xref: /haiku/docs/user/app/Looper.dox

/*
 * Copyright 2008-2019 Haiku, Inc. All rights reserved.
 * Distributed under the terms of the MIT License.
 *
 * Authors:
 *		Niels Sascha Reedijk, niels.reedijk@gmail.com
 *		John Scipione, jscipione@gmail.com
 *		Adrien Destugues, pulkomandy@pulkomandy.tk
 *
 * Corresponds to:
 *		headers/os/app/Looper.h	hrev52501
 *		src/kits/app/Looper.cpp	hrev52501
 */


/*!
	\file Looper.h
	\ingroup app
	\ingroup libbe
	\brief Provides the BLooper class.
*/


/*!
	\def B_LOOPER_PORT_DEFAULT_CAPACITY
	\brief The default size of the port of a BLooper.

	\since BeOS R3
*/


/*!
	\class BLooper
	\ingroup app
	\ingroup libbe
	\brief Receive and process messages in a separate thread.

	When an object of this class is created, the message loop can be started
	with Run(). This spawns the thread that receives messages and processes
	messages. Messages are actually passed on to \link BHandler handlers \endlink
	that are associated with this looper. By default there is always one
	handler available: the looper itself. To 'quit' a looper, you should pass
	a \c B_QUIT_REQUESTED message using one of the message post functions. When
	a looper receives such a request, it will \b delete itself. As such, looper
	should <em>always be created on the heap</em> (with \c new), and never on
	the stack.

	Posting messages can be done using the various PostMessage() methods.
	Whenever a message is posted, it will be added through to the message
	queue. It is possible to apply filters (see AddCommonFilter()) to filter
	out any messages that correspond with certain criteria. The method will
	copy the contents of the message and this copy is processed, so make sure
	you delete the original messages in case you create them on the heap.
	The handler for the message is chosen using the following criteria:

	-# If PostMessage() or the BMessenger is set to a specific handler, and
		this handler is associated with this looper, than the message is
		processed by that handler.
	-# Else, the preferred handler is used. You can set this using
		SetPreferredHandler().
	-# If there is no preferred handler, then the looper itself will process
		the message.

	Because a looper usually is used in multiple threads, you should make sure
	you Lock() and Unlock() it during most operations. Locking calls can be
	recursive (so multiple locks can come from a single thread), but make sure
	you pair every Lock() with an Unlock() call. Failing to do so will
	inevitably cause a deadlock.

	Because a looper provides a separate thread, and the inherited handler is
	usually a default handler, you will most often use this class by
	subclassing it. For example, you are likely to subclass BWindow (which is
	derived from BLooper) to customize your window and handle the messages
	sent to that window. You can override Run() in case you want to perform
	additional tasks before (or right after) starting the message loop. You can
	override QuitRequested() if you want to decline quitting in certain
	circumstances. You can override Quit() in case you want to perform
	additional procedures during closing time. You can also override
	DispatchMessage() if you want to do something with all incoming messages
	before they are dispatched to a handler.

	BLooper is one of the major base classes of the Haiku application
	programmers interface. Closely related classes are BMessage, BHandler and
	BMessenger. It is used in the interface kit, for example by the BWindow
	class, which makes sure every window runs it its own thread.

	BLooper is a part of the chain in the eloquent messaging structure. For a
	proper understanding of all its facets, have a look at the \ref app_messaging
	"messaging overview".

	\since BeOS R3
*/


/*!
	\fn BLooper::BLooper(const char* name, int32 priority, int32 portCapacity)
	\brief Construct a new BLooper with a \a priority and an \a capacity.

	The new looper is, by default, not running yet. If you have set up
	everything properly, you may call Run().

	\attention Remember that loopers should be created on the heap, because
	           they will \c delete themselves in the Quit() method.

	\param name The name of the looper.
	\param priority The priority of the message thread of this looper. The
	       default priority should be good enough for most tasks. Also, some
	       derived versions of BLooper will use a specialized priority. So it
	       is advised to leave this setting at the default, unless you know
	       why you would like another setting.
	\param portCapacity Loopers use ports to send and receive messages (see
	       the kernel kit). Ports have a maximum capacity; if there are so many
	       messages queued that the port is full, all other incoming messages
	       are dropped. There are situations where the size of the port should
	       be different from the default. This might be when your looper
	       receives a lot of messages, or if the message handling thread runs
	       at a lower priority than normal, which would decrease the processing
	       speed. Finding a suitable value for these custom scenarios would be
	       done by testing.

