Handles messages that are passed on by a BLooper. More...

Inheritance diagram for BHandler:

Public Member Functions
	BHandler (const char *name=NULL)
	Construct a new handler with a name.
virtual	~BHandler ()
	Free the filters of this handler, as well as the list of observers.
virtual status_t	AllArchived (BMessage *archive) const
	Method relating to the use of `BArchiver`.
virtual status_t	AllUnarchived (const BMessage *archive)
	Method relating to the use of `BUnarchiver`.
virtual status_t	Perform (perform_code d, void *arg)
	Perform some action (Internal method defined for binary compatibility purposes).
Archiving
BHandler inherits the BArchivable class, and as such implements support for archiving and unarchiving handlers.
	BHandler (BMessage *data)
	Construct a handler from an archived message.
virtual status_t	Archive (BMessage *data, bool deep=true) const
	Archive a handler to a message.
Core Handler Functionality
virtual void	MessageReceived (BMessage *message)
	Handle message that has been received by the associated looper.
BLooper *	Looper () const
	Return a pointer to the looper that this handler is associated with.
void	SetName (const char *name)
	Set or change the name of this handler.
const char *	Name () const
	Return the name of this handler.
virtual void	SetNextHandler (BHandler *handler)
	Set the next handler in the chain that the message is passed on to if this handler cannot process it.
BHandler *	NextHandler () const
	Return the next hander in the chain to which the message is passed on.
Message Filtering
virtual void	AddFilter (BMessageFilter *filter)
	Add filter as a prerequisite to this handler.
virtual bool	RemoveFilter (BMessageFilter *filter)
	Remove filter from the filter list.
virtual void	SetFilterList (BList *filters)
	Set the internal list of filters to filters.
BList *	FilterList ()
	Return a pointer to the list of filters.
Locking
This class provides some utility functions to look the looper associated with this handler.
bool	LockLooper ()
	Lock the looper associated with this handler.
status_t	LockLooperWithTimeout (bigtime_t timeout)
	Lock the looper associated with this handler, with a time out value.
void	UnlockLooper ()
	Unlock the looper.
Scripting
virtual BHandler *	ResolveSpecifier (BMessage msg, int32 index, BMessage specifier, int32 form, const char *property)
	Determine the proper handler for a scripting message.
virtual status_t	GetSupportedSuites (BMessage *data)
	Reports the suites of messages and specifiers that derived classes understand.
Observing
Handlers can function as state machines, which emit messages to observers when the state changes. Use the following methods to subscribe to these notifications. Note that there is a semantic difference between the two StartWatching() methods. The overloaded method that accepts a BHandler, expects as argument an observer that watches this handler. The method that accepts a BMessenger, expects a target that emits the state changes to this handler.
status_t	StartWatching (BMessenger target, uint32 what)
	Subscribe this handler to watch a specific state change of a target.
status_t	StartWatchingAll (BMessenger target)
	Subscribe this handler to watch a target for all events.
status_t	StopWatching (BMessenger target, uint32 what)
	Unsubscribe this handler from watching a specific state.
status_t	StopWatchingAll (BMessenger target)
	Unsubscribe this handler from watching all states.
status_t	StartWatching (BHandler *observer, uint32 what)
	Subscribe an observer for a specific state change of this handler.
status_t	StartWatchingAll (BHandler *observer)
	Subscribe an observer for a all state changes.
status_t	StopWatching (BHandler *observer, uint32 what)
	Unsubscribe an observer from watching a specific state.
status_t	StopWatchingAll (BHandler *observer)
	Unsubscribe an observer from watching all states.
Emitting State Changes
If your handler functions as a state machine, and it has observers (which subscribed using the StartWatching() method), you can emit these state changes.
virtual void	SendNotices (uint32 what, const BMessage *notice=NULL)
	Emit a state change to the observers.
bool	IsWatched () const
	Check if there are any observers watching this handler.
Static Public Member Functions
static BArchivable *	Instantiate (BMessage *data)
	Static method to instantiate a handler from an archived message.

Detailed Description

Handles messages that are passed on by a BLooper.

