Architecture Overview
=====================

The address bar is implemented as a *model-view-controller* (MVC) system. One of
the scopes of this architecture is to allow easy replacement of its components,
for easier experimentation.

Each search is represented by a unique object, the *UrlbarQueryContext*. This
object, created by the *View*, describes the search and is passed through all of
the components, along the way it gets augmented with additional information.
The *UrlbarQueryContext* is passed to the *Controller*, and finally to the
*Model*.  The model appends results to a property of *UrlbarQueryContext* in
chunks, it sorts them through a *Muxer* and then notifies the *Controller*.

See the specific components below, for additional details about each one's tasks
and responsibilities.


The UrlbarQueryContext
----------------------

The *UrlbarQueryContext* object describes a single instance of a search.
It is augmented as it progresses through the system, with various information:

.. code:: JavaScript

  UrlbarQueryContext {
    allowAutofill; // {boolean} If true, providers are allowed to return
                   // autofill results.  Even if true, it's up to providers
                   // whether to include autofill results, but when false, no
                   // provider should include them.
    isPrivate; // {boolean} Whether the search started in a private context.
    maxResults; // {integer} The maximum number of results requested. It is
                // possible to request more results than the shown ones, and
                // do additional filtering at the View level.
    searchString; // {string} The user typed string.
    userContextId; // {integer} The user context ID (containers feature).

    // Optional properties.
    muxer; // {string} Name of a registered muxer. Muxers can be registered
           // through the UrlbarProvidersManager.
    providers; // {array} List of registered provider names. Providers can be
               // registered through the UrlbarProvidersManager.
    sources: {array} list of accepted UrlbarUtils.RESULT_SOURCE for the context.
            // This allows to switch between different search modes. If not
            // provided, a default will be generated by the Model, depending on
            // the search string.
    engineName: // {string} if sources is restricting to just SEARCH, this
                // property can be used to pick a specific search engine, by
                // setting it to the name under which the engine is registered
                // with the search service.
    currentPage: // {string} url of the page that was loaded when the search
                 // began.
    prohibitRemoteResults:
      // {boolean} This provides a short-circuit override for
      // context.allowRemoteResults(). If it's false, then allowRemoteResults()
      // will do its usual checks to determine whether remote results are
      // allowed. If it's true, then allowRemoteResults() will immediately
      // return false. Defaults to false.

    // Properties added by the Model.
    results; // {array} list of UrlbarResult objects.
    tokens; // {array} tokens extracted from the searchString, each token is an
            // object in the form {type, value, lowerCaseValue}.
  }


The Model
---------

The *Model* is the component responsible for retrieving search results based on
the user's input, and sorting them accordingly to their importance.
At the core is the `UrlbarProvidersManager <https://searchfox.org/mozilla-central/source/browser/components/urlbar/UrlbarProvidersManager.sys.mjs>`_,
a component tracking all the available search providers, and managing searches
across them.

The *UrlbarProvidersManager* is a singleton, it registers internal providers on
startup and can register/unregister providers on the fly.
It can manage multiple concurrent queries, and tracks them internally as
separate *Query* objects.

The *Controller* starts and stops queries through the *UrlbarProvidersManager*.
It's possible to wait for the promise returned by *startQuery* to know when no
more results will be returned, it is not mandatory though.
Queries can be canceled.

.. note::

  Canceling a query will issue an interrupt() on the database connection,
  terminating any running and future SQL query, unless a query is running inside
  a *runInCriticalSection* task.

The *searchString* gets tokenized by the `UrlbarTokenizer <https://searchfox.org/mozilla-central/source/browser/components/urlbar/UrlbarTokenizer.sys.mjs>`_
component into tokens, some of these tokens have a special meaning and can be
used by the user to restrict the search to specific result type (See the
*UrlbarTokenizer::TYPE* enum).

.. caution::

  The tokenizer uses heuristics to determine each token's type, as such the
  consumer may want to check the value before applying filters.