	\see Run()

	\since BeOS R3
*/


/*!
	\fn BLooper::~BLooper()
	\brief Destruct the looper.

	You will never delete a looper yourself. You should pass a
	\c B_QUIT_REQUESTED message, or if you are destroying the looper from
	inside its own message handling thread, you should call Quit().

	\see Quit()

	\since BeOS R3
*/


///// Archiving /////


/*!
	\name Archiving
*/


//! @{


/*!
	\fn BLooper::BLooper(BMessage* data)
	\brief Construct a looper from an archived message.

	The \a data message has to be constructed by a BLooper::Archive() call.
	Note that the data that is restored, is merely the port capacity and the
	name of the looper/handler. Other data, such as filters, is not archived by
	the default archiver.

	\warning This constructor does no type check whatsoever. Since you can pass
	         any BMessage, you should - if you are not sure about the exact
	         type - use the Instantiate() method, which does check the type.

	\see Instantiate()
	\see Archive()

	\since BeOS R3
*/


/*!
	\fn BArchivable* BLooper::Instantiate(BMessage* data)
	\brief Static method to instantiate a looper from an archived message.

	\return A pointer to the instantiated looper, or \c NULL if the \a data
	        is not a valid archived BLooper object.

	\see BLooper(BMessage* data)

	\since BeOS R3
*/


/*!
	\fn status_t BLooper::Archive(BMessage* data, bool deep) const
	\brief Archive a looper to a message

	Currently, only the name and the port capacity are archived. Any other
	data, such as the filters, is not stored.

	\param data The message to archive the object in.
	\param deep This parameter is ignored, as BLooper does not have children.

	\retval B_OK Archiving succeeded.
	\retval B_BAD_VALUE The \a data parameter is not a valid message.

	\see BLooper::Instantiate(BMessage* data)

	\since BeOS R3
*/


//! @}


/*!
	\name Message Mechanics
*/


//! @{


/*!
	\fn status_t BLooper::PostMessage(uint32 command)
	\brief Post a message with the \a command as \c what identifier to this
	       looper.

	Posting a message puts it in the message queue. The message passes through
	the default handler chain.

	\param command The \c what identifier of the message to be sent.

	\return A status code.
	\retval B_OK The operation succeeded, and the message is sent to the port.
	\retval B_ERROR There was a general operation error.
	\retval B_BAD_VALUE This looper is not yet running and therefore cannot
	        receive messages.

	\see PostMessage(BMessage *) if you want to send a message with data
	     members.
	\see PostMessage(uint32, BHandler *, BHandler *) if you want to send a
	     message to a specific handler, and request a reply.
	\see PostMessage(BMessage *, BHandler *, BHandler *) for the same thing,
		but with a complete message.

	\since BeOS R5
*/


/*!
	\fn status_t BLooper::PostMessage(BMessage* message)
	\brief Post a \a message to this looper.

	Posting a message puts it in the message queue. The message passes through
	the default handler chain.

	The \a message is copied, and as such, you should make sure you will not
	leak it. The best way to send messages is like this:
\code
	BMessage message;
	message.what = B_DO_SOMETHING;
	message.AddString("some_data", "This is data")

	aLooper->PostMessage(&message);
\endcode

	\param message The message you would like to pass to this method.

	\return A status code.
	\retval B_OK The operation succeeded, and the message is sent to the port.
	\retval B_ERROR There was a general operation error.
	\retval B_BAD_VALUE This looper is not yet running and therefore cannot
	        receive messages.

	\see PostMessage(uint32) if you want to send a message without data
	     members.
	\see PostMessage(uint32, BHandler *, BHandler *) if you want to send a
	     message to a specific handler, and request a reply.
	\see PostMessage(BMessage *, BHandler *, BHandler *) for the same thing,
	     but with a complete message.

	\since BeOS R5
*/


/*!
	\fn status_t BLooper::PostMessage(uint32 command, BHandler* handler,
		BHandler* replyTo)
	\brief Sends a message with \a command \c what identifier to the
	      \a handler associated with this looper. A response may be sent to the
	      \a replyTo handler asynchronously.

