chrome.events

Description

The chrome.events namespace contains common types used by APIs dispatching events to notify you when something interesting happens.

Concepts and usage

An Event is an object that lets you be notified when something interesting happens. Here's an example of using the chrome.alarms.onAlarm event to be notified whenever an alarm has elapsed:

chrome.alarms.onAlarm.addListener((alarm) => {
  appendToLog(`alarms.onAlarm -- name: ${alarm.name}, scheduledTime: ${alarm.scheduledTime}`);
});

As the example shows, you register for notification using addListener(). The argument to addListener() is always a function that you define to handle the event, but the parameters to the function depend on which event you're handling. Checking the documentation for alarms.onAlarm, you can see that the function has a single parameter: an alarms.Alarm object that has details about the elapsed alarm.

Example APIs using Events: alarms, i18n, identity, runtime. Most chrome APIs do.

Declarative Event Handlers

The declarative event handlers provide a means to define rules consisting of declarative conditions and actions. Conditions are evaluated in the browser rather than the JavaScript engine which reduces roundtrip latencies and allows for very high efficiency.

Declarative event handlers are used for example in the Declarative Content API. This page describes the underlying concepts of all declarative event handlers.

Rules

The simplest possible rule consists of one or more conditions and one or more actions:

const rule = {
  conditions: [ /* my conditions */ ],
  actions: [ /* my actions */ ]
};

If any of the conditions is fulfilled, all actions are executed.

In addition to conditions and actions you may give each rule an identifier, which simplifies unregistering previously registered rules, and a priority to define precedences among rules. Priorities are only considered if rules conflict each other or need to be executed in a specific order. Actions are executed in descending order of the priority of their rules.

const rule = {
  id: "my rule",  // optional, will be generated if not set.
  priority: 100,  // optional, defaults to 100.
  conditions: [ /* my conditions */ ],
  actions: [ /* my actions */ ]
};

Event objects

Event objects may support rules. These event objects don't call a callback function when events happen but test whether any registered rule has at least one fulfilled condition and execute the actions associated with this rule. Event objects supporting the declarative API have three relevant methods: events.Event.addRules(), events.Event.removeRules(), and events.Event.getRules().

Add rules

To add rules call the addRules() function of the event object. It takes an array of rule instances as its first parameter and a callback function that is called on completion.

const rule_list = [rule1, rule2, ...];
addRules(rule_list, (details) => {...});

If the rules were inserted successfully, the details parameter contains an array of inserted rules appearing in the same order as in the passed rule_list where the optional parameters id and priority were filled with the generated values. If any rule is invalid, for example, because it contained an invalid condition or action, none of the rules are added and the runtime.lastError variable is set when the callback function is called. Each rule in rule_list must contain a unique identifier that is not already used by another rule or an empty identifier.

Note: Rules are persistent across browsing sessions. Therefore, you should install rules during extension installation time using the runtime.onInstalled event. Note that this event is also triggered when an extension is updated. Therefore, you should first clear previously installed rules and then register new rules.

Remove rules

To remove rules call the removeRules() function. It accepts an optional array of rule identifiers as its first parameter and a callback function as its second parameter.

const rule_ids = ["id1", "id2", ...];
removeRules(rule_ids, () => {...});

If rule_ids is an array of identifiers, all rules having identifiers listed in the array are removed. If rule_ids lists an identifier, that is unknown, this identifier is silently ignored. If rule_ids is undefined, all registered rules of this extension are removed. The callback() function is called when the rules were removed.

Retrieve rules

To retrieve a list of registered rules, call the getRules() function. It accepts an optional array of rule identifiers with the same semantics as removeRules() and a callback function.

const rule_ids = ["id1", "id2", ...];
getRules(rule_ids, (details) => {...});

The details parameter passed to the callback() function refers to an array of rules including filled optional parameters.

Performance

To achieve maximum performance, you should keep the following guidelines in mind.

Register and unregister rules in bulk. After each registration or unregistration, Chrome needs to update internal data structures. This update is an expensive operation.

