@percy/core/README.md

The core component of Percy's CLI and SDKs that handles creating builds, discovering snapshot assets, uploading snapshots, and finalizing builds. Uses `@percy/client` for API communication, a Puppeteer browser for asset discovery, and starts a local API s

# @percy/core

The core component of Percy's CLI and SDKs that handles creating builds, discovering snapshot
assets, uploading snapshots, and finalizing builds. Uses `@percy/client` for API communication, a
Puppeteer browser for asset discovery, and starts a local API server for posting snapshots from
other processes.

- [Usage](#usage)
  - [`#start()`](#start)
  - [`#stop()`](#stopforce)
  - [`#idle()`](#idle)
  - [`#snapshot()`](#snapshotoptions)
- [Advanced](#advanced)
  - [Download discovery browser on install](#download-discovery-browser-on-install)
  - [Skipping discovery browser download](#skipping-discovery-browser-download)

## Usage

A `Percy` class instance can manage a Percy build, take page snapshots, and perform snapshot asset
discovery. It also hosts a local API server for Percy SDKs to communicate with.

``` js
import Percy from '@percy/core'

// create a new instance
const percy = new Percy(percyOptions)

// create a new instance and start it
const percy = await Percy.start(percyOptions)
```

#### Options

- `token` — Your project's `PERCY_TOKEN` (**default** `process.env.PERCY_TOKEN`)
- `loglevel` — Logger level, one of `"info"`, `"warn"`, `"error"`, `"debug"` (**default** `"info"`)
- `server` — Controls whether an API server is created (**default** `true`)
- `port` — API server port (**default** `5338`)
- `clientInfo` — Client info sent to Percy via a user-agent string
- `environmentInfo` — Environment info also sent with the user-agent string
- `deferUploads` — Defer creating a build and uploading snapshots until later
- `skipUploads` — Skip creating a build and uploading snapshots altogether

The following options can also be defined within a Percy config file

- `snapshot` — Snapshot options applied to each snapshot
  - `widths` — Widths to take screenshots at (**default** `[375, 1280]`)
  - `minHeight` — Minimum screenshot height (**default** `1024`)
  - `percyCSS` — Percy specific CSS to inject into the snapshot
  - `enableJavaScript` — Enable JavaScript for screenshots (**default** `false`)
- `discovery` — Asset discovery options
  - `allowedHostnames` — Array of allowed hostnames to capture assets from
  - `requestHeaders` — Request headers used when discovering snapshot assets
  - `authorization` — Basic auth `username` and `password` for protected snapshot assets
  - `disableCache` — Disable asset caching (**default** `false`)
  - `userAgent` — Custom user-agent string used when requesting assets
  - `cookies` — Browser cookies to use when requesting assets
  - `networkIdleTimeout` — Milliseconds to wait for the network to idle (**default** `100`)
  - `concurrency` — Asset discovery concerrency (**default** `5`)
  - `launchOptions` — Asset discovery browser launch options
    - `executable` — Browser executable path (**default** `process.env.PERCY_BROWSER_EXECUTABLE`)
    - `timeout` — Discovery launch timeout, in milliseconds (**default** `30000`)
    - `args` — Additional browser process arguments
    - `headless` — Runs the browser headlessy (**default** `true`)
    
Additional Percy config file options are also allowed and will override any options defined by a
local config file. These config file options are also made available to SDKs via the local API
health check endpoint.

### `#start()`

Starting a `Percy` instance will start a local API server, start the asset discovery browser, and
create a new Percy build. If an asset discovery browser is not found, one will be downloaded.

``` js
await percy.start()
// [percy] Percy has started!
```

#### API Server

Starting a `Percy` instance will start a local API server unless `server` is `false`. The server can
be found at `http://localhost:5338/` or at the provided `port` number.