	The target \a handler should be associated with this looper. This method
	bypasses the default message queue.

	\param command The value you want as the message's \c what identifier.
	\param handler The handler you would like to pass this message to.
	\param replyTo If you would like to request a reply, pass the handler to
	       which this reply should be directed to. If you pass \c NULL, you
	       will not receive a reply.

	\return A status code.
	\retval B_OK The operation succeeded, and the message is sent to the port.
	\retval B_ERROR There was a general operation error.
	\retval B_BAD_VALUE This looper is not yet running and therefore cannot
	        receive messages.
	\retval B_MISMATCHED_VALUES The \a handler is not associated with this
	        looper.

	\see PostMessage(uint32) if you want to send a message without data
	     members.
	\see PostMessage(BMessage *) if you want to send a message with data
	     members.
	\see PostMessage(BMessage *, BHandler *, BHandler *) if you want to send a
	     message to a specific handler, and request a reply.

	\since BeOS R5
*/


/*!
	\fn status_t BLooper::PostMessage(BMessage* message, BHandler* handler,
		BHandler* replyTo)
	\brief Send a \a message to the \a handler associated with this looper.
	       A response may be sent to the \a replyTo handler asynchronously.

	The target \a handler should be associated with this looper. This method
	bypasses the default message queue.

	The \a message is copied, and as such, you should make sure you will not
	leak it. The best way to send messages is like this:
\code
	BMessage message;
	message.what = B_DO_SOMETHING;
	message.AddString("some_data", "This is data")

	aLooper->PostMessage(&message, aHandler);
\endcode

	\param message The message you want to pass.
	\param handler The handler you would like to pass this message to.
	\param replyTo If you would like to request a reply, pass the handler to
	       which this reply should be directed to. If you pass \c NULL, you
	       will not receive a reply.

	\return A status code.
	\retval B_OK The operation succeeded, and the message is sent to the port.
	\retval B_ERROR There was a general operation error.
	\retval B_BAD_VALUE This looper is not yet running and therefore cannot
	        receive messages.
	\retval B_MISMATCHED_VALUES The \a handler is not associated with this
	        looper.

	\see PostMessage(uint32) if you want to send a message without data
	     members.
	\see PostMessage(BMessage *) if you want to send a message with data
	     members.
	\see PostMessage(uint32, BHandler *, BHandler *) if you want to send a
	     message without data to a specific handler, and request a reply.

	\since BeOS R5
*/


//! @}


/*!
	\name Message Processing
*/


//! @{


/*!
	\fn void BLooper::DispatchMessage(BMessage *message, BHandler *handler)
	\brief Dispatch a message to a handler. Override if there are messages that
	       you want to catch before they are sent to the handlers.

	This method is called by the message looping thread to dispatch a message
	to \a handler. If you implement the BLooper class and your looper receives
	messages that absolutely have to be processed by the looper instead of any
	of the handlers, override this method. For example, the default
	implementation catches B_QUIT_REQUESTED messages before they are sent to
	the handlers, so that the looper will quit at those messages.

	You are discouraged from using this method to filter out any messages you
	do not want to process. For this, there is a more generic method using
	the BMessageFilter class. If you want to skip messages with certain
	patterns, have a look at the AddCommonFilter() and SetCommonFilterList()
	methods.

	If you do override this method, please remember to call the
	DispatchMessage() method of the parent class.

	\since BeOS R3
*/


/*!
	\fn void BLooper::MessageReceived(BMessage* message)
	\brief Process a message received by the internal handler of this looper.

	Reimplemented from BHandler::MessageReceived();

	\since BeOS R5
*/


/*!
	\fn BMessage* BLooper::CurrentMessage() const
	\brief Retrieve the current message.

	\attention Only call this method from within the thread that processes the
	           messages. It contains a pointer to the message that is currently
	           being handled. Due to the multithreaded nature of the operating
	           system, this method will not safely let you read the message
	           that is being processed by this handler from outside the context
	           of the processing. If you do want to use a message outside of
	           the processing thread, have a look at DetachCurrentMessage() to
	           safely retrieve a message.

	\return A pointer to the message that is currently being processed. Note
	        that calling it from outside the thread that processes the message,
	        could give you a \c NULL pointer or an invalid pointer.

