% Copyright 2006 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{Key Management}
\label{section-keys}

This section describes the package |pgfkeys|. It is loaded
automatically by both \pgfname\ and \tikzname.

\begin{package}{pgfkeys}
  This package can be used independently of \pgfname. Note that no
  other package of \pgfname\ needs to be loaded (so neither the
  emulation layer nor the system layer is needed). The Con\TeX t
  abbreviation is |pgfkey| if |pgfmod| is not loaded.
\end{package}



\subsection{Introduction}

\subsubsection{Comparison to Other Packages}

The |pgfkeys| package defines a key--value management system that is in
some sense similar to the more light-weight |keyval| system and the
improved |xkeyval| system. However, |pgfkeys| uses a slightly
different philosophy than these systems and it will coexist peacefully
with both of them.

The main differences between |pgfkeys| and |xkeyval| are the
following:

\begin{itemize}
\item |pgfkeys| organizes keys in a tree, while |keyval| and |xkeyval|
  use families. In |pgfkeys| the families correspond to the root
  entries of the key tree.
\item For efficiency reasons, |pgfkeys| does not directly support
  setting keys drawn from multiple families as |xkeyval| does. This
  can be emulated if necessary, but it will be slower than |xkeyval|'s
  native support.
\item |pgfkeys| has no save-stack impact (you will have to read the
  \TeX Book very carefully to appreciate this).
\item |pgfkeys| is slightly slower than |keyval|, but not much.
\item |pgfkeys| supports styles. This means that keys can just stand
  for other keys (which can stand for other keys in turn or which can
  also just execute some code). \tikzname\ uses this mechanism heavily.
\item |pgfkeys| supports multi-argument key code. This can, however,
  be emulated in |keyval|.
\item |pgfkeys| supports handlers. These are call-backs that are
  called when a key is not known. They are very flexible, in fact even
  defining keys in different ways is handled by, well, handlers.
\end{itemize}


\subsubsection{Quick Guide to Using the Key Mechanism}

The following quick guide to \pgfname's key mechanism only treats the
most commonly used features. For an in-depth discussion of what is
going on, please consult the remainder of this section.

Keys are organized in a large tree that is reminiscent of the Unix
file tree. A typical key might be, say, |/tikz/coordinate system/x|
or just |/x|. Again as in Unix, when you specify keys you can provide
the complete path of the key, but you usually just provide the name of
the key (corresponding to the file name without any path) and the path
is added automatically.

Typically (but not necessarily) some code is associated with a key. To
execute this code, you use the |\pgfkeys| command. This command takes
a list of so-called key--value pairs. Each pair is of the form
\meta{key}|=|\meta{value}. For each pair the |\pgfkeys| command will
execute the code stored for the \meta{key} with its parameter set to
\meta{value}.

Here is a typical example of how the |\pgfkeys| command is used:
\begin{codeexample}[code only]
\pgfkeys{/my key=hallo,/your keys/main key=something\strange,
         key name without path=something else}
\end{codeexample}

Now, to set the code that is stored in a key you do not need to learn
a new command. Rather, the |\pgfkeys| command can also be used to set
the code of a key. This is done using so-called \emph{handlers}. They
look like keys whose names look like ``hidden files in Unix'' since
they start with a dot. The handler for setting the code of a key is
appropriately called |.code| and it is used as follows:
\begin{codeexample}[]
\pgfkeys{/my key/.code=The value is '#1'.}
\pgfkeys{/my key=hi!}
\end{codeexample}
As you can see, in the first line we defined the code for the key
|/my key|. In the second line we executed this code with the parameter
set to |hi!|.

There are numerous handlers for defining a key. For instance, we can
also define a key whose value actually consists of more than one
parameter. 
\begin{codeexample}[]
\pgfkeys{/my key/.code 2 args=The values are '#1' and '#2'.}
\pgfkeys{/my key={a1}{a2}}
\end{codeexample}

We often want to have keys where the code is called with some default
value if the user does not provide a value. Not surprisingly, this is
also done using a handler, this time called |.default|.
\begin{codeexample}[]
\pgfkeys{/my key/.code=(#1)}
\pgfkeys{/my key/.default=hello}
\pgfkeys{/my key=hallo,/my key}
\end{codeexample}

The other way round, it is also possible to specify that a value
\emph{must} be specified, using a handler called
|.value required|. Finally, you can also require that no value
\emph{may} be specified using |.value forbidden|.

