:linkattrs:
= Gerrit Code Review - JavaScript Plugin Development and API

Gerrit Code Review supports an API for JavaScript plugins to interact
with the web UI and the server process.

[[loading]]
== Plugin loading and initialization

JavaScript is loaded using a standard `<script src='...'>` HTML tag.
Plugins should protect the global namespace by defining their code
within an anonymous function passed to `Gerrit.install()`. The plugin
will be passed an object describing its registration with Gerrit.

* The plugin provides pluginname.js, and can be a standalone file or a static
  asset in a jar as a link:dev-plugins.html#deployment[Web UI plugin].
* pluginname.js contains a call to `Gerrit.install()`. There should
  only be a single `Gerrit.install()` call per file.
* The Gerrit web app imports pluginname.js.
* For standalone plugins, the entry point file is a `pluginname.js` file
  located in `gerrit-site/plugins` folder, where `pluginname` is an alphanumeric
  plugin name.

=== Examples
Here's a recommended starter `myplugin.js`:

``` js
Gerrit.install(plugin => {
  // Your code here.
});
```

== TypeScript API ==

Gerrit provides a TypeScript plugin API.

For a plugin built inline, its `tsconfig.json` can extends Gerrit plugin
TypeScript configuration:

`tsconfig.json`:
``` json
{
  "extends": "../tsconfig-plugins-base.json"
}
```

For standalone plugins (outside of a Gerrit tree), a TypeScript plugin API is
published:
link:https://www.npmjs.com/package/@gerritcodereview/typescript-api[@gerritcodereview/typescript-api].
It provides a TypeScript configuration `tsconfig-plugins-base.json` which can
be used in your plugin `tsconfig.json`:

``` json
{
  "extends": "node_modules/@gerritcodereview/typescript-api/tsconfig-plugins-base.json",
  // your custom configuration and overrides
}
```

[[low-level-api-concepts]]
== Low-level DOM API concepts

Basically, the DOM is the API surface. Low-level API provides methods for
decorating, replacing, and styling DOM elements exposed through a set of
link:pg-plugin-endpoints.html[endpoints].

Gerrit provides a simple way for accessing the DOM via DOM hooks API. A DOM
hook is a custom element that is instantiated for the plugin endpoint. In the
decoration case, a hook is set with a `content` attribute that points to the DOM
element.

1. Get the DOM hook API instance via `plugin.hook(endpointName)`
2. Set up an `onAttached` callback
3. Callback is called when the hook element is created and inserted into DOM
4. Use element.content to get UI element

``` js
Gerrit.install(plugin => {
  const domHook = plugin.hook('reply-text');
  domHook.onAttached(element => {
    if (!element.content) { return; }
    // element.content is a reply dialog text area.
  });
});
```

[[low-level-decorating]]
=== Decorating DOM Elements

For each endpoint, Gerrit provides a list of DOM properties (such as
attributes and events) that are supported in the long-term.

``` js
Gerrit.install(plugin => {
  const domHook = plugin.hook('reply-text');
  domHook.onAttached(element => {
    if (!element.content) { return; }
    element.content.style.border = '1px red dashed';
  });
});
```

[[low-level-replacing]]
=== Replacing DOM Elements

An endpoint's contents can be replaced by passing the replace attribute as an
option.

``` js
Gerrit.install(plugin => {
  const domHook = plugin.hook('header-title', {replace: true});
  domHook.onAttached(element => {
    element.appendChild(document.createElement('my-site-header'));
  });
});
```

[[low-level-style]]
=== Styling DOM Elements

Gerrit only offers customized CSS styling by setting
link:https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_[custom_properties]
(aka css variables).

See link:https://gerrit.googlesource.com/gerrit/+/master/polygerrit-ui/app/styles/themes/[app-theme.ts]
for the list of available variables.