	\since BeOS R5
*/


/*!
	\fn BMessage* BLooper::DetachCurrentMessage()
	\brief Get ownership of the message currently being processed.

	Retrieve the current message and gain ownership of it. This means that the
	message will not be deleted as soon as the looper is done processing it.
	You can then use it for different purposes.

	\attention Only call this method from within the thread that processes the
	           messages. Due to the multithreaded nature of the operating
	           system, calling it from another thread is very likely to give
	           you an invalid or a \c NULL pointer.

	\since BeOS R5
*/


/*!
	\fn void BLooper::DispatchExternalMessage(BMessage* message,
		BHandler* handler, bool& _detached)
	\brief Internal method to support single-threaded GUI toolkits

	\since Haiku R1
*/


/*!
	\fn BMessageQueue* BLooper::MessageQueue() const
	\brief Get a pointer to the internal message queue of this looper.

	You can use this pointer to manipulate the message queue. Note that the
	message that is being processed is already detached from this queue.

	\return A pointer to the internal message queue.

	\since BeOS R5
*/


/*!
	\fn bool BLooper::IsMessageWaiting() const
	\brief Check if there is a message waiting.

	\return \c true if there are still messages to be processed,
	        \c false if there is no message waiting.
*/


//! @}


/*!
	\name Handler Management
*/


//! @{


/*!
	\fn void BLooper::AddHandler(BHandler* handler)
	\brief Associate a \a handler to this looper.

	The \a handler will be associated to this looper. By default, the handler
	in this looper will be chained to the supplied \a handler.

	\param handler The handler to associate with this looper. If the handler
	       is already associated to another looper, the operation will fail
	       silently. Check beforehand if you cannot be sure that the
	       \a handler is unassociated.

	\see RemoveHandler()

	\since BeOS R3
*/


/*!
	\fn bool BLooper::RemoveHandler(BHandler* handler)
	\brief Disassociate a \a handler from this looper.

	If the handler is disassociated, it can be reassociated to another looper.

	\return \c true if the \a handler has been removed from this looper,
	        \c false The \a handler was invalid or the handler was not
	           associated to this looper.

	\see AddHandler()

	\since BeOS R3
*/


/*!
	\fn int32 BLooper::CountHandlers() const
	\brief Get the number of handlers associated with this looper.

	\see HandlerAt()
	\see IndexOf()

	\since BeOS R3
*/


/*!
	\fn BHandler* BLooper::HandlerAt(int32 index) const
	\brief Get the handler at an \a index of the list of associated handlers.

	\return A pointer to the handler at that \a index, or \c NULL if the
	        \a index is out of range.

	\see CountHandlers()
	\see IndexOf()

	\since BeOS R3
*/


/*!
	\fn int32 BLooper::IndexOf(BHandler* handler) const
	\brief Get the index of the \a handler that is in the associated handler
	       list.

	\return The index of the handler in the list if the \a handler is in the
	        list, else this method will return -1.

	\since BeOS R3
*/


/*!
	\fn BHandler* BLooper::PreferredHandler() const
	\brief Get the preferred handler.

	\return A pointer to the preferred handler, or \c NULL if none is set.

	\see SetPreferredHandler()

	\since BeOS R3
*/


/*!
	\fn void BLooper::SetPreferredHandler(BHandler* handler)
	\brief Set a preferred handler.

	If messages are posted to this looper using one of the PostMessage()
	methods without a specific BHandler argument, the messages will be handled
	by the looper itself (since a looper is a subclass of BHandler, this is
	perfectly possible). If you want to override that behavior, you should set
	a preferred handler. This handler will be called if incoming messages do
	not ask to be directly passed on to a specific handler.

	\param handler The preferred handler you want undesignated messages to be
	       handled by. If you want to unset the preferred handler, pass
	       \c NULL. If the supplied \a handler is not associated with this
	       looper, this call will fail silently and the current preferred
	       handler will be unset.

	\see PreferredHandler()

	\since BeOS R3
*/


//! @}


/*!
	\name Loop Control
*/


//! @{


/*!
	\fn thread_id BLooper::Run()
	\brief Start the event loop.

	After the looper has been constructed, it needs to be started using this
	method. A thread will be spawned, which will receive messages.

	Make sure the looper is not yet running before you call this method.

