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[mkws-moved-to-github.git] / doc / mkws-developer.markdown

% The MasterKey Widget Set developer's guide
% Mike Taylor
% November 2014


Introduction
============

This manual is for people who want to build the widget set from
source, develop the widget set's core code, or (more likely) create
their own widgets as extensions to the main set.

Those who want to use existing widgets should read
[The MKWS manual: embedded metasearching with the MasterKey Widget
Set](mkws-manual.html) instead.


Required development tools
==========================

If you are building the widget set, you will need the following Debian
packages (or their equivalents on your operating system):

        $ sudo apt-get install curl git make unzip apache2 pandoc

You also need Node.js, but unfortunately the `node-js` package is not
available for Debian wheezy. You can either get it from
wheezy-backports or download the source from
http://nodejs.org/download/ and build it yourself. You need both Node
itself and its package manager NPM: `make install` puts them into
`/usr/local/bin`.


Concepts
========

Code structure
--------------

The code of the widget set is in four main layers, described here from
the bottom up:

1. The core code, which manages the set of widget teams, default
options, authentication onto the Service Proxy, and the creation of
widgets from HTML elements.
This code is in `mkws-core.js`

2. The team code, which manages teams of widgets. This is responsible
for the collections of widgets that make up teams, event queues, and
handling search-and-retrieval events
This code is in `mkws-team.js`

3. The generic widget code, which handles the creation of widget
objects, parsing configuration attributes from their HTML elements,
and firing off automatic searches.

4. The code for individual widgets, which is specific to those
widgets. It often involves subscribing to events and responding to
them by setting the HTML of the widget element, but need not do
so. The code for many of the most important widgets is in
`mkws-widget-main.js`, but certain other widgets are defined in other
files beginning with the prefix `mkws-widget-`.

In addition to this code, there are several source files containing
support code:

* `mkws-filter.js` contains support routine implementing the
filter-set data structure, which contains information about which
filters (e.g. by target, or by facet) are in force.

* `mkws-handlebars.js` contains Handlebars helpers which can be used
by the HTML templates.

* `mkws-popup.js` defines a special widget for creating popup
  windows. These may, but need not, contain other MKWS widgets,
  forming a popup searching application.

The final component of the source code is the set of Handlebars
templates, in the `templates` directory, which are used to emit the
HTML of the various widgets' contents. These are compiled into the
file `mkws-templates.js`.



Event passing
-------------

The primary method of communication between components of the widget
set -- specifically, between teams and their widgets -- is event
passing. Widgets subscribe to named events; when something relevant
happens (such as the reception of a message from metasearch
middleware), the event is published, along with the relevant data. All
widgets that susbcribed to the event are then notified, and can take
appropriate action.

Different kinds of events have different data associated with
them. This data is passed when the event is published, and so is made
available to the subscribing code.

The possible events, and their associated data, are described
[below](#events).


Defining new types of widget
----------------------------

Development with MKWS consists primarily of defining new types of
widgets. This is done using exactly the same API as the the widgets
that come as part of the set: they have no privileged access.

You create a new widget type by calling the `mkws.registerWidgetType`
function, passing in the widget name and a function. The name is used
to recognise HTML elements as being widgets of this type -- for
example, if you register a `foo` widget, elements like
`<div class="mkws-foo">` will become widgets of this type.

The function promotes a bare widget object (which is created by the
core widget code and passed in as `this`) into a
widget of the appropriate type. MKWS doesn't use classes or explicit
prototypes: it just makes objects that have the necessary
behaviours. There are _no_ behaviours that Widgets are obliged to
provide: you can make a doesn't-do-anything-at-all widget if you like:

        mkws.registerWidgetType('sluggard', function() {});

More commonly, widgets will subscribe to one or more events, so that
they're notified when something interesting happens. For example, the
`log` widget asks to be notified when a `log` event happens, and
appends the logged message to its node, as follows:

        mkws.registerWidgetType('log', function() {
          var that = this;

          this.team.queue("log").subscribe(function(teamName, timestamp, message) {
            $(that.node).append(teamName + ": " + timestamp + message + "<br/>");
          });
        });

This simple widget illustrates several important points:

* The base widget object (`this`) has several baked-in properties and
  methods that are available to individual widgets. These include
  `this.team` (the team that this widget is a part of) and `this.node`
  (the DOM element of the widget). See below for a full list.

