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authorNorbert Preining <norbert@preining.info>2019-09-02 13:46:59 +0900
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+% Copyright 2008 by Till Tantau
+%
+% This file may be distributed and/or modified
+%
+% 1. under the LaTeX Project Public License and/or
+% 2. under the GNU Free Documentation License.
+%
+% See the file doc/generic/pgf/licenses/LICENSE for more details.
+
+
+\section{Object-Oriented Programming}
+\label{section-oop}
+
+This section describes the |oo| module.
+
+\begin{pgfmodule}{oo}
+ This module defines a relatively small set of \TeX\ commands for defining
+ classes, methods, attributes and objects in the sense of object-oriented
+ programming.
+\end{pgfmodule}
+
+In this chapter it is assumed that you are familiar with the basics of a
+typical object-oriented programming language like Java, C++ or Eiffel.
+
+
+\subsection{Overview}
+
+\TeX\ does not support object-oriented programming, presumably because it was
+written at a time when this style of programming was not yet ``en vogue''. When
+one is used to the object-oriented style of thinking, some programming
+constructs in \TeX\ often seem overly complicated. The object-oriented
+programming module of \pgfname\ may help here. It is written completely using
+simple \TeX\ macros and is, thus, perfectly portable. This also means, however,
+that it is not particularly fast (but not too slow either), so you should use
+it only for non-time-critical things.
+
+Basically, the oo-system supports \emph{classes} (in the object-oriented sense,
+this has nothing to do with \LaTeX-classes), \emph{methods},
+\emph{constructors}, \emph{attributes}, \emph{objects}, \emph{object
+identities}, and (thanks to Sa\v o \v Zivanovi\'c) \emph{inheritance} and
+\emph{overloading.}
+
+The first step is to define a class, using the macro |\pgfooclass| (all normal
+macros in \pgfname's object-oriented system start with |\pgfoo|). This macro
+gets the name of a class and in its body a number of \emph{methods} are
+defined. These are defined using the |\method| macro (which is defined only
+inside such a class definition) and they look a bit like method definitions in,
+say, Java. Object attributes are declared using the |\attribute| command, which
+is also defined only inside a class definition.
+
+Once a class has been defined, you can create objects of this class. Objects
+are created using |\pgfoonew|. Such an object has many characteristics of
+objects in a normal object-oriented programming language: Each object has a
+\emph{unique identity}, so when you create another object, this object is
+completely distinct from all other objects. Each object also has a set of
+private attributes, which may change over time. Suppose, for instance, that we
+have a |point| class. Then creating a new object (called an instance) of this
+class would typically have an |x|-attribute and a |y|-attribute. These can be
+changed over time. Creating another instance of the |point| class creates
+another object with its own |x|- and |y|-attributes.
+
+Given an object, you can call a method for this object. Inside the method the
+attributes of the object for which the method is being called can be accessed.
+
+The life of an object always ends with the end of the \TeX\ scope in which it
+was created. However, changes to attribute values are not local to scopes, so
+when you change an attribute anywhere, this change persists till the end of the
+life of the object or until the attribute is changed again.
+
+
+\subsection{A Running Example: The Stamp Class}
+
+As a running example we will develop a |stamp| class and |stamp| objects. The
+idea is that a stamp object is able to ``stamp something'' on a picture. This
+means that a stamp object has an attribute storing the ``stamp text'' and there
+is a method that asks the object to place this text somewhere on a canvas. The
+method can be called repeatedly and there can be several different stamp
+objects, each producing a different text. Stamp objects can either be created
+dynamically when needed or a library might define many such objects in an outer
+scope.
+
+Such stamps are similar to many things present in \pgfname\ such as arrow tips,
+patterns, or shadings and, indeed, these could all have been implemented in
+this object-oriented fashion (which might have been better, but the
+object-oriented subsystem is a fairly new addition to \pgfname).
