MVC: how do I stop mutual recursion?

6 Message(s) by 5 Author(s) originally posted in java gui

From: Mungo Henning

Date: Sunday, September 23, 2007

Hi Folks,
Okays, although I'm not the oldest person here I'm struggling to understand
an issue in MVC

1. <programming> {Model View Controller}.

	2. <filename extension> The {filename extension} of {JPEG}
	images output by {Sony}'s {Mavica} range of {digital cameras}.

	(2002-05-28)

with a Swing JAVA application

1. {application program}.

	2. {function application}.

.
If someone could point

1. <unit, text> (Sometimes abbreviated "pt") The unit of
	length used in {typography} to specify text character height,
	{rule} width, and other small measurements.

	There are six slightly different definitions: {Truchet point},
	{Didot point}, {ATA point}, {TeX point}, {Postscript point},
	and {IN point}.

	In Europe, the most commonly used is Didot and in the US, the
	formerly standard ATA point has essentially been replaced by
	the PostScript point due to the demise of traditional
	typesetting systems and rise of desktop computer based systems
	running software such as {QuarkXPress}, {Adobe InDesign} and
	{Adobe Pagemaker}.

	There are 20 {twips} in a point and 12 points in a {pica}
	(known as a "Cicero" in the Didot system).

	{Different point systems
	(http://www.vakcer.com/oberon/dtp/fonts/point.htm)}.

	(2004-12-23)

	2. <hardware> To move a {pointing device} so that the
	on-screen pointer is positioned over a certain object on the
	screen such as a {button} in a {graphical user interface}.  In
	most {window systems} it is then necessary to {click} a
	(physical) button on the pointing device to activate or select
	the object.  In some systems, just pointing to an object is
	known as "mouse-over" {event} which may cause some help text
	(called a "tool tip" in {Windows}) to be displayed.

	(2001-05-21)

me in the right direction I'd be obliged.

Here's the problem: I'm writing a little application and the idea of MVC
tickles me such that I
want to learn about it and implement it to fully appreciate MVC.
Hypothetically, if I have a plain "String" object

<object-oriented> In {object-oriented programming}, an
	instance of the data structure and behaviour defined by the
	object's {class}.  Each object has its own values for the
	{instance variables} of its class and can respond to the
	{methods} defined by its class.

	For example, an object of the "Point" class might have
	instance variables "x" and "y" and might respond to the "plot"
	method by drawing a dot on the screen at those coordinates.

	(2004-01-26)

in my model and a
"JTextField" object on my display, I
would software

<programming> (Or "computer program", "program", "code") The
	instructions executed by a computer, as opposed to the
	physical device on which they run (the "{hardware}").

	The term was coined by the eminent statistician, {John Tukey}.

	Programs stored on {non-volatile storage} built from
	{integrated circuits} (e.g. {ROM} or {PROM}) are usually
	called {firmware}.

	Software can be split into two main types - {system software}
	and application software or {application programs}.  System
	software is any software required to support the production or
	execution of application programs but which is not specific to
	any particular application.  Examples of system software would
	include the {operating system}, {compilers}, editors and
	sorting programs.

	Examples of application programs would include an accounts
	package or a {CAD} program.  Other broad classes of
	application software include {real-time} software, {business
	software}, scientific and engineering software, {embedded
	software}, personal computer software and {artificial
	intelligence} software.

	Software includes both {source code} written by humans and
	executable {machine code} produced by {assemblers} or
	{compilers}.  It does not usually include the data processed
	by programs unless this is in a format such as {multimedia}
	which depends on the use of computers for its presentation.
	This distinction becomes unclear in cases such as {spread
	sheets} which can contain both instructions (formulae and
	{macros}) and data.  There are also various intermediate
	compiled or {semi-compiled}, forms of software such as
	{library} files and {byte-code}.