* The team object (`this.team`) also has baked-in properties and
  methods. These include the `queue` function, which takes an event-name
  as its argument. See below for a full list.

* You can add functionality to a widget by subscribing it to an
  event's queue using `this.team.queue("EVENT").subscribe`. The
  argument is a function which is called whenever the event is
  published.

* As with so much JavaScript programming, the value of the special
  variable `this` is lost inside the `subscribe` callback function,
  so it must be saved if it's to be used inside that callback
  (typically as a local variable named `that`).


Widget specialisation (inheritance)
-----------------------------------

Many widgets are simple specialisations of existing widgets. For
example, the `images` widget is the same as the `records` widget
except that it defaults to using the `images` template for displaying
its result list. It's defined as follows:

        mkws.registerWidgetType('images', function() {
          mkws.promotionFunction('records').call(this);
          if (!this.config.template) this.config.template = 'images';
        });

Remember that when a promotion function is called, it's passed a base
widget object that's not specialised for any particular task. To make
a specialised widget, you first promote that base widget into the type
that you want to specialise from -- in this case, `Records` -- using
the promotion function that's been registered for that type.

Once this has been done, the specialisations can be introduced. In
this case, it's a very simple matter of changing the `template`
configuration setting to `'images'` unless it's already been given an
explicit value. (That would occur if the HTML used an element like
`<div class="mkws-images" template="my-images">` to use a customised
template.


Reference Guide
===============


Widget properties and methods
-----------------------------

The following properties and methods exist in the bare widget object
that is passed into `registerWidgetType`'s callback function, and can
be used by the derived widget.

* `String this.type` --
        A string containing the type of the widget (`search`,
        `switch`, etc.)

* `Team this.team` --
        The team object to which this widget belongs. The team has
        several additional important properties and methods, described
        below.

* `DOMElement this.node` --
        The DOM element of the widget. Most often used for inserting
        HTML into the widget element.

* `Hash this.config` --
        A table of configuration values for the widget. This table
        inherits missing values from the team's configuration, which
        in turn inherits from the top-level MKWS configuration, which
        inherits from the default configuration. Instances of widgets
        in HTML can set configuration items as HTML attributes: for
        example, the HTML element
        `<div class="mkwsRecords" maxrecs="10">`
        creates a widget for which `this.config.maxrecs` is set to 10.

* `String this.toString()` --
        A function returning a string that briefly names this
        widget. Can be useful in logging.

* `Void this.log(string)` --
        A function to log a string for debugging purposes. The string
        is written on the browser console, and also published to any
        subcribers to the `log` event.

* `String this.value()` --
        A function returning the value of the widget's HTML element.

* `VOID autosearch()` --
        Registers that this kind of widget is one that requires an
        automatic search to be run for it if an `autosearch` attribute
        is provided on the HTML element. This is appropriate for
        widgets such as `Records` and `Facet` that display some part
        of a search result.

* `subwidget(type, overrides, defaults)` --
        Returns the HTML of a subwidget of the specified type, which
        can then be inserted into the widget using the
        `this.node.html` function. The subwidget is given the same
        attributes at the parent widget that invokes this function,
        except where overrides are passed in. If defaults are also
        provided, then these are used when the parent widget provides
        no values. Both the `overrides` and `defaults` arguments are
        hashes: the latter is optional. This can be used to assemble
        compound widgets containing several subwidgets.

In addition to these properties and methods of the bare widget object,
some kinds of specific widget add other properties of their own. For
example, the `builder` widget uses a `callback` property as the
function that it use to publish the widget definition that it
constructs. This defaults to the builtin function `alert`, but can be
overridden by derived widgets such as `console-builder`.


Team methods
------------

Since the team object is supposed to be opaque to widgets, all access
is via the following API methods rather than direct access to
properties.

* `String team.name()`
* `Bool team.submitted()`
* `Num team.perpage()`
* `Num team.totalRecordCount()`
* `Num team.currentPage();`
* `String team.currentRecordId()`
* `String team.currentRecordData()`

These are all simple accessor functions that provide the ability to
read properties of the team. `submitted` is initially false, then
becomes true when the first search is submitted (manually or
automatically).

