easejs/lib/class.js

/**
 * Contains basic inheritance mechanism
 *
 *  Copyright (C) 2010 Mike Gerwitz
 *
 *  This file is part of ease.js.
 *
 *  ease.js is free software: you can redistribute it and/or modify it under the
 *  terms of the GNU Lesser General Public License as published by the Free
 *  Software Foundation, either version 3 of the License, or (at your option)
 *  any later version.
 *
 *  This program is distributed in the hope that it will be useful, but WITHOUT
 *  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 *  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License
 *  for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * @author  Mike Gerwitz
 * @package core
 */

var util           = require( __dirname + '/util' ),
    class_builder  = require( __dirname + '/class_builder' )
;


/**
 * This module may be invoked in order to provide a more natural looking class
 * definition mechanism
 *
 * This may not be used to extend existing classes. To extend an existing class,
 * use the class's extend() method. If unavailable (or extending a non-ease.js
 * class/object), use the module's extend() method.
 *
 * @param  {string=}  name  optional name
 * @param  {Object}   def   class definition
 *
 * @return  {Class}  new class
 */
module.exports = function()
{
    var type   = typeof arguments[ 0 ],
        result = null
    ;

    switch ( type )
    {
        // anonymous class
        case 'object':
            result = createAnonymousClass.apply( null, arguments );
            break;

        // named class
        case 'string':
            result = createNamedClass.apply( null, arguments );
            break;

        default:
            // we don't know what to do!
            throw TypeError(
                "Expecting anonymous class definition or named class definition"
            );
    }

    return result;
};


/**
 * Creates a class, inheriting either from the provided base class or the
 * default base class
 *
 * @param  {Object}  base  object to extend (extends Class by default)
 *
 * @return  {Object}  extended class
 */
module.exports.extend = function( base )
{
    return extend.apply( this, arguments );
};


/**
 * Implements an interface or set of interfaces
 *
 * @param  {...Interface}  interfaces  interfaces to implement
 *
 * @return  {Class}  new class containing interface abstractions
 */
module.exports.implement = function()
{
    // implement on empty base
    return createImplement(
        null,
        Array.prototype.slice.call( arguments )
    );
};


/**
 * Determines whether the provided object is a class created through ease.js
 *
 * @param  {Object}  obj  object to test
 *
 * @return  {boolean}  true if class (created through ease.js), otherwise false
 */
module.exports.isClass = function( obj )
{
    obj = obj || {};

    return ( obj.prototype instanceof class_builder.ClassBase )
        ? true
        : false
    ;
};


/**
 * Determines whether the provided object is an instance of a class created
 * through ease.js
 *
 * @param  {Object}  obj  object to test
 *
 * @return  {boolean}  true if instance of class (created through ease.js),
 *                     otherwise false
 */
module.exports.isClassInstance = function( obj )
{
    obj = obj || {};

    return ( obj instanceof class_builder.ClassBase )
        ? true
        : false;
};


/**
 * Determines if the class is an instance of the given type
 *
 * The given type can be a class, interface, trait or any other type of object.
 * It may be used in place of the 'instanceof' operator and contains additional
 * enhancements that the operator is unable to provide due to prototypal
 * restrictions.
 *
 * @param  {Object}  type      expected type
 * @param  {Object}  instance  instance to check
 *
 * @return  {boolean}  true if instance is an instance of type, otherwise false
 */
module.exports.isInstanceOf = class_builder.isInstanceOf;


/**
 * Alias for isInstanceOf()
 *
 * May read better in certain situations (e.g. Cat.isA( Mammal )) and more
 * accurately conveys the act of inheritance, implementing interfaces and
 * traits, etc.
 */
module.exports.isA = module.exports.isInstanceOf;