Instead of

const rule1 = {...};
const rule2 = {...};
chrome.declarativeWebRequest.onRequest.addRules([rule1]);
chrome.declarativeWebRequest.onRequest.addRules([rule2]);

Prefer

const rule1 = {...};
const rule2 = {...};
chrome.declarativeWebRequest.onRequest.addRules([rule1, rule2]);

Prefer substring matching over regular expressions in an events.UrlFilter. Substring based matching is extremely fast.

Instead of

const match = new chrome.declarativeWebRequest.RequestMatcher({
  url: {urlMatches: "example.com/[^?]*foo" }
});

Prefer

const match = new chrome.declarativeWebRequest.RequestMatcher({
  url: {hostSuffix: "example.com", pathContains: "foo"}
});

If there are many rules that share the same actions, merge the rules into one. Rules trigger their actions as soon as a single condition is fulfilled. This speeds up the matching and reduces memory consumption for duplicate action sets.

Instead of

const condition1 = new chrome.declarativeWebRequest.RequestMatcher({
  url: { hostSuffix: 'example.com' }
});
const condition2 = new chrome.declarativeWebRequest.RequestMatcher({
  url: { hostSuffix: 'foobar.com' }
});
const rule1 = { conditions: [condition1],
                actions: [new chrome.declarativeWebRequest.CancelRequest()]
              };
const rule2 = { conditions: [condition2],
                actions: [new chrome.declarativeWebRequest.CancelRequest()]
              };
chrome.declarativeWebRequest.onRequest.addRules([rule1, rule2]);

Prefer

const condition1 = new chrome.declarativeWebRequest.RequestMatcher({
  url: { hostSuffix: 'example.com' }
});
const condition2 = new chrome.declarativeWebRequest.RequestMatcher({
  url: { hostSuffix: 'foobar.com' }
});
const rule = { conditions: [condition1, condition2],
              actions: [new chrome.declarativeWebRequest.CancelRequest()]
             };
chrome.declarativeWebRequest.onRequest.addRules([rule]);

Filtered events

Filtered events are a mechanism that allows listeners to specify a subset of events that they are interested in. A listener that uses a filter won't be invoked for events that don't pass the filter, which makes the listening code more declarative and efficient. A service worker need not be woken up to handle events it doesn't care about.

Filtered events are intended to allow a transition from manual filtering code.

Instead of

chrome.webNavigation.onCommitted.addListener((event) => {
  if (hasHostSuffix(event.url, 'google.com') ||
      hasHostSuffix(event.url, 'google.com.au')) {
    // ...
  }
});

Prefer

chrome.webNavigation.onCommitted.addListener((event) => {
  // ...
}, {url: [{hostSuffix: 'google.com'},
          {hostSuffix: 'google.com.au'}]});

Events support specific filters that are meaningful to that event. The list of filters that an event supports will be listed in the documentation for that event in the "filters" section.

When matching URLs (as in the example above), event filters support the same URL matching capabilities as expressible with a events.UrlFilter, except for scheme and port matching.

Types

Event

An object which allows the addition and removal of listeners for a Chrome event.

Properties

• addListener

  void

  Registers an event listener callback to an event.

  The addListener function looks like:

  (callback: H) => {...}

  • callback

    H

    Called when an event occurs. The parameters of this function depend on the type of event.

• addRules

  void

  Registers rules to handle events.

  The addRules function looks like:

  (rules: Rule<anyany>[], callback?: function) => {...}

  • rules

    Rule<anyany>[]

    Rules to be registered. These do not replace previously registered rules.

  • callback

    function optional

    The callback parameter looks like:

    (rules: Rule<anyany>[]) => void

    • rules

      Rule<anyany>[]

      Rules that were registered, the optional parameters are filled with values.

• getRules

  void

  Returns currently registered rules.

  The getRules function looks like:

  (ruleIdentifiers?: string[], callback: function) => {...}

  • ruleIdentifiers

    string[] optional

    If an array is passed, only rules with identifiers contained in this array are returned.

  • callback

    function

    The callback parameter looks like:

    (rules: Rule<anyany>[]) => void

    • rules

      Rule<anyany>[]

      Rules that were registered, the optional parameters are filled with values.

• hasListener

  void

  The hasListener function looks like:

  (callback: H) => {...}

  • callback

    H

    Listener whose registration status shall be tested.

