breaking changes

Most importantly, the command itself has changed and all previous arguments are no longer accepted.

  • The percy-storybook command has been replaced with a percy CLI subcommand, percy storybook.

  • The previous --build_dir flag is now a command argument and there is no default build directory. If you relied on the default, it must now be explicitly provided.

    # before
$ percy-storybook
# after
$ percy storybook ./storybook-static

# before
$ percy-storybook --build_dir ./build
# after
$ percy storybook ./build

  • The --widths flag is no longer accepted. Widths can be set using the respective widths Percy config file snapshot option or percy Storybook parameter.

  • The --minimum_height flag is no longer accepted and therefore no longer defaults to 800px. The default minimum height shared by all SDKs is 1024px. The minimum height can be set using the respective min-height Percy config file snapshot option or percy Storybook parameter.

  • The --debug flag is now --verbose, inherited from the CLI.

  • The --output-format flag is no longer accepted and has no alternative. If you relied on this flag, please open an issue.

  • The --rtl and --rtl_regex flags are no longer accepted. The --rtl flag duplicated stories and set the direction=rtl query parameter for the duplicate's URL. The --rtl_regex flag was used to determine when to create this RTL duplicate story.

    // .storybook/preview.js

export const parameters = {
  percy: {
    // tell percy to take an additional RTL snapshot for matching stories
    additionalSnapshots: [{
      suffix: ' [RTL]',
      queryParams: { direction: 'rtl' },
      include: ['^FormElement: .*']
    }]
  }
};

Performance

The old SDK did not take DOM snapshots or perform asset discovery, as all other modern Percy SDKs do. This sometimes resulted in flakey snapshots or snapshots with missing assets. However, DOM snapshots and asset discovery add an overhead cost of performance. Where the old SDK was very quick to simply upload the local build directory, the new SDK can be a little slower while it is capturing the real DOM and relevant assets of each story.

Unexpected diffs

Because the old SDK did not take DOM snapshots, JavaScript had to be enabled in our rendering environment for Storybook to properly load. This is in contrast to all of our other SDKs, where JavaScript is disabled by default to prevent flakey diffs caused by animations or other JavaScript running on the page. With the new SDK and real DOM snapshots, JS is disabled by default. If you upgrade and experience diffs due to the lack of JavaScript, it can be re-enabled using the matching Percy config file or per-snapshot option, enableJavaScript.

Development

  • Current package.json and yarn.lock install storybook v7 as devDependency and hence require node 16 for development.
  • There are separate package.json and config files for storybook v6, for testing purposes. Please check prepare-storybook-v6-tests.sh file for more details around changes for storybook v6 testing.
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    percy-admin
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