+
+
+\subsection{Classes}
+
+We start with the definition of the |stamp| class. This is done using the
+|\pgfooclass| macro:
+
+\begin{command}{\pgfooclass\opt{|(|\meta{list of superclasses}|)|}\marg{class name}\marg{body}}
+ This command defines a class named \meta{class name}. The name of the class
+ can contain spaces and most other characters, but no periods. So, valid
+ class names are |MyClass| or |my class| or |Class_C++_emulation??1|. The
+ \meta{list of superclasses} is optional just like the parenthesis around
+ it.
+
+ The \meta{body} is actually just executed, so any normal \TeX-code is
+ permissible here. However, while the \meta{body} is being executed, the
+ macros |\method| and |\attribute| are set up so that they can be used to
+ define methods and attributes for this class (the original meanings are
+ restored afterward).
+
+ The definition of a class is local to the scope where the class has been
+ defined.
+ %
+\begin{codeexample}[code only]
+\pgfooclass{stamp}{
+ % This is the class stamp
+
+ \attribute text;
+ \attribute rotation angle=20;
+
+ \method stamp(#1) { % The constructor
+ ...
+ }
+
+ \method apply(#1,#2) { % Causes the stamp to be shown at coordinate (#1,#2)
+ ...
+ }
+}
+
+% We can now create objects of type "stamp"
+\end{codeexample}
+
+ Concerning the list of base classes, the Method Resolution Order
+ (\textsc{mro}) is computed using the C3 algorithm also used in Python, v2.3
+ and higher. The linearization computed by the algorithm respects both
+ local precedence ordering and monotonicity. Resolution of both methods and
+ attributes depends on the \textsc{mro}: when a method method name is called
+ on an object of class $C$, the system invokes method method name from the
+ first class in the \textsc{mro} of $C$ which defines method method name;
+ when an object is created, each attribute |attr| is initialized to the
+ value specified in the first class in the \textsc{mro} of $C$ which
+ declares attribute |attr|.
+\end{command}
+
+The \meta{body} of a class usually just consists of calls to the macros
+|\attribute| and |\method|, which will be discussed in more detail in later
+sections.
+
+
+\subsection{Objects}
+
+Once a class has been declared, we can start creating objects for this class.
+For this the |\pgfoonew| command can be used, which has a peculiar syntax:
+
+\begin{command}{\pgfoonew\opt{\meta{object handle or attribute}|=|}|new |\meta{class name}|(|\meta{constructor arguments}|)|}
+ Causes a new object to be created. The class of the object will be
+ \meta{class name}, which must previously have been declared using
+ |\pgfooclass|. Once the object has been created, the constructor method of
+ the object will be called with the parameter list set to \meta{constructor
+ arguments}.
+
+ The resulting object is stored internally and its lifetime will end exactly
+ at the end of the current scope.
+
+ Here is an example in which three stamp objects are created.
+ %
+\begin{codeexample}[code only]
+\pgfoonew \firststamp=new stamp()
+\pgfoonew \secondstamp=new stamp()
+{
+ \pgfoonew \thirdstamp=new stamp()
+ ...
+}
+% \thirdstamp no longer exists, but \firststamp and \secondstamp do
+% even if you try to store \thirdstamp in a global variable, trying
+% to access it will result in an error.
+\end{codeexample}
+
+ The optional \meta{object handle or attribute} can either be an
+ \meta{object handle} or an \meta{attribute}. When an \meta{object handle}
+ is given, it must be a normal \TeX\ macro name that will ``point'' to the
+ object (handles are discussed in more detail in
+ Section~\ref{section-identities}). You can use this macro to call methods
+ of the object as discussed in the following section. When an
+ \meta{attribute} is given, it must be given in curly braces (the curly
+ braces are used to detect the presence of an attribute). In this case, a
+ handle to the newly created object is stored in this attribute.