	Some claim that {documentation} (both paper and electronic) is
	also software.  Others go further and define software to be
	programs plus documentation though this does not correspond
	with common usage.

	The noun "program" describes a single, complete and
	more-or-less self-contained list of instructions, often stored
	in a single {file}, whereas "code" and "software" are
	uncountable nouns describing some number of instructions which
	may constitute one or more programs or part thereof.  Most
	programs, however, rely heavily on various kinds of {operating
	system} software for their execution.  The nounds "code" and
	"software" both refer to the same thing but "code" tends to
	suggest an interest in the implementation details whereas
	"software" is more of a user's term.

	(2002-07-21)

connect

<library, networking> {Unix} socket library routine to connect
	a socket that has been created on the local hosts to one at a
	specified socket address on the remote host.

	{Unix manual pages}: connect(2), accept(2).

	(1995-03-21)

these by telling the Model when the end user changes
the text

1. Executable code, especially a "pure code" portion shared
	between multiple instances of a program running in a
	{multitasking} {operating system}.

	Compare {English}.

	2. Textual material in the mainstream sense; data in ordinary
	{ASCII} or {EBCDIC} representation (see {flat ASCII}).  "Those
	are text files; you can review them using the editor."

	These two contradictory senses confuse hackers too.

	[{Jargon File}]

	(1995-03-16)

field, and using
the Observer pattern tell the JTextField (and any other viewers) when the
model data

<data, data processing, jargon> /day't*/ (Or "raw data")
	Numbers, {characters}, {images}, or other method of recording,
	in a form which can be assessed by a human or (especially)
	input into a {computer}, stored and {processed} there, or
	transmitted on some {digital channel}.  Computers nearly
	always represent data in {binary}.

	Data on its own has no meaning, only when interpreted by some
	kind of {data processing system} does it take on meaning and
	become {information}.

	People or computers can find patterns in data to perceive
	information, and information can be used to enhance
	{knowledge}.  Since knowledge is prerequisite to wisdom, we
	always want more data and information.  But, as modern
	societies verge on {information overload}, we especially need
	better ways to find patterns.

	1234567.89 is data.

	"Your bank balance has jumped 8087% to $1234567.89" is
	information.

	"Nobody owes me that much money" is knowledge.

	"I'd better talk to the bank before I spend it, because of
	what has happened to other people" is wisdom.

	(1999-04-30)

changes.

So if the end user types the first character

<character> An {atom} in a {character repertoire}.

	Compare with {glyph}.

	(1998-10-18)

in the JTextField then this
JTextField nudges the associated
Model and says "data has changed; you'd better update your String".
The model then updates its String object and then it has to tell ALL its
viewers that its data has changed.
These viewers include

[{Usenet}] 1. To duplicate a portion (or whole) of another's
	message (typically with attribution to the source) in a reply
	or followup, for clarifying the context of one's response.
	See the discussion of inclusion styles under "Hacker Writing
	Style".

	2. [{C}] "#include <disclaimer.h>" has appeared in {sig
	blocks} to refer to a notional "standard {disclaimer} file".

	[{Jargon File}]

the originating JTextField, hence we get a mutual
loop: A calls B and then B calls A which
leads to A calling B again...

Knowing that others have solved this, how do people break

1. To cause to be {broken} (in any sense).  "Your latest patch
	to the editor broke the paragraph commands."

	2. (Of a program) To stop temporarily, so that it may
	debugged.  The place where it stops is a "{breakpoint}".

	3. To send an {EIA-232} break (two character widths of line
	high) over a {serial line}.

	4. [Unix] To strike whatever key currently causes the tty
	driver to send SIGINT to the current process.  Normally,
	break, delete or {control-C} does this.

	5. "break break" may be said to interrupt a conversation (this
	is an example of verb doubling).  This usage comes from radio
	communications, which in turn probably came from landline
	telegraph/teleprinter usage, as badly abused in the Citizen's
	Band craze a few years ago.