  • returns

    boolean

    True if callback is registered to the event.

• hasListeners

  void

  The hasListeners function looks like:

  () => {...}

  • returns

    boolean

    True if any event listeners are registered to the event.

• removeListener

  void

  Deregisters an event listener callback from an event.

  The removeListener function looks like:

  (callback: H) => {...}

  • callback

    H

    Listener that shall be unregistered.

• removeRules

  void

  Unregisters currently registered rules.

  The removeRules function looks like:

  (ruleIdentifiers?: string[], callback?: function) => {...}

  • ruleIdentifiers

    string[] optional

    If an array is passed, only rules with identifiers contained in this array are unregistered.

  • callback

    function optional

    The callback parameter looks like:

    () => void

Rule

Description of a declarative rule for handling events.

Properties

• actions

  any[]

  List of actions that are triggered if one of the conditions is fulfilled.

• conditions

  any[]

  List of conditions that can trigger the actions.

• id

  string optional

  Optional identifier that allows referencing this rule.

• priority

  number optional

  Optional priority of this rule. Defaults to 100.

• tags

  string[] optional

  Tags can be used to annotate rules and perform operations on sets of rules.

UrlFilter

Filters URLs for various criteria. See event filtering. All criteria are case sensitive.

Properties

• cidrBlocks

  string[] optional

  Chrome 123+

  Matches if the host part of the URL is an IP address and is contained in any of the CIDR blocks specified in the array.

• hostContains

  string optional

  Matches if the host name of the URL contains a specified string. To test whether a host name component has a prefix 'foo', use hostContains: '.foo'. This matches 'www.foobar.com' and 'foo.com', because an implicit dot is added at the beginning of the host name. Similarly, hostContains can be used to match against component suffix ('foo.') and to exactly match against components ('.foo.'). Suffix- and exact-matching for the last components need to be done separately using hostSuffix, because no implicit dot is added at the end of the host name.

• hostEquals

  string optional

  Matches if the host name of the URL is equal to a specified string.

• hostPrefix

  string optional

  Matches if the host name of the URL starts with a specified string.

• hostSuffix

  string optional

  Matches if the host name of the URL ends with a specified string.

• originAndPathMatches

  string optional

  Matches if the URL without query segment and fragment identifier matches a specified regular expression. Port numbers are stripped from the URL if they match the default port number. The regular expressions use the RE2 syntax.

• pathContains

  string optional

  Matches if the path segment of the URL contains a specified string.

• pathEquals

  string optional

  Matches if the path segment of the URL is equal to a specified string.

• pathPrefix

  string optional

  Matches if the path segment of the URL starts with a specified string.

• pathSuffix

  string optional

  Matches if the path segment of the URL ends with a specified string.

• ports

  (number | number[])[] optional

  Matches if the port of the URL is contained in any of the specified port lists. For example [80, 443, [1000, 1200]] matches all requests on port 80, 443 and in the range 1000-1200.

• queryContains

  string optional

  Matches if the query segment of the URL contains a specified string.

• queryEquals

  string optional

  Matches if the query segment of the URL is equal to a specified string.

• queryPrefix

  string optional

  Matches if the query segment of the URL starts with a specified string.

• querySuffix

  string optional

  Matches if the query segment of the URL ends with a specified string.

• schemes

  string[] optional

  Matches if the scheme of the URL is equal to any of the schemes specified in the array.

• urlContains

  string optional

  Matches if the URL (without fragment identifier) contains a specified string. Port numbers are stripped from the URL if they match the default port number.

• urlEquals

  string optional

  Matches if the URL (without fragment identifier) is equal to a specified string. Port numbers are stripped from the URL if they match the default port number.

• urlMatches

  string optional

  Matches if the URL (without fragment identifier) matches a specified regular expression. Port numbers are stripped from the URL if they match the default port number. The regular expressions use the RE2 syntax.

• urlPrefix

  string optional

  Matches if the URL (without fragment identifier) starts with a specified string. Port numbers are stripped from the URL if they match the default port number.

• urlSuffix

  string optional

  Matches if the URL (without fragment identifier) ends with a specified string. Port numbers are stripped from the URL if they match the default port number.