+ %
+\begin{codeexample}[code only]
+\pgfooclass{foo}
+{
+ \attribute stamp obj;
+ \attribute another object;
+
+ \method foo() {
+ \pgfoonew{stamp obj}=new stamp()
+ \pgfoonew{another object}=new bar()
+ }
+ ...
+}
+\end{codeexample}
+ %
+\end{command}
+
+\begin{command}{\pgfoogc}
+ This command causes the ``garbage collector'' to be invoked. The job of
+ this garbage collector is to free the global \TeX-macros that are used by
+ ``dead'' objects (objects whose life-time has ended). This macro is called
+ automatically after every scope in which an object has been created, so you
+ normally do not need to call this macro yourself.
+\end{command}
+
+
+\subsection{Methods}
+
+Methods are defined inside the body of classes using the following command:
+
+\begin{command}{\method \meta{method name}|(|\meta{parameter list}|)|\marg{method body}}
+ This macro, which is only defined inside a class definition, defines a new
+ method named \meta{method name}. Just like class names, method names can
+ contain spaces and other characters, so \meta{method names} like
+ |put_stamp_here| or |put stamp here| are both legal.
+
+ Three method names are special: First, a method having either the same name
+ as the class or having the name |init| is called the \emph{constructor} of
+ the class. There are (currently) no destructors; objects simply become
+ ``undefined'' at the end of the scope in which they have been created. The
+ other two methods are called |get id| and |get handle|, which are always
+ automatically defined and which you cannot redefine. They are discussed in
+ Section~\ref{section-identities}.
+
+ Overloading of methods by differing numbers of parameters is not possible,
+ that is, it is illegal to have two methods inside a single class with the
+ same name (despite possibly different parameter lists). However, two
+ different classes may contain a method with the same name, that is, classes
+ form namespaces for methods. Also, a class can (re)implement a method from
+ a superclass.
+
+ The \meta{method name} must be followed by a \meta{parameter list} in
+ parentheses, which must be present even when the \meta{parameter list} is
+ empty. The \meta{parameter list} is actually a normal \TeX\ parameter list
+ that will be matched against the parameters inside the parentheses upon
+ method invocation and, thus, could be something like |#1#2 foo #3 bar.|,
+ but a list like |#1,#2,#3| is more customary. By setting the parameter list
+ to just |#1| and then calling, say, |\pgfkeys{#1}| at the beginning of a
+ method, you can implement Objective-C-like named parameters.
+
+ When a method is called, the \meta{body} of the method will be executed.
+ The main difference to a normal macro is that while the \meta{body} is
+ executed, a special macro called |\pgfoothis| is set up in such a way that
+ it references the object for which the method is executed.
+\end{command}
+
+In order to call a method for an object, you first need to create the object
+and you need a handle for this object. In order to invoke a method for this
+object, a special syntax is used that is similar to Java or C++ syntax:
+
+\begin{pgfmanualentry}
+ \pgfmanualentryheadline{\meta{object handle}\opt{|.|\meta{super class}}|.|\meta{method name}|(|\meta{parameters}|)|}%
+ \pgfmanualbody
+ This causes the method \meta{method name} to be called for the object
+ referenced by the \meta{object handle}. The method is the one defined in
+ the class of the object or, if it is not defined there, the method defined
+ in the superclasses of the object's class (if there are several
+ superclasses that define the same method, the method resolution order is
+ used to determine which one gets called). If the optional \meta{super
+ class} is specified, the method implementation of that class will be used
+ rather than the implementation in the object's class. The \meta{parameters}
+ are matched against the parameters of the method and, then, the method body
+ is executed. The execution of the method body is \emph{not} done inside a
+ scope, so the effects of a method body persist.