.. code:: JavaScript

  UrlbarProvidersManager {
    registerProvider(providerObj);
    unregisterProvider(providerObj);
    registerMuxer(muxerObj);
    unregisterMuxer(muxerObjOrName);
    async startQuery(queryContext);
    cancelQuery(queryContext);
    // Can be used by providers to run uninterruptible queries.
    runInCriticalSection(taskFn);
  }

UrlbarProvider
~~~~~~~~~~~~~~

A provider is specialized into searching and returning results from different
information sources. Internal providers are usually implemented in separate
*sys.mjs* modules with a *UrlbarProvider* name prefix. External providers can be
registered as *Objects* through the *UrlbarProvidersManager*.
Each provider is independent and must satisfy a base API, while internal
implementation details may vary deeply among different providers.

.. important::

  Providers are singleton, and must track concurrent searches internally, for
  example mapping them by UrlbarQueryContext.

.. note::

  Internal providers can access the Places database through the
  *PlacesUtils.promiseLargeCacheDBConnection* utility.

.. code:: JavaScript

  class UrlbarProvider {
    /**
     * Unique name for the provider, used by the context to filter on providers.
     * Not using a unique name will cause the newest registration to win.
     * @abstract
     */
    get name() {
      return "UrlbarProviderBase";
    }
    /**
     * The type of the provider, must be one of UrlbarUtils.PROVIDER_TYPE.
     *
     * @returns {Values<typeof UrlbarUtils.PROVIDER_TYPE>}
     * @abstract
     */
    get type() {
      throw new Error("Trying to access the base class, must be overridden");
    }
    /**
     * Whether this provider should be invoked for the given context.
     * If this method returns false, the providers manager won't start a query
     * with this provider, to save on resources.
     *
     * @param {UrlbarQueryContext} queryContext
     *   The query context object
     * @param {UrlbarController} controller
     *   The current controller.
     * @returns {Promise<boolean>}
     *   Whether this provider should be invoked for the search.
     * @abstract
     */
    isActive(queryContext) {
      throw new Error("Trying to access the base class, must be overridden");
    }
    /**
     * Gets the provider's priority.  Priorities are numeric values starting at
     * zero and increasing in value.  Smaller values are lower priorities, and
     * larger values are higher priorities.  For a given query, `startQuery` is
     * called on only the active and highest-priority providers.
     * @param {UrlbarQueryContext} queryContext The query context object
     * @returns {number} The provider's priority for the given query.
     * @abstract
     */
    getPriority(queryContext) {
      // By default, all providers share the lowest priority.
      return 0;
    }
    /**
     * Starts querying.
     * @param {UrlbarQueryContext} queryContext The query context object
     * @param {function} addCallback Callback invoked by the provider to add a new
     *        result. A UrlbarResult should be passed to it.
     * @note Extended classes should return a Promise resolved when the provider
     *       is done searching AND returning results.
     * @abstract
     */
    startQuery(queryContext, addCallback) {
      throw new Error("Trying to access the base class, must be overridden");
    }
    /**
     * Cancels a running query,
     * @param {UrlbarQueryContext} queryContext The query context object to cancel
     *        query for.
     * @abstract
     */
    cancelQuery(queryContext) {
      throw new Error("Trying to access the base class, must be overridden");
    }
  }

UrlbarMuxer
~~~~~~~~~~~

The *Muxer* is responsible for sorting results based on their importance and
additional rules that depend on the UrlbarQueryContext. The muxer to use is
indicated by the UrlbarQueryContext.muxer property.

.. caution::

  The Muxer is a replaceable component, as such what is described here is a
  reference for the default View, but may not be valid for other implementations.

.. code:: JavaScript

  class UrlbarMuxer {
    /**
     * Unique name for the muxer, used by the context to sort results.
     * Not using a unique name will cause the newest registration to win.
     * @abstract
     */
    get name() {
      return "UrlbarMuxerBase";
    }
    /**
     * Sorts UrlbarQueryContext results in-place.
     * @param {UrlbarQueryContext} queryContext the context to sort results for.
     * @abstract
     */
    sort(queryContext) {
      throw new Error("Trying to access the base class, must be overridden");
    }
  }


The Controller
--------------

`UrlbarController <https://searchfox.org/mozilla-central/source/browser/components/urlbar/UrlbarController.sys.mjs>`_
is the component responsible for reacting to user's input, by communicating
proper course of action to the Model (e.g. starting/stopping a query) and the
View (e.g. showing/hiding a panel). It is also responsible for reporting Telemetry.

.. note::

  Each *View* has a different *Controller* instance.

.. code:: JavaScript

  UrlbarController {
    async startQuery(queryContext);
    cancelQuery(queryContext);
    // Invoked by the ProvidersManager when results are available.
    receiveResults(queryContext);
    // Used by the View to listen for results.
    addListener(listener);
    removeListener(listener);
  }


The View
--------

The View is the component responsible for presenting search results to the
user and handling their input.

.. caution

  The View is a replaceable component, as such what is described here is a
  reference for the default View, but may not be valid for other implementations.