/**
 * Creates a new anonymous Class from the given class definition
 *
 * @param  {Object}  def  class definition
 *
 * @return  {Class}  new anonymous class
 */
function createAnonymousClass( def )
{
    // ensure we have the proper number of arguments (if they passed in
    // too many, it may signify that they don't know what they're doing,
    // and likely they're not getting the result they're looking for)
    if ( arguments.length > 1 )
    {
        throw Error(
            "Expecting one argument for anonymous Class definition; " +
                arguments.length + " given."
        );
    }

    return extend( def );
}


/**
 * Creates a new named Class from the given class definition
 *
 * @param  {string}  name  class name
 * @param  {Object}  def   class definition
 *
 * @return  {Class}  new named class
 */
function createNamedClass( name, def )
{
    // if too many arguments were provided, it's likely that they're
    // expecting some result that they're not going to get
    if ( arguments.length > 2 )
    {
        throw Error(
            "Expecting at most two arguments for definition of named Class '" +
                name + "'; " + arguments.length + " given."
        );
    }

    // if no definition was given, return a staging object, to apply the name to
    // the class once it is actually created
    if ( def === undefined )
    {
        return createStaging( name );
    }
    // the definition must be an object
    else if ( typeof def !== 'object' )
    {
        throw TypeError(
            "Unexpected value for definition of named Class '" + name +
                "'; object expected"
        );
    }

    // add the name to the definition
    def.__name = name;

    return extend( def );
}


/**
 * Creates a staging object to stage a class name
 *
 * The class name will be applied to the class generated by operations performed
 * on the staging object. This allows applying names to classes that need to be
 * extended or need to implement interfaces.
 *
 * @param  {string}  cname  desired class name
 *
 * @return  {Object}  object staging the given class name
 */
function createStaging( cname )
{
    return {
        extend: function()
        {
            var args = Array.prototype.slice.apply( arguments );

            // extend() takes a maximum of two arguments. If only one
            // argument is provided, then it is to be the class definition.
            // Otherwise, the first argument is the supertype and the second
            // argument is the class definition. Either way you look at it,
            // the class definition is always the final argument.
            //
            // We want to add the name to the definition.
            args[ args.length - 1 ].__name = cname;

            return extend.apply( null, args );
        },

        implement: function()
        {
            // implement on empty base, providing the class name to be used once
            // extended
            return createImplement(
                null,
                Array.prototype.slice.call( arguments ),
                cname
            );
        },
    };
}


/**
 * Creates an intermediate object to permit implementing interfaces
 *
 * This object defers processing until extend() is called. This intermediate
 * object ensures that a usable class is not generated until after extend() is
 * called, as it does not make sense to create a class without any
 * body/definition.
 *
 * @param  {Object}   base    base class to implement atop of, or null
 * @param  {Array}    ifaces  interfaces to implement
 * @param  {string=}  cname   optional class name once extended
 *
 * @return  {Object}  intermediate implementation object
 */
function createImplement( base, ifaces, cname )
{
    // Defer processing until after extend(). This also ensures that implement()
    // returns nothing usable.
    return {
        extend: function()
        {
            var args     = Array.prototype.slice.call( arguments ),
                def      = args.pop(),
                ext_base = args.pop()
            ;

            // if any arguments remain, then they likely misunderstood what this
            // method does
            if ( args.length > 0 )
            {
                throw Error(
                    "Expecting no more than two arguments for extend()"
                );
            }

            // if a base was already provided for extending, don't allow them to
            // give us yet another one (doesn't make sense)
            if ( base && ext_base )
            {
                throw Error(
                    "Cannot override parent " + base.toString() + " with " +
                    ext_base.toString() + " via extend()"
                );
            }

            // if a name was provided, use it
            if ( cname )
            {
                def.__name = cname;
            }

            // If a base was provided when createImplement() was called, use
            // that. Otherwise, use the extend() base passed to this function.
            // If neither of those are available, extend from an empty class.
            ifaces.push( base || ext_base || extend( {} ) );

            return extend.apply( null, [
                implement.apply( this, ifaces ),
                def
            ] );
        },
    };
}


/**
 * Mimics class inheritance
 *
 * This method will mimic inheritance by setting up the prototype with the
 * provided base class (or, by default, Class) and copying the additional
 * properties atop of it.
 *
 * The class to inherit from (the first argument) is optional. If omitted, the
 * first argument will be considered to be the properties list.
 *
 * @return  {Object}  extended class
 */
function extend()
{
    // set up the new class
    var data = class_builder.build.apply( null, arguments ),

        new_class        = data[ 'class' ],
        abstract_methods = data.abstractMethods,
        class_id         = data.classId
    ;