+ %
+\begin{codeexample}[code only]
+\pgfooclass{stamp}{
+ % This is the class stamp
+
+ \method stamp() { % The constructor
+ }
+
+ \method apply(#1,#2) { % Causes the stamp to be shown at coordinate (#1,#2)
+ % Draw the stamp:
+ \node [rotate=20,font=\huge] at (#1,#2) {Passed};
+ }
+}
+
+\pgfoonew \mystamp=new stamp()
+
+\begin{tikzpicture}
+ \mystamp.apply(1,2)
+ \mystamp.apply(3,4)
+\end{tikzpicture}
+\end{codeexample}
+
+ Inside a method, you can call other methods. If you have a handle for
+ another object, you can simply call it in the manner described above. In
+ order to call a method of the current object, you can use the special
+ object handle |\pgfoothis|.
+
+ \begin{command}{\pgfoothis}
+ This object handle is well-defined only when a method is being executed.
+ There, it is then set to point to the object for which the method is
+ being called, which allows you to call another method for the same
+ object.
+ %
+\begin{codeexample}[code only]
+\pgfooclass{stamp}{
+ % This is the class stamp
+
+ \method stamp() {}
+
+ \method apply(#1,#2) {
+ \pgfoothis.shift origin(#1,#2)
+
+ % Draw the stamp:
+ \node [rotate=20,font=\huge] {Passed};
+ }
+
+ % Private method:
+ \method shift origin(#1,#2) {
+ \tikzset{xshift=#1,yshift=#2}
+ }
+}
+\end{codeexample}
+ \end{command}
+\end{pgfmanualentry}
+
+\begin{command}{\pgfoosuper|(|\meta{class},\meta{object handle}|).|\meta{method name}|(|\meta{arguments}|)|}
+ This macro gives you finer control over which method gets invoked in case
+ of multiple inheritance. This macro calls \meta{method name} of the object
+ specified by \meta{object handle}, but which implementation of the method
+ is called is determined as follows: it will be the implementation in the
+ first class (in the method resolution order) after \meta{class} that
+ defines \meta{method name}.
+\end{command}
+
+
+\subsection{Attributes}
+
+Every object has a set of attributes, which may change over time. Attributes
+are declared using the |\attribute| command, which, like the |\method| command,
+is defined only inside the scope of |\pgfooclass|. Attributes can be modified
+(only) by methods. To take the |stamp| example, an attribute of a |stamp|
+object might be the text that should be stamped when the |apply| method is
+called.
+
+When an attribute is changed, this change is \emph{not} local to the current
+\TeX\ group. Changes will persist till the end of the object's life or until
+the attribute is changed once more.
+
+To declare an attribute you should use the |\attribute| command:
+%
+\begin{command}{\attribute \meta{attribute name}\opt{|=|\meta{initial value}}|;|}
+ This command can only be given inside the body of an |\pgfooclass| command.
+ It declares the attribute named \meta{attribute name}. This name, like
+ method or class names, can be quite arbitrary, but should not contain
+ periods. Valid names are |an_attribute?| or |my attribute|.
+
+ You can optionally specify an \meta{initial value} for the attribute; if
+ none is given, the empty string is used automatically. The initial value is
+ the value that the attribute will have just after the object has been
+ created and before the constructor is called.
+ %
+\begin{codeexample}[code only]
+\pgfooclass{stamp}{
+ % This is the class stamp
+
+ \attribute text;
+ \attribute rotation angle = 20;
+
+ \method stamp(#1) {
+ \pgfooset{text}{#1} % Set the text
+ }
+
+ \method apply(#1,#2) {
+ \pgfoothis.shift origin(#1,#2)
+
+ % Draw the stamp:
+ \node [rotate=\pgfoovalueof{rotation angle},font=\huge]
+ {\pgfoovalueof{text}};
+ }
+
+ \method shift origin(#1,#2) { ... }
+
+ \method set rotation (#1) {
+ \pgfooset{rotation angle}{#1}
+ }
+}
+\end{codeexample}
+ %
+\end{command}
+
+Attributes can be set and read only inside methods, it is not possible to do so
+using an object handle. Spoken in terms of traditional object-oriented
+programming, attributes are always private. You need to define getter and
+setter methods if you wish to read or modify attributes.