`UrlbarInput.sys.mjs <https://searchfox.org/mozilla-central/source/browser/components/urlbar/UrlbarInput.sys.mjs>`_
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Implements an input box *View*, owns an *UrlbarView*.

.. code:: JavaScript

  UrlbarInput {
    constructor(options = { textbox, panel });
    // Uses UrlbarValueFormatter to highlight the base host, search aliases
    // and to keep the host visible on overflow.
    formatValue(val);
    openResults();
    // Converts an internal URI (e.g. a URI with a username or password) into
    // one which we can expose to the user.
    makeURIReadable(uri);
    // Handles an event which would cause a url or text to be opened.
    handleCommand();
    // Called by the view when a result is selected.
    resultsSelected();
    // The underlying textbox
    textbox;
    // The results panel.
    panel;
    // The containing window.
    window;
    // The containing document.
    document;
    // An UrlbarController instance.
    controller;
    // An UrlbarView instance.
    view;
    // Whether the current value was typed by the user.
    valueIsTyped;
    // Whether the context is in Private Browsing mode.
    isPrivate;
    // Whether the input box is focused.
    focused;
    // The go button element.
    goButton;
    // The current value, can also be set.
    value;
  }

`UrlbarView.sys.mjs <https://searchfox.org/mozilla-central/source/browser/components/urlbar/UrlbarView.sys.mjs>`_
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Represents the base *View* implementation, communicates with the *Controller*.

.. code:: JavaScript

  UrlbarView {
    // Manage View visibility.
    open();
    close();
    // Invoked when the query starts.
    onQueryStarted(queryContext);
    // Invoked when new results are available.
    onQueryResults(queryContext);
    // Invoked when the query has been canceled.
    onQueryCancelled(queryContext);
    // Invoked when the query is done. This is invoked in any case, even if the
    // query was canceled earlier.
    onQueryFinished(queryContext);
    // Invoked when the view opens.
    onViewOpen();
    // Invoked when the view closes.
    onViewClose();
  }


UrlbarResult
------------

An `UrlbarResult <https://searchfox.org/mozilla-central/source/browser/components/urlbar/UrlbarResult.sys.mjs>`_
instance represents a single search result with a result type, that
identifies specific kind of results.
Each kind has its own properties, that the *View* may support, and a few common
properties, supported by all of the results.

.. note::

  Result types are also enumerated by *UrlbarUtils.RESULT_TYPE*.

.. code-block:: JavaScript

  UrlbarResult {
    constructor(resultType, payload);

    type: {integer} One of UrlbarUtils.RESULT_TYPE.
    source: {integer} One of UrlbarUtils.RESULT_SOURCE.
    title: {string} A title that may be used as a label for this result.
    icon: {string} Url of an icon for this result.
    payload: {object} Object containing properties for the specific RESULT_TYPE.
    autofill: {object} An object describing the text that should be
              autofilled in the input when the result is selected, if any.
    autofill.value: {string} The autofill value.
    autofill.selectionStart: {integer} The first index in the autofill
                             selection.
    autofill.selectionEnd: {integer} The last index in the autofill selection.
    suggestedIndex: {integer} Suggest a preferred position for this result
                    within the result set. Undefined if none.
    isSuggestedIndexRelativeToGroup: {boolean} Whether the suggestedIndex
                                     property is relative to the result's group
                                     instead of the entire result set.
  }

The following RESULT_TYPEs are supported:

.. code:: JavaScript

    // An open tab.
    // Payload: { icon, url, userContextId }
    TAB_SWITCH: 1,
    // A search suggestion or engine.
    // Payload: { icon, suggestion, keyword, query, providesSearchMode, inPrivateWindow, isPrivateEngine }
    SEARCH: 2,
    // A common url/title tuple, may be a bookmark with tags.
    // Payload: { icon, url, title, tags }
    URL: 3,
    // A bookmark keyword.
    // Payload: { icon, url, keyword, postData }
    KEYWORD: 4,
    // A WebExtension Omnibox result.
    // Payload: { icon, keyword, title, content }
    OMNIBOX: 5,
    // A tab from another synced device.
    // Payload: { icon, url, device, title }
    REMOTE_TAB: 6,
    // An actionable message to help the user with their query.
    // Payload: { buttons, helpL10n, helpUrl, icon, titleL10n, type }
    TIP: 7,
    // A type of result which layout is defined at runtime.
    // Payload: { dynamicType }
    DYNAMIC: 8,
