chrome.events

Description: The chrome.events namespace contains common types used by APIs dispatching events to notify you when something interesting happens.
Availability: Since Chrome 21.

An Event is an object that allows you to 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(function(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 Web Request API and 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:

      var 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.

      var 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.

Adding 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.

      var rule_list = [rule1, rule2, ...];
      function addRules(rule_list, function callback(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, e.g., 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 currently 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.

Removing 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.

      var rule_ids = ["id1", "id2", ...];
      function removeRules(rule_ids, function callback() {...});
      

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.

Retrieving rules

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

      var rule_ids = ["id1", "id2", ...];
      function getRules(rule_ids, function callback(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

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

    prefer to write

          var rule1 = {...};
          var rule2 = {...};
          chrome.declarativeWebRequest.onRequest.addRules([rule1, rule2]);
  • Prefer substring matching over matching using regular expressions in a events.UrlFilter. Substring based matching is extremely fast.

    Instead of

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

    prefer to write

          var match = new chrome.declarativeWebRequest.RequestMatcher({
              url: {hostSuffix: "example.com", pathContains: "foo"} });
  • If you have many rules that all share the same actions, you may merge the rules into one because 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

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

    prefer to write

          var 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 makes use of a filter won't be invoked for events that don't pass the filter, which makes the listening code more declarative and efficient - an event page page 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 like this:

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

into this:

      chrome.webNavigation.onCommitted.addListener(function(e) {
        // ...
      }, {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.

Summary

Types
Rule
Event
UrlFilter

Types

Rule

Description of a declarative rule for handling events.
properties
string (optional) id

Optional identifier that allows referencing this rule.

array of string (optional) tags

Since Chrome 28.

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

array of any conditions

List of conditions that can trigger the actions.

array of any actions

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

integer (optional) priority

Optional priority of this rule. Defaults to 100.

Event

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

addListener

Event.addListener(function callback)

Registers an event listener callback to an event.

Parameters
function callback

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

The callback parameter should be a function that looks like this:

function() {...};

removeListener

Event.removeListener(function callback)

Deregisters an event listener callback from an event.

Parameters
function callback

Listener that shall be unregistered.

The callback parameter should be a function that looks like this:

function() {...};

hasListener

boolean Event.hasListener(function callback)
Parameters
function callback

Listener whose registration status shall be tested.

The callback parameter should be a function that looks like this:

function() {...};

hasListeners

boolean Event.hasListeners()

addRules

Event.addRules(array of Rule rules, function callback)

Registers rules to handle events.

Parameters
array of Rule rules

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

function (optional) callback

Called with registered rules.

If you specify the callback parameter, it should be a function that looks like this:

function(array of Rule rules) {...};
array of Rule rules

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

getRules

Event.getRules(array of string ruleIdentifiers, function callback)

Returns currently registered rules.

Parameters
array of string (optional) ruleIdentifiers

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

function callback

Called with registered rules.

The callback parameter should be a function that looks like this:

function(array of Rule rules) {...};
array of Rule rules

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

removeRules

Event.removeRules(array of string ruleIdentifiers, function callback)

Unregisters currently registered rules.

Parameters
array of string (optional) ruleIdentifiers

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

function (optional) callback

Called when rules were unregistered.

If you specify the callback parameter, it should be a function that looks like this:

function() {...};

UrlFilter

Filters URLs for various criteria. See event filtering. All criteria are case sensitive.
properties
string (optional) hostContains

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.

string (optional) hostEquals

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

string (optional) hostPrefix

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

string (optional) hostSuffix

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

string (optional) pathContains

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

string (optional) pathEquals

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

string (optional) pathPrefix

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

string (optional) pathSuffix

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

string (optional) queryContains

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

string (optional) queryEquals

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

string (optional) queryPrefix

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

string (optional) querySuffix

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

string (optional) urlContains

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.

string (optional) urlEquals

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.

string (optional) urlMatches

Since Chrome 23.

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.

string (optional) originAndPathMatches

Since Chrome 28.

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.

string (optional) urlPrefix

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.

string (optional) urlSuffix

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.

array of string (optional) schemes

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

array of integer or array of integer (optional) ports

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.