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------------------------ How to develop a filter in AXEL  -------------------------
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St├ęphane Sire
Last update: 2012-12-03

Summary
=======
Filter functionalities
======================

Filters are objects that can be "applied" on a primitive editor plugin to replace some of its methods with new methods implemented by the filter.

Filters are declared on the "filter" parameter of the xt:use element. The example below applies the 'wiki' filter and then the 'capitalize' filter on the 'text' primitive editor.
<xt:use types='text' param="filter=wiki capitalize"/>
The declaration order of the filter element is significant as filtered plugin methods will be redefined applying filter methods in declaration order (from left to right). Hence if both 'wiki' and 'capitalize' filters redefine the 'onLoad' method, this is the 'onLoad' method of the capitalize filter that will be called with the declaration order above. The choice of which methods to filter depends on the desired effects. Basically the filter can belong to one of the following categories: - category A : filters with no side effect on the edited content, such filters will generally look for some specific data inside the content model and forward it to other components; such filters may build and maintain a toc, generate collaboration events outside of the application, etc. (e.g. the 'event' filter that generates a custom DOM event for some specific life cycle methods call) - category B : some filters that change the user text entry but not the handle nor the target content model; such filters may do some small typographical corrections such as capitalizing some words, expanding some abbreviations, proposing some contextual input aids while typing, etc. (e.g. to implement autocomplete) - category C : some filters that change the handle and/or the target content model; such filter can improve the presentation of the edited content (e.g. displaying a video player if the input is a video URL), they can also allow to edit sub-content model using special rules to interpret the input (e.g. the 'wiki' filter in 'wiki.js', the 'date' filter that dynamically replaces the input device of a 'text' editor with a datepicker device to select a date)
Filter registration
===================

The $axel.filter.register method takes as parameter a filter identifier, a hash with a set of options applying to the plugin, a hash with a list of parameters with their default values, and the filter methods.
$axel.filter.register( 'identifier', { chain : ['names'] } | null, // option hash { key : value } | null // parameter hash _FilterMethods // method hash );
The option hash only supports a single 'chain' option. The 'chain' option value is an array of method names as explained in the section "Creating delegation method call chains". You can replace the hash with a 'null' value if the filter does not chain method calls. The parameter hash defines the default values for the extra parameters which can be declared on the param attribute of the xt:use or xt:attribute element holding the filter's declaration. This is explained in the "Filter parameters" section. The method hash declares the filter's methods inside the following structure : - overriden life cycle methods are stored under first level keys of the hash ("onInit", "onAwake", "onLoad", "onSave") - overriden native plugin methods are stored under a first level "api" key that must contain a hash with the methods - all other overriden methods (or filter new methods) are stored under a first level "methods" key that must contain a hash with the methods you can omit a method hash entry if you do not define any method within that entry. The "this" object inside overriden methods will refer to the plugin instance. Note that it is always possible to use a closure to define filter's private variables and methods. According to the specification above, there are most chance that Category A filters can be shared on many different primitive editors. Category B may eventually be shared on all the primitive editor plugins that define a very similar entry field (e.g. a text entry field). Category C are most probably specific to a given primitive editor plugin because they depend on the structure of its handle.
Filter application
==================

In addition to being registered, the filter must be applied to a plugin before a template author can use it inside a template. This decoupling between registration and application allows to share filter's code between different plugins.
$axel.filter.applyTo({'name' : 'plugin'}); // applies 'name' filter to 'plugin' class or $axel.filter.applyTo({'name' : ['plugin1', 'plugin2', ...]}); // applies 'name' filter to several 'plugin' classes
The $axel.filter.applyTo method takes as parameter a hash where each key must be a previously registered filter. The value associated with the key is either a string identifier representing the target plugin, or a list of string identifiers representing the target plugins. The second form is a syntactic sugar to apply the filter to several plugins. The filter application must be declared after the filter registration, otherwise the filter identifier will not be recognized. It must be declared after the plugin's registration for the same reason. Filters are usually applied to their target plugins directly within the filter source file, however there are some situations where you may need to declare it elsewhere due to the loading order of source files. Note that we are planning a deferred mechanism to become independent of load order in the future.
Creating delegation method call chains
======================================

It is the responsability of a filter to call the previous filter's method (or ultimately the original plugin's method if  the filter is the first in the list or the only one). For that purpose, when registering the filter you must declare the names of the plugin methods that you wish to chain in a 'chain' parameter (see the filter file skeleton below).