	6. {pipeline break}.

	7. {break statement}.

	[{Jargon File}]

	(2004-03-24)

this recursion?
I'd think that whomever informs the Model that the data has changed ought
not be on the list

<data> A data structure holding many values, possibly of
	different types, which is usually accessed sequentially,
	working from the head to the end of the tail - an "ordered
	list".  This contrasts with a (one-dimensional) {array}, any
	element of which can be accessed equally quickly.

	Lists are often stored using a cell and pointer arrangement
	where each value is stored in a cell along with an associated
	pointer to the next cell.  A special pointer, e.g. zero, marks
	the end of the list.  This is known as a (singly) "linked
	list".  A doubly linked list has pointers from each cell to
	both next and previous cells.

	An unordered list is a {set}.

	(1998-11-12)

of Viewers that
the model should subsequently contact.

MVC sounds wonderful; someone enlighten my ignorance please.

Thanks in advance

Mungo :-)

From: Roedy Green

Date: Sunday, September 23, 2007

On Sun, 23 Sep 2007 18:29:37 +0100, "Mungo Henning" <mungoh@xxxxxxxxxxx>
wrote in message, quoted or indirectly quoted someone who said :

Knowing that others have solved this, how do people break this recursion?

I had a similar problem in JColourChooser. I simply created a boolean
called "propInProgress". When it was true, it suppressed propagation
of changes in a spreadsheet-like network

<networking> Hardware and software data communication systems.

	The {OSI} seven layer model attempts to provide a way of
	partitioning any computer network into independent modules
	from the lowest (physical) layer to the highest (application)
	layer.  Many different specifications exist at each of these
	layers.

	Networks are often also classified according to their
	geographical extent: {local area network} (LAN), {metropolitan
	area network} (MAN), {wide area network} (WAN) and also
	according to the {protocols} used.

	See {BITNET}, {Ethernet}, {Internet}, {Novell}, {PSTN},
	{network, the}.

	[Tanenbaum, A., "Computer Networks; 2nd ed.", Prentice Hall,
	Englewood Cliffs, NJ, 1989.]

	(1995-03-10)

of elements.

In a simple case, you can sometimes simply ignore or stop listening to
event

1. <software> An occurrence or happening of significance to a
	task or program, such as the completion of an asynchronous
	input/output operation.  A task may wait for an event or any
	of a set of events or it may (request to) receive asynchronous
	notification (a {signal} or {interrupt}) that the event has
	occurred.

	See also {event-driven}.

	2. <data> A transaction or other activity that affects the
	records in a file.

	(2000-02-09)

s at A B when each is triggering unwanted changes in the other.
--
Roedy Green

A language proposed by Cii {Honeywell-Bull} to meet the DoD
	{Ironman} requirements which led to {Ada}.  This language won
	in 1979.

	["On the GREEN Language Submitted to the DoD", E.W. Dijkstra,
	SIGPLAN Notices 13(10):16-21 (Oct 1978)].

	(1994-12-02)

Canadian Mind Products
The JAVA Glossary
http://mindprod.com

From: Mungo Henning

Date: Sunday, September 23, 2007

Hi Roedy,
Obliged for your speedy reply.

On Sun, 23 Sep 2007 18:29:37 +0100, "Mungo Henning" <mungoh@xxxxxxxxxxx>
wrote in message, quoted or indirectly quoted someone who said :
Knowing that others have solved this, how do people break this recursion?
I had a similar problem in JColourChooser. I simply created a boolean
called "propInProgress". When it was true, it suppressed propagation
of changes in a spreadsheet-like network of elements.
In a simple case, you can sometimes simply ignore or stop listening to
events at A B when each is triggering unwanted changes in the other.

Hmmm... but that means that each side of the fence

1. A sequence of one or more distinguished ({out-of-band})
	characters (or other data items), used to delimit a piece of
	data intended to be treated as a unit (the computer-science
	literature calls this a "sentinel").  The NUL (ASCII 0000000)
	character that terminates strings in C is a fence.  {Hex} FF
	is also (though slightly less frequently) used this way.  See
	{zigamorph}.