	\return A (positive) thread id if spawning the thread succeeded, or an
	        error code.

	\since BeOS R3
*/


/*!
	\fn thread_id BLooper::Loop()
	\brief Run the event loop in the current thread.

	This method runs the event loop in an already existing thread. It blocks
	until the looper stops looping. This can
	be used to turn an existing thread into a BLooper.

	Make sure the looper is not yet running before you call this method.

	\since Haiku R1
*/


/*!
	\fn void BLooper::Quit()
	\brief Hook method that is called after a \c B_QUIT_REQUESTED message.

	If you want to quit and delete the looper, you should post a
	\c B_QUIT_REQUESTED message. This will first call the hook method
	QuitRequested(), which can be overridden in child classes in case there
	are conditions that would prevent the looper to be quit. If you really
	know what you are doing, and you definitely want to quit this looper,
	you may call this method, but only after performing a Lock() operation.

	Override this method if your subclass needs to perform specific clean-up
	tasks. Remember to call the base class implementation when you're done.

	\attention You will not have to delete the looper object, if a looper quits
	           it will delete itself.

	\since BeOS R3
*/


/*!
	\fn bool BLooper::QuitRequested()
	\brief Hook method that is called during a \c B_QUIT_REQUESTED message.

	This hook function is called by the looper thread when a
	\c B_QUIT_REQUESTED is received. The default implementation always accepts
	the message, but if your subclass needs a special condition to be met
	before actually accepting a quit message, you can test for that condition
	in this hook method. A good example is a window (which is a derivative of
	BLooper), which contains a modified document. The condition may be that a
	modal dialog requesting a path of action is closed.

	\return \c true if the looper can be quit and destroyed,
	        \c false if this method does not accept the quit message
	           and continue processing messages.

	\since BeOS R3
*/


/*!
	\fn bool BLooper::Lock()
	\brief Lock the looper. 

	For most operations involving the internal data of the looper, you need to
	hold the lock. Each looper implements a global lock, which you can use to
	perform operations on internal data in a thread-safe manner.

	Do not forget to pair each Lock() request with an Unlock() request. Lock()
	requests can be stacked, which means that recursively locking a looper from
	a thread that actually holds the lock, will not cause a deadlock. See
	BLocker for more information on locking internals.

	\return \c true if the locking request succeeded,
	        \c false if the locking request could not be completed. There are a
	           variety of reasons for this to happen, for example when the
	           looper is destroyed.

	\see Unlock()
	\see LockWithTimeout()
	\see IsLocked()

	\since BeOS R5
*/


/*!
	\fn void BLooper::Unlock()
	\brief Unlock a locked looper.

	Use this method paired with Lock() calls, to release a lock. Make sure that
	this method is only called on a locked looper.

	\see Lock()
	\see LockWithTimeout()
	\see IsLocked()

	\since BeOS R5
*/


/*!
	\fn bool BLooper::IsLocked() const
	\brief Check if a looper is locked.

	\return \c true if the looper is locked,
	        \c false if the looper is not locked, or the looper has been
	           deleted.

	\see Lock()
	\see Unlock()
	\see LockWithTimeout()

	\since BeOS R5
*/


/*!
	\fn status_t BLooper::LockWithTimeout(bigtime_t timeout)
	\brief Lock a looper with a \a timeout.

	This method locks the looper like Lock(), but if the locking request does
	not succeed within the provided \a timeout, the method will return.

	\param timeout The maximum time to wait for the lock request to succeed.

	\return A status code.
	\retval B_OK The lock is acquired.
	\retval B_BAD_VALUE The looper has been destroyed.
	\retval "other errors" There was an error acquiring the lock.

	\see Lock()
	\see Unlock()
	\see IsLocked()

	\since BeOS R5
*/


/*!
	\fn thread_id BLooper::Thread() const
	\brief Return the thread id of the internal message looper thread.

	If the looper is not yet running, this method will return 0.

	\see Run()

	\since BeOS R3
*/


/*!
	\fn team_id BLooper::Team() const
	\brief Return the team id in which this looper exists.

	\since BeOS R3
*/


/*!
	\fn BLooper* BLooper::LooperForThread(thread_id thread)
	\brief Static method to retrieve a BLooper for a specified \a thread.