The BHandler class implements two important pieces of functionality. It provides the foundations for handling messages, and it serves as a state machine that sends out notifications of the state changes.

The most common use of this class is to handle messages. Handlers can be tied to loopers, which are the objects that send and receive messages. As soon as a message is received, the looper passes through its list of associated handlers and tries them in a certain order until the message is handled, or the options are exhausted.

You should know that a looper is a subclass of a handler, and as such, loopers can be self-contained and do not need additional handlers. In many cases, this construction will suffice. You will simply subclass the looper, override its MessageReceived() hook and handle the messages you receive. In some cases, you might opt in for a more ingenious construction. A real-world example is the interface kit. Within that kit, the windows are represented by a BLooper, and all the views and controls in that kit are derived from BHandler. If you put a control in a window, then whenever messages such as clicks are received, the window loops the handlers until there is a handler that is at the screen position the click was in. It is not unlikely that you will some day want to use this functionality of the API.

If your handler is limited to a certain type of messages, you can set a filter that the looper will apply to your message before passing it on to your overridden MessageReceived() method. The BMessageFilter class provides the framework for the flexible filtering options, and using AddFilter() you can apply filters to this handler. Note that a filter object should only be applied to one handler. They cannot be shared.

For more information on the handling chain, have a look at the documentation of the BLooper class.

Using BHandler as a state machine is a second area of functionality. Since handlers process messages, and perform actions associated with those, they are the center of keeping track on the current state of things within an application. If you want to synchronize these states between different parts of your application, you could perform this manually by sending messages to the interested components, or you can use the more flexible approach with observers.

Observers watch a certain state. A handler can track one or more different states. Each state is represented by a four byte constant - just like the what property of a message. Using the StartWatching() methods, you can register observers both within your team, and in other applications. As an argument of that method, you can supply the state you want to watch, or you can register an observer using StartWatchingAll() to watch all the states the handler tracks. When the handler needs to emit a state change, you can use SendNotices(). You can specify the exact state change, and some data that you want to be send to the observers. This data is in the form of the very flexible BMessage, as such you are almost free to pass anything you want.

Whenever SendNotices() is called, all interested observers will receive a message of the B_OBSERVER_NOTICE_CHANGE type. Please note that the constant that is associated with the state itself is not transmitted. If you require this information, consider using the message that is passed on to describe the state change.

BHandler is a part of the chain in the eloquent messaging structure. For a proper understanding of all its facets, have a look at the messaging overview.

Constructor & Destructor Documentation

BHandler::BHandler ( const char * name = NULL )

Construct a new handler with a name.

The newly constructed handler is not associated with a looper until you explicitly request this to happen. To associate this handler with a looper, use BLooper::AddHandler().

BHandler::~BHandler ( ) [virtual]

Free the filters of this handler, as well as the list of observers.

This method does not remove the handler from the looper to which this handler is associated. You should do this yourself, using BLooper::RemoveHandler().

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.

BHandler::BHandler ( BMessage * data )

Construct a handler from an archived message.

This data has to be created using the BHandler::Archive() method. Note that only the name is stored. The filters, the associated looper and the observers are not stored, and should be manually added when you are using this object.

Member Function Documentation

void BHandler::AddFilter ( BMessageFilter * filter ) [virtual]

Add filter as a prerequisite to this handler.

If the handler is associated with a looper, this looper needs to be locked in order for this operation to succeed.

Note that the filter is not copied, rather a pointer to the filter is stored. As such, you need to make sure that the filter object exists as long as it is added to this handler.

See also:: RemoveFilter(), SetFilterList()

status_t BArchivable::AllArchived ( BMessage * into ) const [virtual, inherited]

Method relating to the use of 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.; You should archive any objects you own in your Archive() method implementation, and NOT your AllArchived() method.