You can just create `<style>` elements yourself and add them to the
`document.head`, but for your convenience the Plugin API provides a simple
`styleApi().insertCSSRule()` method for doing just that. Typically you would
define a CSS rule for `html`, which is always applied, or for a specific theme
such as `html.lightTheme`. 

``` js
Gerrit.install(plugin => {
  plugin.styleApi().insertCSSRule(`
    html {
      --header-text-color: black;
    }
  `);
  plugin.styleApi().insertCSSRule(`
    html.lightTheme {
      --header-background-color: red;
    }
  `);
  plugin.styleApi().insertCSSRule(`
    html.darkTheme {
      --header-background-color: blue;
    }
  `);
});
```

[[high-level-api-concepts]]
== High-level DOM API concepts

High level API is based on low-level DOM API and is essentially a standardized
way for doing common tasks. It's less flexible, but will be a bit more stable.

The common way to access high-level API is through `plugin` instance passed
into setup callback parameter of `Gerrit.install()`, also sometimes referred to
as `self`.

[[low-level-api]]
== Low-level DOM API

The low-level DOM API methods are the base of all UI customization.

=== attributeHelper
`plugin.attributeHelper(element)`

Alternative for
link:https://polymer-library.polymer-project.org/3.0/docs/devguide/data-binding[Polymer data
binding,role=external,window=_blank] for plugins that don't use Polymer. Can be used to bind element
attribute changes to callbacks.

=== hook
`plugin.hook(endpointName, opt_options)`

See link:pg-plugin-endpoints.html[endpoints].

=== registerCustomComponent
`plugin.registerCustomComponent(endpointName, opt_moduleName, opt_options)`

See link:pg-plugin-endpoints.html[endpoints].

=== registerDynamicCustomComponent
`plugin.registerDynamicCustomComponent(dynamicEndpointName, opt_moduleName,
opt_options)`

See link:pg-plugin-endpoints.html[endpoints].

=== on
Register a JavaScript callback to be invoked when events occur within
the web interface. Signature

``` js
self.on(event, callback);
```

Parameters

* event: A supported event type. See below for description.

* callback: JavaScript function to be invoked when event happens.
  Arguments may be passed to this function, depending on the event.

Supported events:

* `showchange`: Invoked when a change is made visible. A
  link:rest-api-changes.html#change-info[ChangeInfo] and
  link:rest-api-changes.html#revision-info[RevisionInfo]
  are passed as arguments. Gerrit provides a third parameter which
  is an object with a `mergeable` boolean.

* `submitchange`: Invoked when the submit button is clicked
  on a change. A link:rest-api-changes.html#change-info[ChangeInfo]
  and link:rest-api-changes.html#revision-info[RevisionInfo] are
  passed as arguments. Similar to a form submit validation, the
  function must return true to allow the operation to continue, or
  false to prevent it. The function may be called multiple times, for
  example, if submitting a change shows a confirmation dialog, this
  event may be called to validate that the check whether dialog can be
  shown, and called again when the submit is confirmed to check whether
  the actual submission action can proceed.

[[high-level-api]]
== High-level API

Plugin instance provides access to a number of more specific APIs and methods
to be used by plugin authors.

=== admin
`plugin.admin()`

.Params:
- none

.Returns:
- Instance of link:pg-plugin-admin-api.html[GrAdminApi].

=== changeActions
`self.changeActions()`

Returns an instance of the
link:https://gerrit.googlesource.com/gerrit/+/master/polygerrit-ui/app/api/change-actions.ts[ChangeActionsPluginApi].

==== changeActions.add()
Adds a new action to the change actions section. Returns the key of the newly
added action.

``` js
changeActions.add(type, label)
```

* type: The type of the action, either `change` or `revision`.

* label: The label to be used in UI for this action.


==== changeActions.remove()
Removes an action from the change actions section.

``` js
changeActions.remove(key)
```

* key: The key of the action.