- GET `/percy/healthcheck` – Responds with information about the running instance
- GET `/percy/dom.js` – Responds with the [`@percy/dom`](./packages/dom) library
- GET `/percy/idle` - Responds when the running instance is [idle](#idle)
- POST `/percy/snapshot` – Calls [`#snapshot()`](#snapshotoptions) with provided snapshot options
- POST `/percy/stop` - Remotely [stops](#stopforce) the running `Percy` instance

### `#stop([force])`

Stopping a `Percy` instance will wait for any pending snapshots, close the asset discovery browser,
close the local API server, and finalize the current Percy build. When uploads are deferred,
stopping the instance will also trigger processing of the upload queue. When `force` is `true`,
queues are cleared and closed to prevent queued snapshots from running.

``` js
await percy.stop()
// [percy] Processing 3 snapshots...
// [percy] Snapshot taken: Snapshot one
// [percy] Snapshot taken: Snapshot two
// [percy] Snapshot taken: Snapshot three
// [percy] Uploading 3 snapshots...
// [percy] Finalized build #1: https://percy.io/org/project/123

await percy.stop(true)
// [percy] Stopping percy...
// [percy] Finalized build #1: https://percy.io/org/project/123
```

### `#idle()`

This method will resolve shortly after pending snapshots and uploads have completed and no more have
started. Queued tasks are not considered pending unless they are actively running, so deferred
uploads will not be awaited on with this method.

``` js
percy.snapshot(...);
percy.snapshot(...);
percy.snapshot(...);

await percy.idle()
// [percy] Snapshot taken: ...
// [percy] Snapshot taken: ...
// [percy] Snapshot taken: ...
```

### `#snapshot(options)`

Performs asset discovery for a snapshot and queues uploading the snapshot to the associated Percy
build. When an existing DOM snapshot is provided, it is served as the root resource during asset
discovery. When no existing DOM snapshot is provided, a new one will be captured using any provided
snapshot capture options.

``` js
// snapshots can be handled concurrently, no need to await
percy.snapshot({
  name: 'Snapshot 1',
  url: 'http://localhost:3000',
  domSnapshot: domSnapshot,
  clientInfo: 'my-sdk',
  environmentInfo: 'my-lib'
  ...snapshotOptions
})

percy.snapshot({
  name: 'Snapshot 2',
  url: 'http://localhost:3000/',
  ...snapshotOptions,
  
  // without a domSnapshot, capture options will be used to take one
  waitForTimeout: 1000,
  waitForSelector: '.done-loading',
  execute: async () => {},
  additionalSnapshots: [{
    name: 'Snapshot 2.1',
    execute: () => {},
    ...snapshotOptions
  }]
})
```

#### Options

- `url` — Snapshot URL (**required**)
- `name` — Snapshot name
- `domSnapshot` — Snapshot DOM string
- `clientInfo` — Additional client info
- `environmentInfo` — Additional environment info
- `discovery` - Limited snapshot specific discovery options
  - `allowedHostnames`, `requestHeaders`, `authorization`, `disableCache`, `userAgent`

**Capture options** can only be provided when `domSnapshot` is missing.

- `waitForTimeout` — Milliseconds to wait before taking a snapshot
- `waitForSelector` — CSS selector to wait for before taking a snapshot
- `execute` — Function or function body to execute within the page before taking a snapshot
- `additionalSnapshots` — Array of additional sequential snapshots to take of the page
  - `name` — Snapshot name (**required** if no `prefix` or `suffix`)
  - `prefix` — Snapshot name prefix (**required** if no `name` or `suffix`)
  - `suffix` — Snapshot name suffix (**required** if no `name` or `prefix`)
  - `waitForTimeout`, `waitForSelector`, `execute` — See above

Common snapshot options are also accepted and will override instance snapshot options. [See instance
options](#options) for common snapshot and discovery options.

## Advanced

### Download discovery browser on install

By default, the browser is only downloaded when asset discovery is started for the first time. This
is because many features of the CLI do not require a browser at all, and automatically downloading a
browser creates a much heavier footprint than needed for those features. However, if your CI caches
dependencies after the install step, the browser will not be cached and will be downloaded every
time Percy runs without it.

If the environment variable `PERCY_POSTINSTALL_BROWSER` is present and truthy, then the browser will
be downloaded after the package is installed to allow it to be cached. You can also require
`@percy/core/post-install` within another node module to trigger the browser download manually.

### Skipping discovery browser download

If your CI comes with a Chromium binary pre-installed and you wish to skip Percy's own browser
installation, you can set the respective `discovery.launchOptions.executable` config option. When
the executable at the provided path exists, the default download will be skipped and the provided
binary will be used instead. This option can also be set using the `PERCY_BROWSER_EXECUTABLE`
environment variable.

> **Warning!** Percy is only tested against the browser it downloads automatically. When providing a
> custom browser executable, you may experience unexpected issues.