See also:: BArchiver BArchiver::Finish()

Reimplemented in BView, BLayout, BGridLayout, BLayoutItem, BTwoDimensionalLayout, BAbstractLayout, and BGroupLayout.

status_t BArchivable::AllUnarchived ( const BMessage * archive ) [virtual, inherited]

Method relating to the use of 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 also:: BUnarchiver, BUnarchiver::Finish()

Reimplemented in BView, BLayout, BGridLayout, BLayoutItem, BTwoDimensionalLayout, BAbstractLayout, and BGroupLayout.

status_t BHandler::Archive	(	BMessage *	data,
		bool	deep = `true`
	)		const `[virtual]`

Archive a handler to a message.

Currently, only the name is archived. The filters, the associated looper and the observers are not stored.

Parameters:

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

Return values:

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

See also:: BHandler::Instantiate(BMessage *data)

Reimplemented from BArchivable.

Reimplemented in BView, BDirectWindow, BTextView, BAlert, BListView, BApplication, BLooper, BColorControl, BDragger, BControl, BButton, BScrollView, BBox, BChannelControl, and BCheckBox.

BList * BHandler::FilterList ( )

Return a pointer to the list of filters.

Returns:: A pointer to the list of filters. Do not manipulate the list of filters directly, but use the methods provided by this class, in order to maintain internal consistency.

See also:: AddFilter(), RemoveFilter(), SetFilterList().

BArchivable * BHandler::Instantiate ( BMessage * data ) [static]

Static method to instantiate a handler from an archived message.

Returns:: A pointer to the instantiated handler, or NULL if the data is not a valid archived BHandler object.

See also:: BHandler(BMessage* data)

Reimplemented from BArchivable.

Reimplemented in BView, BDirectWindow, BTextView, BAlert, BListView, BApplication, BLooper, BColorControl, BDragger, BControl, BBox, BButton, BScrollView, and BCheckBox.

bool BHandler::LockLooper ( )

Lock the looper associated with this handler.

Return values:

true	The looper is locked.
false	There was an error acquiring the lock.

See also:: LockLooperWithTimeout(), UnlockLooper()

status_t BHandler::LockLooperWithTimeout ( bigtime_t timeout )

Lock the looper associated with this handler, with a time out value.

Parameters:

timeout The time to wait for acquiring the lock in microseconds. You may also use B_INFINITE_TIMEOUT, in which this method will wait as long as it takes to acquire the lock.

Return values:

B_OK	Locking succeeded.
B_BAD_VALUE	This handler is not associated with a looper (anymore).
B_TIMED_OUT	The time specified in timeout has passed without locking the looper.

See also:: LockLooper(), UnlockLooper()

BLooper * BHandler::Looper ( ) const

Return a pointer to the looper that this handler is associated with.

Returns:: If the handler is not yet associated with a looper, it will return NULL.

See also:: BLooper::AddHandler(); LockLooper()

void BHandler::MessageReceived ( BMessage * message ) [virtual]

Handle message that has been received by the associated looper.

This method is reimplemented by subclasses. If the messages that have been received by a looper pass through the filters, then they end up in the MessageReceived() methods.

The example below shows a very common way to handle message. Usually, this involves parsing the BMessage::what constant and then perform an action based on that.

void
ShowImageApp::MessageReceived(BMessage *message)
{
    switch (message->what) {
        case MSG_FILE_OPEN:
            fOpenPanel->Show();
            break;

        case B_CANCEL:
            // File open panel was closed,
            // start checking count of open windows.
            StartPulse();
            break;
        
        default:
            // We do not handle this message, pass it on to the base class.
            BApplication::MessageReceived(message); 
            break;
    }
}

If your handler cannot process this message, you should pass it on to the base class. Eventually, it will reach the base implementation, which will reply with B_MESSAGE_NOT_UNDERSTOOD.

Attention:: If you want to keep or manipulate the message, have a look at BLooper::DetachCurrentMessage() to receive ownership of the message.

Parameters:

message The message that needs to be handled.

Reimplemented in BView, BTextView, BDirectWindow, BListView, BAlert, BScrollView, BApplication, BLooper, BBox, BColorControl, BChannelControl, BDragger, BButton, BCheckBox, and BControl.

const char * BHandler::Name ( ) const

Return the name of this handler.