All keys for a package like, say, \tikzname\ start with the path
|/tikz|. We obviously do not like to write this path down every
time we use a key (so we do not have to write things like
|\draw[/tikz/line width=1cm]|). What we need is to somehow ``change
the default path to a specific location.'' This is done using the
handler |.cd| (for ``change directory''). Once this handler has been
used on a key, all subsequent keys {\itshape in the current call of
  |\pgfkeys| only} are automatically prefixed with this path, if
necessary.

Here is an example:
\begin{codeexample}[code only]
\pgfkeys{/tikz/.cd,line width=1cm,line cap=round}
\end{codeexample}
This makes it easy to define commands like |\tikzset|, which could be
defined as follows (the actual definition is a bit faster, but the
effect is the same):
\begin{codeexample}[code only]
\def\tikzset#1{\pgfkeys{/tikz/.cd,#1}}
\end{codeexample}

When a key is handled, instead of executing some code, the key can
also cause further keys to be executed. Such keys will be called
\emph{styles}. A style is, in essence, just a key list that should be
executed whenever the style is executed. Here is an example:
\begin{codeexample}[]
\pgfkeys{/a/.code=(a:#1)}
\pgfkeys{/b/.code=(b:#1)}
\pgfkeys{/my style/.style={/a=foo,/b=bar,/a=#1}}
\pgfkeys{/my style=wow}
\end{codeexample}
As the above example shows, style can also be parametrized, just like
the normal code keys.

As a typical use of styles, suppose we wish to setup the key |/tikz|
so that it will change the default path to |/tikz|. This can be
achieved as follows:
\begin{codeexample}[code only]
\pgfkeys{/tikz/.style=/tikz/.cd}
\pgfkeys{tikz,line width=1cm,draw=red}
\end{codeexample}

Note that when |\pgfkeys| is executed, the default path is set
to~|/|. This means that the first |tikz| will be completed to
|/tikz|. Then |/tikz| is a style and, thus, replaced by |/tikz/.cd|,
which changes the default path to |/tikz|. Thus, the |line width| is
correctly prefixed with |/tikz|.

\subsection{The Key Tree}

The |pgfkeys| package organizes keys in a so-called \emph{key
  tree}. This tree will be familiar to anyone who has used a Unix
operating system: A key is addressed by a path, which consists of
different parts separated by slashes. A typical key might be
|/tikz/line width| or just |/tikz| or something more complicated like
|/tikz/cs/x/.store in|.

Let us fix some further terminology: Given a key like |/a/b/c|, we
call the part leading up the last slash (|/a/b|) the \emph{path} of
the key. We call everything after the last slash (|c|) the \emph{name}
of the key (in a file system this would be the file name). 

We do not always wish to specify keys completely. Instead, we usually
specify only part of a key (typically only the name) and the
\emph{default path} is then added to the key at the front. So, when
the default path is |/tikz| and you 
refer to the (partial) key |line width|, the actual key that is used
is |/tikz/line width|. There is a simple rule for deciding whether a
key is a partial key or a full key: If it starts with a slash, then it
is a full key and it is not modified; if it does not start with
a slash, then the default path is automatically prefixed.

Note that the default path is not the same as a search path. In
particular, the default path is just a single path. When a partial key
is given, only this single default path is prefixed; |pgfkeys| does
not try to lookup the key in different parts of a search path. It is,
however, possible to emulate search paths, but a much more
complicated mechanism must be used.

When you set keys (to be explained in a moment), you can freely mix
partial and full keys and you can change the default path. This makes
it possible to temporarily use keys from another part of the key tree
(this turns out to be a very useful feature).

Each key (may) store some \emph{tokens} and there exist commands,
described below, for setting, getting, and changing the tokens stored
in a key. However, you will only very seldom use these commands
directly. Rather, the standard way of using keys is the |\pgfkeys|
command or some command that uses it internally like, say,
|\tikzset|. So, you may wish to skip the following commands and
continue with the next subsection.

\begin{command}{\pgfkeyssetvalue\marg{full key}\marg{token text}}
  Stores the \meta{token text} in the \meta{full key}. The \meta{full key}
  may not be a partial key, so no default-path-adding is done. The
  \meta{token text} can be arbitrary tokens and may even contain things
  like |#| or unbalanced \TeX-ifs.
\begin{codeexample}[]
\pgfkeyssetvalue{/my family/my key}{Hello, world!}
\pgfkeysvalueof{/my family/my key}  
\end{codeexample}