+
+Reading and writing attributes is not done using the ``dot-notation'' that is
+used for method calls. This is mostly due to efficiency reasons. Instead, a set
+of special macros is used, all of which can \emph{only be used inside methods}.
+
+\begin{command}{\pgfooset\marg{attribute}\marg{value}}
+ Sets the \meta{attribute} of the current object to \meta{value}.
+ %
+\begin{codeexample}[code only]
+\method set rotation (#1) {
+ \pgfooset{rotation angle}{#1}
+}
+\end{codeexample}
+ %
+\end{command}
+
+\begin{command}{\pgfooeset\marg{attribute}\marg{value}}
+ Performs the same action as |\pgfooset| but in an |\edef| full expansion
+ context.
+\end{command}
+
+\begin{command}{\pgfooappend\marg{attribute}\marg{value}}
+ This method adds the given \meta{value} to the \meta{attribute} at the end.
+\end{command}
+
+\begin{command}{\pgfooprefix\marg{attribute}\marg{value}}
+ This method adds the given \meta{value} to the \meta{attribute} at the
+ beginning.
+\end{command}
+
+\begin{command}{\pgfoolet\marg{attribute}\marg{macro}}
+ Sets the \meta{attribute} of the current value to the current value of
+ \meta{macro} using \TeX's |\let| command.
+ %
+\begin{codeexample}[code only]
+\method foo () {
+ \pgfoolet{my func}\myfunc
+ % Changing \myfunc now has no effect on the value of attribute my func
+}
+\end{codeexample}
+ %
+\end{command}
+
+\begin{command}{\pgfoovalueof\marg{attribute}}
+ Expands (eventually) to the current value of \meta{attribute} of the
+ current object.
+ %
+\begin{codeexample}[code only]
+\method apply(#1,#2) {
+ \pgfoothis.shift origin(#1,#2)
+
+ \node [rotate=\pgfoovalueof{rotation angle},font=\huge]
+ {\pgfoovalueof{text}};
+}
+\end{codeexample}
+ %
+\end{command}
+
+\begin{command}{\pgfooget\marg{attribute}\marg{macro}}
+ Reads the current value of \meta{attribute} and stores the result in
+ \meta{macro}.
+ %
+\begin{codeexample}[code only]
+...
+ \method get rotation (#1) {
+ \pgfooget{rotation angle}{#1}
+ }
+...
+
+\mystamp.get rotation(\therotation)
+``\therotation'' is now ``20'' (or whatever).
+\end{codeexample}
+ %
+\end{command}
+
+
+\subsection{Identities}
+\label{section-identities}
+
+Every object has a unique identity, which is simply an integer. It is possible
+to retrieve the object id using the |get id| method (discussed below), but
+normally you will not need to do so because the id itself cannot be used to
+access an object. Rather, you access objects via their methods and these, in
+turn, can only be called via object handles.
+
+Object handles can be created in four ways:
+%
+\begin{enumerate}
+ \item Calling |\pgfoonew|\meta{object handle}|=...| will cause \meta{object
+ handle} to be a handle to the newly created object.
+ \item Using |\let| to create an alias of an existing object handle: If
+ |\mystamp| is a handle, saying |\let\myotherstamp=\mystamp| creates a
+ second handle to the same object.
+ \item |\pgfooobj|\marg{id} can be used as an object handle to the object
+ with the given \meta{id}.
+ \item Using the |get handle| method to create a handle to a given object.
+\end{enumerate}
+
+Let us have a look at the last two methods.
+
+\begin{command}{\pgfooobj\marg{id}}
+ Provided that \meta{id} is the id of an existing object (an object whose
+ life-time has not expired), calling this command yields a handle to this
+ object. The handle can then be used to call methods:
+ %
+\begin{codeexample}[code only]
+% Create a new object:
+\pgfoonew \mystamp=new stamp()
+
+% Get the object's id and store it in \myid:
+\mystamp.get id(\myid)
+
+% The following two calls have the same effect:
+\mystamp.apply(1,1)
+\pgfooobj{\myid}.apply(1,1)
+\end{codeexample}
+ %
+\end{command}
+
+The |get id| method can be used to retrieve the id of an object. This method is
+predefined for every class and you should not try to define a method of this
+name yourself.
