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== Observer Pattern ==
The Observer is a design pattern used in computer programming. The Observer pattern has two parts, the subject and the observer. The pattern has a one-to-many dependency between a subject object and the observer object(s). All the observer objects are updated when a subject changes state. It is used extensively in the AWT/Swing Listeners in Java.
== UML Example ==
}
}
}
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== Real World Example ==
Here is some code from the Mozilla Internet Browser V 1.7a
<pre>
/* -*- Mode: C; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* ***** BEGIN LICENSE BLOCK *****
* Version: NPL 1.1/GPL 2.0/LGPL 2.1
*
* The contents of this file are subject to the Netscape Public License
* Version 1.1 (the "License"); you may not use this file except in
* compliance with the License. You may obtain a copy of the License at
* http://www.mozilla.org/NPL/
*
* Software distributed under the License is distributed on an "AS IS" basis,
* WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
* for the specific language governing rights and limitations under the
* License.
*
* The Original Code is mozilla.org code.
*
* The Initial Developer of the Original Code is
* Netscape Communications Corporation.
* Portions created by the Initial Developer are Copyright (C) 1998
* the Initial Developer. All Rights Reserved.
*
* Contributor(s):
*
* Alternatively, the contents of this file may be used under the terms of
* either the GNU General Public License Version 2 or later (the "GPL"), or
* the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
* in which case the provisions of the GPL or the LGPL are applicable instead
* of those above. If you wish to allow use of your version of this file only
* under the terms of either the GPL or the LGPL, and not to allow others to
* use your version of this file under the terms of the NPL, indicate your
* decision by deleting the provisions above and replace them with the notice
* and other provisions required by the GPL or the LGPL. If you do not delete
* the provisions above, a recipient may use your version of this file under
* the terms of any one of the NPL, the GPL or the LGPL.
*
* ***** END LICENSE BLOCK ***** */
#include "prmem.h"
#include "xp_obs.h"
#include "prclist.h"
#include "prtypes.h"
#define MK_OUT_OF_MEMORY -1
typedef struct Observer
{
struct PRCListStr mLink;
XP_ObserverProc mCallback;
void* mClosure;
} Observer;
#define NextObserver(_obsptr_) ((Observer*)(_obsptr_)->mLink.next)
#define ObserverLinks(_obsptr_) (&((_obsptr_)->mLink))
struct OpaqueObserverList
{
Observer* mObserverList;
XP_Observable mObservable;
PRBool mNotificationEnabled;
};
/*
Creates a new XP_Observable, to which you can add observers,
who are notified when XP_NotifyObservers is called.
Observerer notification is enabled by default.
*/
NS_Error
XP_NewObserverList(
XP_Observable inObservable,
XP_ObserverList* outObserverList )
{
NS_Error result = 0;
PR_ASSERT(outObserverList != NULL);
*outObserverList = PR_MALLOC(sizeof(struct OpaqueObserverList));
if (*outObserverList != NULL)
{
(*outObserverList)->mObserverList = NULL;
(*outObserverList)->mObservable = inObservable;
(*outObserverList)->mNotificationEnabled = PR_TRUE;
}
else {
result = MK_OUT_OF_MEMORY;
}
return result;
}
/*
Disposes of an XP_Observable. Does nothing with
its observers.
*/
void
XP_DisposeObserverList(
XP_ObserverList inObserverList )
{
Observer *obs, *next = NULL;
PR_ASSERT(inObserverList != NULL);
for (obs = inObserverList->mObserverList; obs != NULL; )
{
next = NextObserver(obs);
if (next == obs) {
break;
}
PR_REMOVE_LINK(ObserverLinks(obs));
inObserverList->mObserverList = next;
PR_FREEIF(obs);
obs = inObserverList->mObserverList;
}
PR_FREEIF(inObserverList);
}
/*
XP_GetObserverListObservable
*/
XP_Observable
XP_GetObserverListObservable(
XP_ObserverList inObserverList)
{
PR_ASSERT(inObserverList != NULL);
return inObserverList->mObservable;
}
void
XP_SetObserverListObservable(
XP_ObserverList inObserverList,
XP_Observable inObservable )
{
PR_ASSERT(inObserverList != NULL);
inObserverList->mObservable = inObservable;
}
/*
Registers a function pointer and void* closure
as an "observer", which will be called whenever
XP_NotifyObservers is called an observer notification
is enabled.
