haiku/docs/user/support/Archivable.dox

/*
 * Copyright 2007, Haiku, Inc. All Rights Reserved.
 * Distributed under the terms of the MIT License.
 *
 * Authors:
 *		Niels Sascha Reedijk, niels.reedijk@gmail.com
 *		Alex Wilson, yourpalal2@gmail.com
 *
 * Proofreader:
 *		David Weizades, ddewbofh@hotmail.com
 *		Thom Holwerda, slakje@quicknet.nl
 *
 * Corresponds to:
 *		/trunk/headers/os/support/Archivable.h rev 37751 
 *		/trunk/src/kits/support/Archivable.cpp rev 37751
 */


/*!	\file Archivable.h
	\brief Provides the BArchivable interface and declares the BArchiver and
		BUnarchiver classes.
*/


/*!	\class BArchivable
	\ingroup support
	\ingroup libbe
	\brief Interface for objects that can be archived into a BMessage.

	BArchivable provides an interface for objects that can be put into message
	archives and extracted into objects in another location. Using this you are
	able to send objects between applications, or even between computers across
	networks.

	BArchivable differs from BFlattenable in that BFlattenable is designed to
	store objects into flat streams of data, the main objective being storage to
	disk. The objective of this interface, however, is to store objects that
	will later be restored as new (but identical) objects. To illustrate this
	point, BArchivable objects can be restored automatically to the correct
	class, whereas BFlattenables have a data type which you need to map to
	classes manually.

	Archiving is done with the Archive() method. If your class supports it, the
	caller can request it to store into a deep archive, meaning that all child
	objects in it will be stored. Extracting the archive works with the
	Instantiate() method, which is static. Since the interface is designed to
	extract objects without the caller knowing what kind of object it actually
	is, the global function #instantiate_object() instantiates a message without
	you manually having to determine the class the message is from. This adds
	considerable flexibility and allows BArchivable to be used in combination
	with other add-ons.

	To provide this interface in your classes you should publicly inherit this
	class. You should implement Archive() and Instantiate(), and provide one
	constructor that takes one BMessage argument.

	If your class holds references to other BArchivable objects that you wish
	to archive, then you should consider using the BArchiver and BUnarchiver
	classes in your Archive() method and archive constructor, respectively.
	You should also consider implementing the AllArchived() and AllUnarchived()
	methods, which were designed to ease archiving and unarchiving in such
	a situation.
*/


/*!
	\fn BArchivable::BArchivable(BMessage* from)
	\brief Constructor. Does important behind-the-scenes work in the unarchiving
		process.
	
	If you inherit this interface you should provide at least one constructor
	that takes one BMessage argument. In that constructor, you should call your
	parent class' archive constructor (even if your parent class is
	BArchivable).
*/


/*!
	\fn BArchivable::BArchivable()
	\brief Constructor. Does nothing.
*/


/*!
	\fn BArchivable::~BArchivable()
	\brief Destructor. Does nothing.
*/


/*!
	\fn virtual status_t BArchivable::Archive(BMessage* into,
		 bool deep = true) const
	\brief Archive the object into a BMessage.
	
	You should call this method from your derived implementation as it adds the
	data needed to instantiate your object to the message.
	
	\param into The message you store your object in.
	\param deep If \c true, all children of this object should be archived as
		well.

	\retval B_OK The archive operation was successful.
	\retval B_BAD_VALUE \c NULL \a archive message.
	\retval B_ERROR The archive operation failed.
*/


/*!
	\fn static BArchivable* BArchivable::Instantiate(BMessage* archive)
	\brief Static member to restore objects from messages.

	You should always check that the \a archive argument actually corresponds to
	your class. The automatic functions, such as #instantiate_object() and
	BUnarchiver::InstantiateObject() will not choose the wrong class but manual
	calls to this member might be faulty. You can verify that \c archive
	stores an object of your calss with the validate_instantiation() function.