  The setting of a key is always local to the current \TeX\ group.
\end{command}

\begin{command}{\pgfkeyslet\marg{full key}\marg{macro}}
  Performs a |\let| statement so the the \meta{full key} pionts to the
  contents of \meta{macro}.
\begin{codeexample}[]
\def\helloworld{Hello, world!}
\pgfkeyslet{/my family/my key}{\helloworld}
\pgfkeysvalueof{/my family/my key}  
\end{codeexample}
  You should never let a key be equal to |\relax|. Such a key may or
  may not be indistinguishable from an undefined key.
\end{command}

\begin{command}{\pgfkeysgetvalue\marg{full key}\marg{macro}}
  Retrieves the tokens stored in the \meta{full key} and lets
  \meta{macro} be equal to these tokens. If the key has
  not been set, the \meta{macro} will be equal to |\relax|. 
\begin{codeexample}[]
\pgfkeyssetvalue{/my family/my key}{Hello, world!}
\pgfkeysgetvalue{/my family/my key}{\helloworld}
\helloworld
\end{codeexample}
\end{command}

\begin{command}{\pgfkeysvalueof\marg{full key}}
  Inserts the value stored in \meta{full key} at the current position
  into the text.

\begin{codeexample}[]
\pgfkeyssetvalue{/my family/my key}{Hello, world!}
\pgfkeysvalueof{/my family/my key}
\end{codeexample}
\end{command}


\begin{command}{\pgfkeysifdefined\marg{full key}\marg{if}\marg{else}}
  Checks whether this key was previously set using either
  |\pgfkeyssetvalue| or |\pgfkeyslet|. If so, the code in \meta{if} is
  executed, otherwise the code in \meta{else}.

  This command will use e\TeX's |\ifcsname| command, if available, for
  efficiency. This means, however, that it may behave differently for
  \TeX\ and for e\TeX\ when you set keys to |\relax|. For this reason
  you should not do so. 
\begin{codeexample}[]
\pgfkeyssetvalue{/my family/my key}{Hello, world!}
\pgfkeysifdefined{/my family/my key}{yes}{no}
\end{codeexample}
\end{command}


\subsection{Setting Keys}

Settings keys is done using a powerful command called |\pgfkeys|. This
command takes a list of so-called \emph{key--value pairs}. These are
pairs of the form \meta{key}|=|\meta{value}. The principle idea is the
following: For each pair in the list, some \emph{action} is
taken. This action can be one of the following:

\begin{enumerate}
\item A command is executed whose argument(s) are \meta{value}. This
  command is stored in a special subkey of \meta{key}.
\item The \meta{value} is stored in the \meta{key} itself.
\item If the key's name (the part after the last slahs) is a known
  \emph{handler}, then this handler will take care of the key.
\item If the key is totally unknown, one of several possible
  \emph{unknown key handlers} is called. 
\end{enumerate}

Addtionally, if the \meta{value} is missing, a default value may or
may not be substituted. Before we plunge into all the details,
let us have a quick look at the command itself.

\begin{command}{\pgfkeys\marg{key list}}
  The \meta{key list} should be a list of key--value pairs, separated
  by commas. A key--value pair can have the following two forms:
  \meta{key}|=|\meta{value} or just \meta{key}. Any spaces around the
  \meta{key} or around the \meta{value} are removed. It is permissible
  to surround both the \meta{key} or the \meta{value} in curly braces,
  which are also removed. Especially putting the \meta{value} in curly
  braces needs to be done quite often, namely whenever the \meta{value}
  contains an equal-sign or a comma.

  The key--value pairs in the list are handled in the order they
  appear. How this handling is done, exactly, is described in the rest
  of this section.

  If a \meta{key} is a partial key, the current value of the default
  path is prefixed to the \meta{key} and this ``upgraded'' key is
  then used. The default path is just the root path |/| when the first
  key is handled, but it may change later on. At the end of the
  command, the default path is reset to the value it had before this
  command was executed. 
  
  Calls of this command may be nested. Thus, it is permissible to call
  |\pgfkeys| inside the code that is executed for a key. Since the
  default path is restored after a call of |\pgfkeys|, the default
  path will not change when you call |\pgfkeys| while executing code
  for a key (which is exactly what you want).
\end{command}

\begin{command}{\pgfqkeys\marg{default path}\marg{key list}}
  This command has the same effect as |\pgfkeys{|\meta{default
      path}|/.cd,|\meta{key list}|}|, it is only marginally
  quicker. This command should not be used in user code, but rather in
  commands like |\tikzset| or |\pgfset| that get called very often.   
\end{command}

\begin{command}{\pgfkeysalso\marg{key list}}
  This command has execatly the same effect as |\pgfkeys|, only the
  default path is not modified before or after the keys are being
  set. This command is mainly intended to be called by the code that
  is being processed for a key.
\end{command}

\begin{command}{\pgfqkeysalso\marg{default path}\marg{key list}}
  This command has the same effect as |\pgfkeysalso{|\meta{default
      path}|/.cd,|\meta{key list}|}|, it is only quicker. Changing the
  default path inside a |\pgfkeyalso| is dangerous, so use with
  care. A rather safe place to call this command is at the beginning
  of a \TeX\ group.
\end{command}


\subsubsection{Default Arguments}

The arguments of the |\pgfkeys| command can either be of the form
\meta{key}|=|\meta{value} or of the form \meta{key} with the
value-part missing. In the second case, the |\pgfkeys| will try to
provide a \emph{default value} for the \meta{value}. If such a default
value is defined, it will be used as if you had written
\meta{key}|=|\meta{default value}.