	2. An extra data value inserted in an array or other data
	structure in order to allow some normal test on the array's
	contents also to function as a termination test.  For example,
	a highly optimised routine for finding a value in an array
	might artificially place a copy of the value to be searched
	for after the last slot of the array, thus allowing the main
	search loop to search for the value without having to check at
	each pass whether the end of the array had been reached.

	3. [among users of optimising compilers] Any technique,
	usually exploiting knowledge about the compiler, that blocks
	certain optimisations.  Used when explicit mechanisms are not
	available or are overkill.  Typically a hack: "I call a dummy
	procedure there to force a flush of the optimiser's
	register-colouring info" can be expressed by the shorter
	"That's a fence procedure".

	[{Jargon File}]

	(1999-01-08)

(well, at least one side
of the fence) has
to have extra code

<software> Instructions for a computer in some programming
	language, often {machine language}.  The word "code" is often
	used to distinguish instructions from {data} (e.g. "The code
	is marked 'read-only'") whereas "{software}" is used in
	contrast with "{hardware}" and may consist of more than just
	code.

	(2000-04-08)

to break the spiral.
So the clean model of "when view changes, alert

<operating system> /*'l*rt/ An audible and/or visual message
	intended to inform a system's users or administrators about a
	change in the operating conditions of that system or about
	some kind of error condition.  In a {graphical user
	interface}, an alert would typically be displayed as a small
	window containing the message and a button to click to dismiss
	the window.

	(1999-03-29)

the model; when model
changes, alert all views" has to become
a little muddied.
Could there not be a case here to tweak

1. To change slightly, usually in reference to a value.  Also
	used synonymously with {twiddle}.  If a program is almost
	correct, rather than figure out the precise problem you might
	just keep tweaking it until it works.  See {frobnicate} and
	{fudge factor}; also see {shotgun debugging}.

	2. To {tune} or {bum} a program; preferred usage in the UK.

	[{Jargon File}]

the "observer" capability: when the
view tells the model that things have
changed the model responds to every other view EXCEPT the triggering view?

Just still trying to find the magic code that takes care of this for me...!

Regards

Mungo :-)

From: ram

Date: Sunday, September 23, 2007

"Mungo Henning" <mungoh@xxxxxxxxxxx> writes:

So the clean model of "when view changes, alert the model; when
model changes, alert all views" has to become a little muddied.

JAVA (Swing) doesn't use MVC.

In MVC, the view never notifies the model.

In MVC, the controller

<hardware> Part of a computer, typically a separate circuit
	board, which allows the computer to use certain kinds of
	{peripheral} devices.  A {disk controller} is used to connect
	{hard disks} and {floppy disks}, a {network controller} is
	used for {Ethernet}.  Other controllers are: {keyboard
	controller}, {interrupt controller} and {graphics controller}.

	(1998-03-16)

notifies the model.

In MVC, there is no loop, because the change initiated by a
user action flows from the controller to the model and then to
the view and then the flow has come to a halt - no loop.

I am aware of the problem you've posed,
but this has to do with Swing - not with MVC.

For Swing Architecture, see:

http://JAVA.sun.com/products/jfc/tsc/articles/architecture/

For MVC, see:

http://st-www.cs.uiuc.edu/users/smarch/st-docs/mvc.html

From: Filip Larsen

Date: Sunday, September 23, 2007

wrote in message

Knowing that others have solved this, how do people break this recursion?Two possible techniques is to either use a flag

1. <programming> A variable or quantity that can take on one
	of two values; a bit, particularly one that is used to
	indicate one of two outcomes or is used to control which of
	two things is to be done.  "This flag controls whether to
	clear the screen before printing the message."  "The program
	status word contains several flag bits."  See also {hidden
	flag}, {mode bit}.