	\param archive The message with the data of the object to restore.
	\retval You should return a pointer to the object you create with
		\c archive, or \c NULL if unarchival fails.
	\warning The default implementation will always return \c NULL. Even though
		it is possible to store plain BArchivable objects, it is impossible to
		restore them. 

	\see instantiate_object(BMessage *from)
	\see BUnarchiver::InstantiateObject()
*/


/*!
	\fn virtual status_t BArchivable::Perform(perform_code d, void* arg)
	\brief Perform some action (Internal method defined for binary
		compatibility purposes).

	\internal This method is defined for binary compatibility purposes, it is
		used to ensure that the correct AllUnarchived() and AllArchived()
		methods are called for objects, as those methods are new to Haiku.

	\param d The perform code.
	\param arg A pointer to store some data.

	\returns A status code.
*/


/*!
	\fn virtual status_t BArchivable::AllUnarchived(const BMessage* archive)
	\brief Method relating to the use of \c BUnarchiver.

	This hook function is called triggered in the BUnarchiver::Finish() method.
	In this method, you can rebuild references to objects that may be direct
	children of your object, or may be children of other objects.
	Implementations of this method should call the implementation of
	their parent class, the same as for the Archive() method.

	\warning To guarantee that your AllUnarchived() method will be called
		during unarchival, you must create a BUnarchiver object in your
		archive constructor.

	\see BUnarchiver, BUnarchiver::Finish()
*/


/*! \fn virtual status_t BArchivable::AllArchived(BMessage* into) const
	\brief Method relating to the use of \c BArchiver.

	This hook function is called once the first BArchiver that was created in
	an archiving session is either destroyed, or has its Finish() method
	called. Implementations of this method can be used, in conjunction with
	BArchiver::IsArchived(), to reference objects in your archive that you
	do not own, depending on whether or not those objects were archived by their
	owners. Implementations of this method should call the implementation of
	their parent class, the same as for the Archive() method.

	\warning To guarantee that your AllArchived() method will be called
		during archival, you must create a BArchiver object in your
		Archive() implementation.

	\warning You should archive any objects you own in your Archive()
		method implementation, and \b NOT your AllArchived() method.

	\see BArchiver BArchiver::Finish()
*/


///// Global methods /////
/*!
	\addtogroup support_globals
	@{
*/


/*!	\typedef typedef BArchivable* (*instantiation_func)(BMessage*)
	\brief Internal definition of a function that can instantiate objects that
		have been created with the BArchivable API.
*/


/*!
	\fn BArchivable* instantiate_object(BMessage *from, image_id *id)
	\brief Instantiate an archived object with the object being defined in a
		different application or library.
	
	This function is similar to instantiate_object(BMessage *from), except that
	it takes the \a id argument referring to an image where the object might be
	stored. 
	
	\note Images are names for executable files. Image id's refer to these
		executable files that have been loaded by your application. Have a look
		at the kernel API for further information.
*/


/*!
	\fn BArchivable* instantiate_object(BMessage *from)
	\brief Instantiate an archived object.
	
	This global function will determine the base class, based on the \a from
	argument, and it will call the Instantiate() function of that object to
	restore it. 
	
	\param from The archived object.
	\return The object returns a pointer to the instantiated object, or \c NULL
		if the instantiation failed. The global \c errno variable will contain
		the reason why it failed.
	\see instantiate_object(BMessage *from, image_id *id)
*/


/*!
	\fn bool validate_instantiation(BMessage* from, const char* className)
	\brief Internal function that checks if the \a className is the same as the
		one stored in the \a from message.
*/


/*!
	\fn instantiation_func find_instantiation_func(const char* className,
		const char* signature)
	\brief Internal function that searches for the instantiation func with a
		specific signature. Use instantiate_object() instead.
*/


/*!
	\fn instantiation_func find_instantiation_func(const char* className)
	\brief Internal function that searches for the instantiation func of a
		specific class. Use instantiate_object() instead.
*/


/*!
	\fn instantiation_func find_instantiation_func(BMessage* archive)
	\brief Internal function that searches for the instantiation func that
		works on the specified \a archive. Use instantiate_object() instead.
*/


//! @}