	\since BeOS R3
*/


//! @}


/*!
	\name Loop Debugging

	These methods may aid you in debugging problems when they occur, but do not
	use these in actual production code. These methods are unreliable because
	they are not thread-safe, and as such are only useful in specific debugging
	situations. Handle with care.
*/


//! @{


/*!
	\fn thread_id BLooper::LockingThread() const
	\brief Return the thread id of the thread that currently holds the lock.

	\since BeOS R3
*/


/*!
	\fn int32 BLooper::CountLocks() const
	\brief Return the number of recursive locks that are currently being held
	       on this looper.

	\since BeOS R3
*/


/*!
	\fn int32 BLooper::CountLockRequests() const
	\brief Return the number of pending locks.

	\since BeOS R3
*/


/*!
	\fn sem_id BLooper::Sem() const
	\brief Return the id of the semaphore that is used to lock this looper.

	\since BeOS R3
*/


//! @}


/*!
	\name Scripting
*/


//! @{
	

/*!
	\fn BHandler* BLooper::ResolveSpecifier(BMessage* message, int32 index,
		BMessage* specifier, int32 what, const char* property)
	\brief Determine the proper handler for a scripting message.

	\copydetails BHandler::ResolveSpecifier()
*/


/*!
	\fn status_t BLooper::GetSupportedSuites(BMessage* data)
	\brief Reports the suites of messages and specifiers that derived classes
		understand.

	\copydetails BHandler::GetSupportedSuites()
*/


//! @}


/*!
	\name Looper Message Filters

	Note that filters added with these methods will be applied to all
	associated handlers. Have a look at the filtering methods of the BHandler
	class to see how filters can be applied to the inherited handler of this
	looper specifically.
*/


//! @{


/*!
	\fn void BLooper::AddCommonFilter(BMessageFilter* filter)
	\brief Add a common filter to the list of filters that are applied to all
	       incoming messages.

	Filters can only be applied once, so they cannot be shared between loopers,
	a handler and a looper or between two handlers.

	The \a filter is not copied; rather a pointer is stored. Keep the \a filter
	alive as long as it is used by a looper.

	\see RemoveCommonFilter()
	\see SetCommonFilterList()
	\see CommonFilterList()

	\since BeOS R3
*/


/*!
	\fn bool BLooper::RemoveCommonFilter(BMessageFilter* filter)
	\brief Remove a \a filter from the common message filter list.

	Note that this will not free the memory used by the \a filter, so you
	should dispose of it yourself. 

	\see AddCommonFilter()
	\see SetCommonFilterList()
	\see CommonFilterList()

	\since BeOS R3
*/


/*!
	\fn void BLooper::SetCommonFilterList(BList* filters)
	\brief Set a new list of \a filters that need to be applied to all
		incoming messages.

	You are responsible for validating that all the items in the list of
	\a filters are actual filters. The old list is discarded; all the filters
	are \b destroyed.

	Note that filters can only be applied to one looper or handler. If any
	of the filters is already associated with another one, this call will fail.

	\see AddCommonFilter()
	\see RemoveCommonFilter()
	\see CommonFilterList()

	\since BeOS R3
*/


/*!
	\fn BList* BLooper::CommonFilterList() const
	\brief Return a list of filters applied to all incoming messages.

	\return A pointer to the internal filter list, or \c NULL if such a list
	        has not yet been created. Please note that you should use the
	        internal list management functions to manipulate the internal
	        filter list, in order to maintain internal consistency.

	\see AddCommonFilter()
	\see RemoveCommonFilter()
	\see SetCommonFilterList()

	\since BeOS R3
*/


//! @}


/*!
	\fn status_t BLooper::Perform(perform_code d, void* arg)
	\brief Internal method.

	\since Haiku R1
*/


/*!
	\fn BMessage* BLooper::MessageFromPort(bigtime_t timeout)
	\brief Hook method to retrieve a message from the looper's port.

	The default implementation is called by the internal message looping thread
	and retrieves the next message from the port that belongs to this looper.

	If you use a looper in a context where it might receive messages from other
	sources, you can override this method in order to insert these methods into
	the message processing. Note that any messages that are returned by this
	method will be deleted by this looper, so make sure you have ownership of
	the message. If you override this method, remember to call the base
	implementation every now and then, in order to retrieve the messages
	arriving at the default port.

	\since Haiku R1
*/