+
+\begin{predefinedmethod}{get id(\meta{macro})}
+ Calling \meta{obj}|.get id(|\meta{macro}|)| stores the id \meta{obj} in
+ \meta{macro}. This is mainly useful when you wish to store an object for a
+ longer time and you cannot guarantee that any handle that you happen to
+ have for this object will be available later on.
+
+ The only way to use the retrieved id later on is to call |\pgfooobj|.
+
+ Different object that are alive (that are still within the scope in which
+ they were created) will always have different ids, so you can use the id to
+ test for equality of objects. However, after an object has been destroyed
+ because its scope has ended, the same id may be used again for newly
+ created objects.
+
+ Here is a typical application where you need to call this method: You wish
+ to collect a list of objects for which you wish to call a specific method
+ from time to time. For the collection process you wish to offer a macro
+ called |\addtoobjectlist|, which takes an object handle as parameter. It is
+ quite easy to store this handle somewhere, but a handle is, well, just a
+ handle. Typically, shortly after the call to |\addtoobjectlist| the handle
+ will no longer be valid or even exist, even though the object still exists.
+ In this case, you wish to store the object id somewhere instead of the
+ handle. Thus, for the object passed to |\addtoobjectlist| you call the
+ |get id| method and store the resulting id, rather than the handle.
+\end{predefinedmethod}
+
+There is a second predefined method, called |get handle|, which is also used to
+create object handles.
+
+\begin{predefinedmethod}{get handle(\marg{macro name})}
+ Calling this method for an object will cause \meta{macro name} to become a
+ handle to the given object. For any object handle |\obj| -- other than
+ |\pgfoothis| -- the following two have the same effect:
+ %
+ \begin{enumerate}
+ \item |\let|\meta{macro name}|=\obj|
+ \item |\obj.get handle(|\meta{macro name}|)|
+ \end{enumerate}
+
+ The first method is simpler and faster. However, for |\pgfoothis| there is
+ a difference: The call |\pgfoothis.get handle(|\meta{macro name}|)| will
+ cause \meta{macro name} to be an object handle to the current object and
+ will persist to be so even after the method is done. By comparison,
+ |\let|\meta{macro name}|=\pgfoothis| causes |\obj| to be the same as the
+ very special macro |\pgfoothis|, so |\obj| will always refer to the current
+ object, which may change over time.
+\end{predefinedmethod}
+
+
+\subsection{The Object Class}
+\label{section-object}
+
+The object-oriented module predefines a basic class |object| that can be used
+as a base class in different context.
+
+\begin{ooclass}{object}
+ This class current only implements one method:
+
+ \begin{method}{copy(\meta{handle})}
+ Creates a new object and initializes the values of its (declared)
+ attributes to the values of the original. The method takes one
+ argument: a control sequence which receives the handle of the copy.
+ \end{method}
+\end{ooclass}
+
+
+\subsection{The Signal Class}
+\label{section-signals}
+
+In addition to the basic mechanism for defining and using classes and object,
+the class |signal| is predefined. It implements a so-called signal--slot
+mechanism.
+
+\begin{ooclass}{signal}
+ This class is used to implement a simple signal--slot mechanism. The idea
+ is the following: From time to time special things happen about which a
+ number of objects need to be informed. Different things can happen and
+ different object will be interested in these things. A |signal| object can
+ be used to signal that such special things of a certain kind have happened.
+ For example, one signal object might be used to signal the event that ``a
+ page has been shipped out''. Another signal might be used to signal that
+ ``a figure is about to be typeset'', and so on.