As a consequent AXEL will create a new method called '__name__method' where "name" is the name of the filter, and "method" is the name of the original method, and it will remap that method to the original one. This way you will be able to invoke the original method with the "this" object from the method that overrides it (without remapping method names there would be an infinite loop).

For instance if you register the hypothetical 'capitalize' filter like this (where _CapitalizeFilter declares an 'update' method to override the original one) :

$axel.filter.register( 'capitalize', { chain : [ 'update' ] }, null _CapitalizeFilter );
then the new 'update' method in your capitalize filter will be able to call the original 'update' method of the plugin (or of the preceeding filter if it has already overridden it) as '__capitalize__update'. There is no rule to determine where to call the predecessor's method, but you must pay lot of attention to potential side effects of different positions. You must also be careful that some filters, usually the one that change the handle and/or the target content model, must break the delegation method call chains and not perpetuate the call to the overriden method. That is the case each time the extended functionalities are incompatible with the default behavior of the plugin. Such filter must document it so that template authors know they cannot put filters before these filters in a filter chain. This delegation pattern is inspired from Alex Russell's blog [1].
Filter parameters
=================

As each plugin can define parameters which can be used by the template author to configure the plugin, each filter can define some extra parameters to configure the filter. These parameters and their default values are declared when registering the filter (see section on "Filter registration").

It is highly recommended to prefix the name of the filter's parameters with 'filter_' where "filter" is the name of the filter. This should avoid parameter name collisions between plugins and filters.

As for the plugin's parameters, the filter's parameters can be retrieved with the native "getParam" method of the plugin object.
Scope of Filters
================

The filter delegate can redefine some methods of the plugin object it is applied to. It can also create new methods inside the filtered object. In terms of object-oriented programming a filtered object is a mixin between the plugin object and the filter object.

The current implementation of filters generates a new mixin class for each combination of plugin and filter chain declaration that replaces the original plugin class. Hence if a filter needs to associate state information with an instance, it can store this data either inside the plugin instance (using the 'this' context object but being careful to avoid property name collisions), or it can maintain it in some private cache as each plugin instance is associated with a unique key identifier accessible with the 'getUniqueKey' method.
Filter file skeleton
====================

The code below can be used as a template for writing a new filter. Note that in all the filter methods, the this object will refer to the filtered plugin instance.

(function ($axel) { // you may use the closure to declare private objects and methods here var _Filter = { ///////////////////////////////////////////////////////// // Overriden Plugin Life cycle methods // // (declare only the methods that you really override) // ///////////////////////////////////////////////////////// onInit : function ( aDefaultData, anOptionAttr, aRepeater ) { }, onAwake : function () { }, onLoad : function (aPoint, aDataSrc) { }, onSave : function (aLogger) { }, ////////////////////////////////////// // Overridden native plugin methods // // (should be rare...) // ////////////////////////////////////// api : { // see plugin howto }, ////////////////////////////////////////////////////// // Overriden specific plugin methods or new methods // ////////////////////////////////////////////////////// methods : { // see plugin howto for most common methods // see each individual plugin for the others } }; $axel.filter.register( 'name', { chain : [ 'name', 'of', 'methods', 'to', 'chain'] }, { key : 'value' }, _Filter ); $axel.filter.applyTo({'name' : 'plugin'}); // applies 'name' filter to 'plugin' class }($axel));
References
==========

[1] http://alex.dojotoolkit.org/2008/10/delegate-delegate-delegate/