* `Array team.filters()` --
        Another accessor function, providing access to the array of
        prevailing filters (which narrow the search results by means
        of Pazpar2 filters and limits). This is really too complicated
        an object for the widgets to be given access to, but it's
        convenient to do it this way. If you have a reason for using
        this, see the `Navi` widget, which is the only place it's used.

* `Bool team.targetFiltered(targetId)` --
        Indicates whether the specified target has been filtered by
        selection as a facet. This is used only by the `Facet` widget,
        and there is probably no reason for you to use it.

* `Hash team.config()` --
        Access to the team's configuration settings. There is
        rarely a need to use this: the settings that haven't
        been overridden are accessible via `this.config`.

* `Void team.set_sortOrder(string)`, `Void team.set_perpage(number)` --
        "Setter" functions for the team's `sortOrder` and `perpage`
        functions. Unlikely to be needed outside of the `Sort` and
        `Perpage` widgets.

* `Queue team.queue(eventName)` --
        Returns the queue associated with the named event: this can be
        used to subscribe to the event (or more rarely to publish
        it). See [the section on events, below](#events).

* `Void team.newSearch(query, sortOrder, maxrecs, perpage, limit, targets, targetfilter)` --
        Starts a new search with the specified parameters. All but the
        query may be omitted, in which case the prevailing defaults
        are used. The meanings of the parameters are those of the
        same-named [configuration
        settings](mkws-manual.html#configuration-settings) described in
        the user's manual.

* `Void team.reShow()` --
        Using the existing search, re-shows the result records after a
        change in sort-order, per-page count, etc.

* `String team.recordElementId(recordId)` --
        Utility function for converting a record identifer (returned
        from Pazpar2) into a version suitable for use as an HTML
        element ID.

* `String team.renderDetails(recordData)` --
        Utility function returns an HTML rendering of the record
        represented by the specified data.

* `Template team.loadTemplate(templateName)` --
        Loads (or retrieves from cache) the named Handlebars template,
        and returns it in a form that can be invoked as a function,
        passed a data-set.

Some of these methods are arguably too low-level and should not be
exposed; others should probably be widget-level methods. The present
infelicities should be fixed in future releases, but backwards
compatibility with the present API will be maintained for at least one
complete major-release cycle.


Events
------

The following events are generated by the widget-set code:

* `authenticated` (authName, realm) --
        When authentication is completed successfully, this event is
        published to _all_ teams. Two parameters are passed: the
        human-readable name of the library that has been authenticated
        onto, and the correponding machine-readable library ID.

* `ready` --
        Published to _all_ teams when they are ready to search. No
        parameters are passed. This event is used to implement
        automatic searching, and should probably not be used by
        application code.

* `stat` (data) --
        Published to a team when a `stat` response is reveived from
        Pazpar2. The data from the response is passed as a parameter.

* `firstrecords` (hitcount) --
        Published to a team when the first set of records is found by
        a search. The number of records found (so far) is passed as
        the parameter.

* `complete` (hitcount) --
        Published to a team when a search is complete, and no more
        records will be found (i.e. all targets have either responded
        or failed with an error). The final number of records found is
        passed as the parameter.

* `targets` (data) --
        Published to a team when a `bytarget` response is reveived from
        Pazpar2. The data from the response is passed as a parameter.

* `facets` (data) --
        Published to a team when a `term` response is reveived from
        Pazpar2. The data from the response is passed as a parameter.

* `pager` (data) --
        Published to a team when a `show` response is reveived from
        Pazpar2. The data from the response is passed as a
        parameter. This event is used to update the pager, showing how
        many records have been found, which page is being displayed,
        etc.

* `records` (data) --
        Also published to a team when a `show` response is reveived
        from Pazpar2. The data from the response is passed as a
        parameter. This event is used to update the displayed records.

* `record` (data) --
        Published to a team when a `record` response is reveived from
        Pazpar2 (i.e. the full data for a single record). The data
        from the response is passed as a parameter.

* `navi` --
        Published to a team when a new search is about to be
        submitted. This is a signal that the navigation area, showing
        which filters are in effect, should be updated. No parameter
        is passed: the event handler should consult `team.filters` to
        see what the prevailing set is.

* `log` (teamName, timestamp, message) --
        Published to a team when a message is about to be logged to
        the console for debugging. Three arguments are passed: the
        name of the team that generated the log message, a timestamp
        string, and the message itself. Note that this event is _not_
        published when the widget-set core code generates a log
        message -- only when a team or a widget does.

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