See also:: SetName()

BHandler * BHandler::NextHandler ( ) const

Return the next hander in the chain to which the message is passed on.

See also:: SetNextHandler()

bool BHandler::RemoveFilter ( BMessageFilter * filter ) [virtual]

Remove filter from the filter list.

If the handler is associated with a looper, this looper needs to be locked in order for this operation to succeed.

Note that the filter is not deleted, merely removed from the list. You need to take care of the memory yourself.

Return values:

true	The filter was in the filter list and is removed.
false	The filter was not found in the filter list.

See also:: AddFilter(), FilterList()

void BHandler::SendNotices	(	uint32	what,
		const BMessage *	msg = `NULL`
	)		`[virtual]`

Emit a state change to the observers.

The actual state (specified by what) will not be transmitted. This is merely for internal bookkeeping. It is not entirely unimaginable that you still want to inform the observers of what actually took place. You can use the msg to transmit this, and any other data you want. Note that the message will be copied and slightly altered: the what member of the message will be B_OBSERVER_NOTICE_CHANGE, and the what constant you specified will be stored in the B_OBSERVE_ORIGINAL_WHAT label.

Parameters:

what	The identifier of the state.
msg	Any data associated with the state change. You retain ownership of this data, so make sure you dispose it when you are done.

void BHandler::SetFilterList ( BList * filters ) [virtual]

Set the internal list of filters to filters.

If the handler is associated with a looper, this looper needs to be locked in order for this operation to succeed.

The internal list will be replaced with the new list of filters. All the existing filters will be deleted.

See also:: AddFilter(), FilterList()

void BHandler::SetName ( const char * name )

Set or change the name of this handler.

See also:: Name()

void BHandler::SetNextHandler ( BHandler * handler ) [virtual]

Set the next handler in the chain that the message is passed on to if this handler cannot process it.

This method has three requirements:

This handler should belong to a looper.
The looper needs to be locked. See LockLooper().
The handler that you pass must be associated with the same looper.

Failure to meet any of these requirements will result in your application crashing.

By default, the handlers are chained in order that they were associated to a looper with BLooper::AddHander().

See also:: NextHandler()

status_t BHandler::StartWatching	(	BMessenger	target,
		uint32	what
	)

Subscribe this handler to watch a specific state change of a target.

Use this method to subscribe messengers to watch state changes in this handler, this also means that observers from other teams can be subscribed.

// Handler B watches Handler A
BHandler A, B;
BMessenger messengerA(&A)

B.StartWatching(messengerA, kNetworkConnection);

Parameters:

target	The messenger from which the notifications would be received.
what	The state that needs to be watched.

Returns:: During the call of this method, a notification will be transmitted using the target. If this works, then this method will return B_OK.

See also:: StartWatchingAll(BMessenger), StopWatching(BMessenger, uint32)

status_t BHandler::StartWatching	(	BHandler *	observer,
		uint32	what
	)

Subscribe an observer for a specific state change of this handler.

Use this method to subscribe observers to watch this handler. State changes of this handler that match the what argument, will be sent.

// Handler B wants to observe Handler A
BHandler A, B;

A.StartWatching(&B, kNetworkConnection);

Since pointers to handlers can only exist in the local namespace, have a look at StartWatching(BMessenger, uint32) for inter-team watching.

Parameters:

observer	The observer for this handler.
what	The state that needs to be watched.

Returns:: During the call of this method, a notification will be transmitted using the observer. If this works, then this method will return B_OK.

See also:: StartWatchingAll(BHandler*), StopWatching(BHandler*, uint32)

status_t BHandler::StartWatchingAll ( BMessenger target )

Subscribe this handler to watch a target for all events.

This method performs the same task as StartWatching(BMessenger, uint32), but it will subscribe to all the state changes the target knows.

See also:: StartWatching(BMessenger, uint32), StopWatchingAll(BMessenger)

status_t BHandler::StartWatchingAll ( BHandler * observer )

Subscribe an observer for a all state changes.

This method performs the same task as StartWatching(BHandler, uint32), but it will subscribe the observer to all the state changes this handler tracks.