chrome.tabs.onCreated.addListener(function(tab) {
  appendToLog('tabs.onCreated --'
              + ' window: ' + tab.windowId
              + ' tab: '    + tab.id
              + ' index: '  + tab.index
              + ' url: '    + tab.url);
});

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 possibly further extension APIs in the future. 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: $ref:events.Event.addRules, $ref:events.Event.removeRules, and $ref: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 $ref: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 $ref: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 $ref: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 $ref:events.UrlFilter, except for scheme and port matching.