haiku/docs/user/app/_app_messaging.dox

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/*
* 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 actuall 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...
*/