*/
NS_Error
XP_AddObserver(
XP_ObserverList inObserverList,
XP_ObserverProc inObserver,
void* inClosure )
{
Observer* obs;
NS_Error result = 0;
PR_ASSERT(inObserverList != NULL);
if (inObserverList == NULL) {
return -1;
}
obs = PR_MALLOC(sizeof (Observer));
if (obs != NULL)
{
obs->mCallback = inObserver;
obs->mClosure = inClosure;
if (inObserverList->mObserverList == NULL)
{
PR_INIT_CLIST(ObserverLinks(obs));
inObserverList->mObserverList = obs;
}
else {
PR_INSERT_AFTER(ObserverLinks(obs), ObserverLinks(inObserverList->mObserverList));
}
} else {
result = MK_OUT_OF_MEMORY;
}
return result;
}
/*
Removes a registered observer. If there are duplicate
(XP_ObserverProc/void* closure) pairs registered,
it is undefined which one will be removed.
Returns false if the observer is not registered.
*/
PRBool
XP_RemoveObserver(
XP_ObserverList inObserverList,
XP_ObserverProc inObserver,
void* inClosure )
{
PRBool result = PR_FALSE;
if ( inObserverList->mObserverList != NULL )
{
Observer *tail = (Observer*) PR_LIST_TAIL(ObserverLinks(inObserverList->mObserverList));
Observer *obs = inObserverList->mObserverList;
do
{
if (obs->mCallback == inObserver && obs->mClosure == inClosure)
{
PR_REMOVE_LINK(ObserverLinks(obs));
if (obs == inObserverList->mObserverList)
{
Observer* next = NextObserver(obs);
inObserverList->mObserverList = (next != obs) ? next : NULL;
}
PR_FREEIF(obs);
result = PR_TRUE;
break;
}
obs = NextObserver(obs);
} while (obs != tail);
}
return result;
}
/*
If observer notification is enabled for this XP_Observable,
this will call each registered observer proc, passing it
the given message and void* ioData, in addition to the
observer's closure void*.
There is no defined order in which observers are called.
*/
void
XP_NotifyObservers(
XP_ObserverList inObserverList,
XP_ObservableMsg inMessage,
void* ioData )
{
Observer* obs;
Observer *tail;
Observer *temp;
PRBool done = PR_FALSE;
if ( ! inObserverList->mNotificationEnabled ||
inObserverList->mObserverList == NULL)
{
return;
}
obs = inObserverList->mObserverList;
tail = (Observer *) PR_LIST_TAIL(ObserverLinks(obs));
do
{
if (obs == tail) done = PR_TRUE;
(obs->mCallback)(inObserverList->mObservable, inMessage, ioData, obs->mClosure);
if (done != PR_TRUE) {
temp = inObserverList->mObserverList; /* just in case this callback free this observable
entry. */
if (temp != obs) {
obs = temp;
tail = (Observer *) PR_LIST_TAIL(ObserverLinks(obs));
}
else
obs = NextObserver(obs);
}
} while (done != PR_TRUE);
}
/*
When called, subsequent calls to XP_NotifyObservers
will do nothing until XP_EnableObserverNotification
is called.
*/
void
XP_DisableObserverNotification(
XP_ObserverList inObserverList )
{
inObserverList->mNotificationEnabled = PR_FALSE;
}
/*
Enables calling observers when XP_NotifyObservers
is invoked.
*/
void
XP_EnableObserverNotification(
XP_ObserverList inObserverList )
{
inObserverList->mNotificationEnabled = PR_TRUE;
}
/*
Returns true if observer notification is enabled.
*/
PRBool
XP_IsObserverNotificationEnabled(
XP_ObserverList inObserverList )
{
return inObserverList->mNotificationEnabled;
}
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