    // important: call after setting prototype
    setupProps( new_class, abstract_methods, class_id );

    // lock down the new class (if supported) to ensure that we can't add
    // members at runtime
    util.freeze( new_class );

    return new_class;
}


/**
 * Implements interface(s) into an object
 *
 * This will copy all of the abstract methods from the interface and merge it
 * into the given object.
 *
 * @param  {Object}        base        base object
 * @param  {...Interface}  interfaces  interfaces to implement into dest
 *
 * @return  {Object}  destination object with interfaces implemented
 */
var implement = function()
{
    var args = Array.prototype.slice.call( arguments ),
        dest = {},
        base = args.pop(),
        len  = args.length,
        arg  = null,

        abstract_list = [],
        implemented   = [];

    // add each of the interfaces
    for ( var i = 0; i < len; i++ )
    {
        arg = args[ i ];

        // copy all interface methods to the class (does not yet deep copy)
        util.propParse( arg.prototype, {
            method: function( name, func, is_abstract, keywords )
            {
                dest[ name ] = func;
            },
        } );
        implemented.push( arg );
    }

    // create a new class with the implemented abstract methods
    var class_new = module.exports.extend( base, dest );
    class_builder.getMeta( class_new ).implemented = implemented;

    return class_new;
}


/**
 * Sets up common properties for the provided function (class)
 *
 * @param  {function()}      func              function (class) to set up
 * @param  {Array.<string>}  abstract_methods  list of abstract method names
 * @param  {number}          class_id          unique id to assign to class
 *
 * @return  {undefined}
 */
function setupProps( func, abstract_methods, class_id )
{
    attachAbstract( func, abstract_methods );
    attachExtend( func );
    attachImplement( func );
    attachId( func, class_id );
}


/**
 * Attaches isAbstract() method to the class
 *
 * @param  {Function}  func     function (class) to attach method to
 * @param  {Array}     methods  abstract method names
 *
 * @return  {undefined}
 */
function attachAbstract( func, methods )
{
    var is_abstract = ( methods.__length > 0 ) ? true: false;

    /**
     * Returns whether the class contains abstract methods (and is therefore
     * abstract)
     *
     * @return  {Boolean}  true if class is abstract, otherwise false
     */
    util.defineSecureProp( func, 'isAbstract', function()
    {
        return is_abstract;
    });
}


/**
 * Attaches extend method to the given function (class)
 *
 * @param  {Function}  func  function (class) to attach method to
 *
 * @return  {undefined}
 */
function attachExtend( func )
{
    /**
     * Shorthand for extending classes
     *
     * This method can be invoked on the object, rather than having to call
     * Class.extend( this ).
     *
     * @param  {Object}  props  properties to add to extended class
     *
     * @return  {Object}  extended class
     */
    util.defineSecureProp( func, 'extend', function( props )
    {
        return extend( this, props );
    });
}


/**
 * Attaches implement method to the given function (class)
 *
 * Please see the implement() export of this module for more information.
 *
 * @param  {function()}  func  function (class) to attach method to
 *
 * @return  {undefined}
 */
function attachImplement( func )
{
    util.defineSecureProp( func, 'implement', function()
    {
        return createImplement(
            func,
            Array.prototype.slice.call( arguments )
        );
    });
}


/**
 * Attaches the unique id to the class and its prototype
 *
 * The unique identifier is used internally to match a class and its instances
 * with the class metadata. Exposing the id breaks encapsulation to a degree,
 * but is a lesser evil when compared to exposing all metadata.
 *
 * @param  {Function}  func  function (class) to attach method to
 * @param  {number}    id    id to assign
 *
 * @return  {undefined}
 */
function attachId( func, id )
{
    util.defineSecureProp( func, '__cid', id );
    util.defineSecureProp( func.prototype, '__cid', id );
}