+
+ Objects can ``tune in'' to signals. They do so by \emph{connecting} one of
+ their methods (then called a \emph{slot}) to the signal. Then, whenever the
+ signal is \emph{emitted}, the method of the connected object(s) get called.
+ Different objects can connect different slots to the same signal as long as
+ the argument lists will fit. For example, the object that is used to signal
+ the ``end of page has been reached'' might emit signals that have, say, the
+ box number in which the finished page can be found as a parameter
+ (actually, the finished page is always in box 255). Then one object could
+ connect a method |handle page(#1)| to this signal, another might connect
+ the method |emergency action(#1)| to this signal, and so on.
+
+ Currently, it is not possible to ``unregister'' or ``detach'' a slot from a
+ signal, that is, once an object has been connect to a signal, it will
+ continue to receive emissions of this signal till the end of the life-time
+ of the signal. This is even true when the object no longer exists (but the
+ signal does), so care must be taken that signal objects are always created
+ after the objects that are listening to them.
+
+ \begin{method}{signal()}
+ The constructor does nothing.
+ \end{method}
+
+ \begin{method}{connect(\meta{object handle},\meta{method name})}
+ This method gets an \meta{object handle} as parameter and a
+ \meta{method name} of this object. It will queue the object-method pair
+ in an internal list and each time the signal emits something, this
+ object's method is called.
+
+ Be careful not to pass |\pgfoothis| as \meta{object handle}. This would
+ cause the signal object to connect to itself. Rather, if you wish to
+ connect a signal to a method of the current object you first need to
+ create an alias using the |get handle| method:
+ %
+\begin{codeexample}[code only]
+\pgfooclass{some class}{
+ \method some class() {
+ \pgfoothis.get handle(\me)
+ \somesignal.connect(\me,foo)
+ \anothersignal.connect(\me,bar)
+ }
+ \method foo () {}
+ \method bar (#1,#2) {}
+}
+\pgfoonew \objA=new some class()
+\pgfoonew \objB=new some class()
+\end{codeexample}
+ \end{method}
+
+ \begin{method}{emit(\meta{arguments})}
+ This method emits a signal to all connected slots. This means that for
+ all objects that have previously been connected via a call of
+ |connect|, the method (slot) that was specified during the call of
+ |connect| is invoked with given \meta{arguments}.
+ %
+\begin{codeexample}[code only]
+\anothersignal.emit(1,2)
+% will call \objA.bar(1,2) and \objB.bar(1,2)
+\end{codeexample}
+ \end{method}
+\end{ooclass}
+
+
+\subsection{Implementation Notes}
+
+For the curious, here are some notes on how the oo-system is implemented:
+%
+\begin{itemize}
+ \item There is an object id counter that gets incremented each time an
+ object is created. However, this counter is local to the current scope,
+ which means that it is reset at the end of each scope, corresponding to
+ the fact that at the end of a scope all objects created in this scope
+ become invalid. Newly created objects will then have the same id as
+ ``deleted'' objects.
+ \item Attributes are stored globally. For each attribute of each object
+ there is a macro whose name is composed of the object's id and the
+ attribute name. Changes to object attributes are always global.
+ \item A call to the garbage collector causes a loop to be executed that
+ tries to find objects whose object number is larger than the current
+ maximum alive objects. The global attributes of these objects are then
+ freed (set to |\relax|) by calling a special internal method of these
+ (dead) objects.
+
+ The garbage collector is automatically called after each group in
+ which an object was created using |\aftergroup|.
+ \item When a method is called, before the method call some code is executed
+ that sets a global counter storing the current object id to the object
+ id of the object being called. After the method call some code is
+ inserted that restores the global counter to its original value. This
+ is done without scopes, so some tricky |\expandafter| magic is needed.
+ Note that, because of this process, you cannot use commands like
+ |\pgfutil@ifnextchar| at the end of a method.
+ \item An object handle contains just the code to set up and restore the
+ current object number to the number of the object being called.
+\end{itemize}