	2. {command line option}.

	[{Jargon File}]

	(1998-05-02)

in the text field

<data, database> An area of a {database} {record}, or
	{graphical user interface} {form}, into which a particular
	item of data is entered.

	Example usage: "The telephone number field is not really a
	numerical field", "Why do we need a four-digit field for the
	year?".

	A {database} {column} is the set of all instances of a given
	field from all records in a {table}.

	(1999-04-26)

listener to indicate an update is in progress and avoid changing the
text field if this flag is set, or to rely on all the models in use will
avoid emitting a change event if setting the same value as previously,
or both.

The first approach requires that you code it yourself, while the second
is support

After-sale handholding; something many software vendors
	promise but few deliver.  To hackers, most support people are
	useless - because by the time a hacker calls support he or she
	will usually know the software and the relevant manuals better
	than the support people (sadly, this is *not* a joke or
	exaggeration).  A hacker's idea of "support" is a tte--tte
	or exchange of {electronic mail} with the software's designer.

	[{Jargon File}]

	(1995-02-15)

ed as standard

<standard> Standards are necessary for {interworking},
	{portability}, and {reusability}.  They may be {de facto
	standards} for various communities, or officially recognised
	national or international standards.

	{Andrew Tanenbaum}, in his Computer Networks book, once said,
	"The nice thing about standards is that there are so many of
	them to choose from", a reference to the fact that competing
	standards become a source of confusion, division,
	obsolescence, and duplication of effort instead of an
	enhancement to the usefulness of products.

	Some bodies concerned in one way or another with computing
	standards are {IAB} ({RFC} and {STD}), {ISO}, {ANSI}, {DoD},
	{ECMA}, {IEEE}, {IETF}, {OSF}, {W3C}.

	(1999-07-06)

by class

1. <programming> The prototype for an {object} in an
	{object-oriented language}; analogous to a {derived type} in a
	{procedural language}.  A class may also be considered to be a
	set of objects which share a common structure and behaviour.
	The structure of a class is determined by the {class
	variables} which represent the {state} of an object of that
	class and the behaviour is given by a set of {methods}
	associated with the class.

	Classes are related in a {class hierarchy}.  One class may be
	a specialisation (a "{subclass}") of another (one of its
	"{superclasses}") or it may be composed of other classes or it
	may use other classes in a {client-server} relationship.  A
	class may be an {abstract class} or a {concrete class}.

	See also {signature}.

	2. <programming> See {type class}.

	3. <networking> One of three types of {Internet addresses}
	distinguished by their most significant bits.

	3. <language> A language developed by the {Andrew Project}.
	It was one of the first attempts to add {object-oriented}
	features to {C}.

	(1995-05-01)

es that use
JAVA.beans.PropertyChangeSupport for emitting change events.

If I recall correctly, not many swing models support suppression of
change events when the new value is "equal" to the old, so there you
basically has to use the first approach.Regards,
--
Filip Larsen

From: ralpe

Date: Monday, September 24, 2007

I solved this problem by creating my own text field class that derives
from JTextField and overrides the setText method. The overriden method
only invokes super.setText if the new string

<programming> A sequence of {data} values, usually {bytes},
	which usually stand for {characters} (a "character string").
	The {mapping} between values and characters is determined by
	the {character set} which is itself specified implcitly or
	explicitly by the environment in which the string is being
	interpreted.

	The most common character set is {ASCII} but, since the late
	1990s, there has been increased interest in larger character
	sets such as {Unicode} where each character is represented by
	more than eight {bits}.

	Most programming languages consider strings (e.g.
	"124:shabooya:\n", "hello world") basically distinct from
	numbers which are typically stored in fixed-length {binary} or
	{floating-point} representation.

	A {bit string} is a sequence of {bits}.

	(1999-12-21)

is different from
getText().

hth
ralph

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