xref: /haiku/docs/user/app/_app_messaging.dox (revision 46b7da1f4f40f7157d74fc7fb26ff9ec7f2416f2)

/*
 * Copyright 2007 Haiku, Inc. All rights reserved.
 * Distributed under the terms of the MIT License.
 *
 * Authors:
 *		Niels Sascha Reedijk, niels.reedijk@gmail.com
 */

/*!
	\page app_messaging Messaging Foundations

	One of the foundations of the Haiku API is the messaging system. This
	framework is the basis for the efficient multithreaded Haiku applications,
	because it solves one of the fundamental issues of multithreading: it
	allows you to easily and securely communicate between threads. The
	framework allows inter-application messaging as well as
	intra-application messaging, and it will always use the most effective
	mechanism for the communication automatically.

	This page will introduce you to the subject of messaging. It is meant as a
	broad overview to the classes, rather than a tutorial. If you are looking
	for effective messaging techniques or a tutorial on messaging, have a look
	at the developer section of the Haiku website.

	<b>Table of contents</b>
	- Overview of the Messaging Classes
	- Receiving and Handling Messages
	- Sending messages

	\section app_messaging_overview Overview of the Messaging Classes

	\subsection app_messaging_overview_bmessage BMessage

	The BMessage class is the class that is in the center of all the messenger
	operations, because it represents a message. A message is nothing more than
	an object that contains:
	- The \c what member, an \c uint32 that determines the type of message.
	  Some constants are defined by the Haiku API, for example B_MOUSE_DOWN or
	  B_QUIT_REQUESTED.
	- Zero or more data objects. BMessage is a powerful data container that
	  keeps track of different sorts of data. BMessage provides many convenient
	  Add*() methods, for example BMessage::AddBool(). With the corresponding
	  Find*() method (in this example,
	  \link BMessage::FindBool(const char *, int32, bool *) const FindBool() \endlink)
	  you can retrieve the data.

	BMessage itself is generic, its syntax and semantics are determined by the
	context. The Haiku API defines several messages and their required data
	members. Several applications provide a scripting interface with defined
	message syntax. You can do the same for your application.

	\subsection app_messaging_overview_blooper BLooper

	Objects of the BLooper type are objects that run message loops. Every
	object runs in its own thread. The BLooper objects continually check for
	incoming messages. To process the messages, the looper looks for message
	handlers that handle the messages within the thread's context. Message
	handling within a looper is synchronous.

	BLooper inherits BHandler, the base class for message handling. However, it
	is possible to chain additional handlers to the object. For example, if you
	have an application that understands different networking protocols, and
	you support extensions that understand the base protocol, these extensions
	can provide handlers that you can chain in your general message parser
	thread. See AddHandler() and SetPreferredHandler() for information on
	handlers.

	Messages can be posted to the looper by using the object's PostMessage()
	method. This method puts the message in the BMessageQueue of the looper.
	Since PostMessage() is asynchronous, the message might not be handled
	immediately. See \ref app_messaging_overview_bmessenger "BMessenger"
	for a synchronous implementation.

	Loopers can have a generic filter that discards messages based on
	user-definable characteristics. The BMessageFilter class provides the
	foundation for the qualifying of messages. See AddCommonFilterList() and
	SetCommonFilterList() for more information.

	To get the most out of the functionality of BLooper, it is usually
	subclassed to create a self-contained event 'machine'. Most of the time,
	these subclasses also perform the message handling, which is possible
	due to the fact that it is also a subclass of BHandler.

	In the Haiku API, there are two major classes that inherit BLooper:
	the base application class, BApplication, and the window class, BWindow.
	Because they inherit BLooper, each application and each window has its
	own message loop. This makes every window quick and responsive. To keep
	your applications running smoothly, it is advisable to make sure that
	event handling that requires more processing power, is done within its own
	BLooper context. Networking usually qualifies as a candidate for its own
	thread.

	\subsection app_messaging_overview_bhandler BHandler

	Objects of the BHandler type are associated to BLoopers. When they are
	created, they should be passed to the BLooper::AddHandler() method of the
	looper they want to handle messages for. They can then either be set as
	preferred handlers (by chaining them with BLooper::SetPreferredHandler()),
	or they can be added to other BHandlers with the SetNextHandler() method.

	The magic of the class happens in the MessageReceived() method. In your
	subclasses you override this method, to check the incoming BMessage.
	Usually, you check the \c what member of the message in a switch statement.
	If your handler cannot handle the object, it will pass the message on to
	the parent class.

	\warning Don't forget to actually call the MessageReceived() method of the
		base class. Failing to do this will mean that the message chain will
		not completely be followed, which can lead to unhandled messages. There
		might be some internal system messages that the Haiku API classes
		handle, and not actually handling these messages could lead to
		inconsistent internal behavior.

	\subsection app_messaging_overview_bmessenger BMessenger

	BMessenger objects can send messages to both local and remote targets. For
	local targets, a BMessenger provides an advantage over directly calling
	the BLooper::PostMessage() method: some variants of the
	BMessenger::SendMessage() methods allow for synchronous replies. So, the
	call will actually verify the handling thread processes the message, and
	reply to the sender.

	The other feature of BMessenger is that it is able to be constructed with
	the signature of another application as argument. This allows the messenger
	to pass messages to other applications. It facilitates inter-application
	communication.

	\subsection app_messaging-overview-other Other messaging classes

	There are several convenience classes supplied with the application kit,
	which can make your life easier in some specific cases.

	- BInvoker binds together a message and a target. By calling
	  BInvoker::Invoke(), the message will be sent. This class is inherited by
	  the controls in the interface kit, such as BButton.
	- A BMessageRunner object will send messages to a specified target with
	  specified intervals in between.
	- BMessageQueue is a class that is also internally used by BLooper. It
	  provides a queue of messages, with convenience functions of managing
	  this queue.
	- BMessageFilter is the base class of the filters. Filters can be applied
	  to BLoopers to filter all incoming messages, or to BHandlers to filter
	  messages that could be handled by that object. The filter object can be
	  subclassed and extended by overriding the \link BMessageFilter::Filter()
	  Filter() \endlink method.

	\section app-messaging-receiving Receiving Messages

	To do...

	\section app-messaging-sending Sending Messages

	To do...
*/