==== changeActions.addTapListener()
Adds a tap listener to an action that will be invoked when the action
is tapped.

``` js
changeActions.addTapListener(key, callback)
```

* key: The key of the action.

* callback: JavaScript function to be invoked when action tapped.


==== changeActions.removeTapListener()
Removes an existing tap listener on an action.

``` js
changeActions.removeTapListener(key, callback)
```

* key: The key of the action.

* callback: JavaScript function to be removed.


==== changeActions.setLabel()
Sets the label for an action.

``` js
changeActions.setLabel(key, label)
```

* key: The key of the action.

* label: The label of the action.


==== changeActions.setTitle()
Sets the title for an action.

``` js
changeActions.setTitle(key, title)
```

* key: The key of the action.

* title: The title of the action.


==== changeActions.setIcon()
Sets an icon for an action.

``` js
changeActions.setIcon(key, icon)
```

* key: The key of the action.

* icon: The name of the icon.


==== changeActions.setEnabled()
Sets an action to enabled or disabled.

``` js
changeActions.setEnabled(key, enabled)
```

* key: The key of the action.

* enabled: The status of the action, true to enable.


==== changeActions.setActionHidden()
Sets an action to be hidden.

``` js
changeActions.setActionHidden(type, key, hidden)
```

* type: The type of the action.

* key: The key of the action.

* hidden: True to hide the action, false to show the action.


==== changeActions.setActionOverflow()
Sets an action to show in overflow menu.

``` js
changeActions.setActionOverflow(type, key, overflow)
```

* type: The type of the action.

* key: The key of the action.

* overflow: True to move the action to overflow menu, false to move
  the action out of the overflow menu.


=== changeReply
`plugin.changeReply()`

Returns an instance of the
link:https://gerrit.googlesource.com/gerrit/+/master/polygerrit-ui/app/api/change-reply.ts[ChangeReplyPluginApi].

[[checks]]
=== checks
`plugin.checks()`

Returns an instance of the link:pg-plugin-checks-api.html[ChecksApi].

=== getPluginName
`plugin.getPluginName()`

Returns the name this plugin was installed as by the server
administrator. The plugin name is required to access REST API
views installed by the plugin, or to access resources.

=== getServerInfo
`plugin.getServerInfo()`

Returns the host config as a link:rest-api-config.html#server-info[ServerInfo]
object.

=== popup
`plugin.popup(moduleName)`

Creates a popup that contains the given web components. Can be controlled with
calling `open()` and `close()` on the return value.

[[plugin-rest-api]]
=== restApi
`plugin.restApi(opt_prefix)`

.Params:
- (optional) URL prefix, for easy switching into plugin URL space,
  e.g. `changes/1/revisions/1/cookbook~say-hello`

.Returns:
- Instance of link:pg-plugin-rest-api.html[RestPluginApi].

[[plugin-screen]]
=== screen
`plugin.screen(screenName, opt_moduleName)`

Registers a web component as a dedicated top-level page that the router
understands and that has a URL (/x/pluginname/screenname) that can be navigated
to. Extension screens are usually linked from the
link:dev-plugins.html#top-menu-extensions[top menu].

.Params:
- `*string* screenName` URL path fragment of the screen, e.g.
`/x/pluginname/*screenname*`
- `*string* opt_moduleName` (Optional) Web component to be instantiated for this
screen.

.Returns:
- Instance of HookApi.

=== url
`plugin.url(opt_path)`

Returns a URL within the plugin's URL space. If invoked with no
parameter the URL of the plugin is returned. If passed a string
the argument is appended to the plugin URL.

A plugin's URL is where this plugin is loaded, it doesn't
necessary to be the same as the Gerrit host. Use `window.location`
if you need to access the Gerrit host info.

``` js
self.url();                    // "https://gerrit-review.googlesource.com/plugins/demo/"
self.url('/static/icon.png');  // "https://gerrit-review.googlesource.com/plugins/demo/static/icon.png"
```