In the following, the details of how default values are determined is
described; however, you should normally use the handlers |.default|
and |.value required| as described in
Section~\ref{section-default-handlers} and you can may wish to skip
the following details.

When |\pgfkeys| encounters a \meta{key} without an equal-sign, the
following happens:
\begin{enumerate}
\item The input is replaced by \meta{key}|=\pgfkeysnovalue|. In
  particular, the commands |\pgfkeys{my key}| and
  |\pgfkeys{my key=\pgfkeysnovalue}| have exactly the same effect and
  you can ``simulate'' a missing value by providing the value
  |\pgfkeysnovalue|, which is sometimes useful. 
\item If the \meta{value} is |\pgfkeysnovalue|, then it is checked
  whether the subkey \meta{key}|/.@def| exists. For instance, if you
  write |\pgfkeys{/my key}|, then it is checked whether the key
  |/my key/.@def| exists.
\item If the key \meta{key}|/.@def| exists, then the tokens stored in
  this key are used as \meta{value}.
\item If the key does not exist, then |\pgfkeysnovalue| is used as the
  \meta{value}.
\item At the end, if the \meta{value} is now equal to
  |\pgfkeysvaluerequired|, then the code  (or something fairly equivalent)
  |\pgfkeys{/errors/value required=|\meta{key}|{}}|
  is executed. Thus, by changing this key you can change the error
  message that is printed or you can handle the missing value in some
  other way.
\end{enumerate}



\subsubsection{Keys That Execute Commands}
\label{section-key-code}

After the transformation process described in the previous subsection,
we arrive at a key of the form \meta{key}=\meta{value}, where
\meta{key} is a full key. Different things can now happen, but always
the macro |\pgfkeyscurrentkey| will have been setup to expand to the
text of the \meta{key} that is currently being processed.

The first things that is tested is whether the key \meta{key}|/.@cmd|
exists. If this is the case, then it is assumed that this key stores
the code of a macro and this macro is executed. The argument of this
macro is \meta{value} directly followed by |\pgfeov|, which stands for
``end of value.'' The \meta{value} is not surrounded by braces. After
this code has been executed, |\pgfkeys| continues with the next key in
the \meta{key list}.

It may seem quite peculiar that the macro stored in the key
\meta{key}|/.@cmd| is not simply executed with the argument
|{|\meta{value}|}|. However, the approach taken in the |pgfkeys|
packages allows for more flexibility. For instance, assume that you
have a key that expects a \meta{value} of the form
``\meta{text}|+|\meta{more text}'' and wishes to store \meta{text} and
\meta{more text} in two different macros. This can be achieved as
follows:
\begin{codeexample}[]
\def\mystore#1+#2\pgfeov{\def\a{#1}\def\b{#2}}
\pgfkeyslet{/my key/.@cmd}{\mystore}
\pgfkeys{/my key=hello+world}

|\a| is \a, |\b| is \b.
\end{codeexample}

Naturally, defining the code to be stored in a key in the above manner
is too awkward. The following commands simplify things a bit, but the
usual manner of setting up code for a key is to use one of the
handlers described in Section~\ref{section-code-handlers}.

\begin{command}{\pgfkeysdef\marg{key}\marg{code}}
  This command temporarily defines a \TeX-macro with the argument list
  |#1\pgfeov| and then lets \meta{key}|/.@cmd| be equal to this
  macro. The net effect of all this is that you have then setup code
  for the key \meta{key} so that when you write
  |\pgfkeys{|\meta{key}|=|\meta{value}|}|, then the \meta{code} is
  executed with all occurrences of |#1| in \meta{code} being replaced
  by \meta{value}. (This behaviour is quite similar to the
  |\define@key| command of |keyval| and |xkeyval|).

\begin{codeexample}[]
\pgfkeysdef{/my key}{#1, #1.}
\pgfkeys{/my key=hello}
\end{codeexample}
\end{command}

\begin{command}{\pgfkeysedef\marg{key}\marg{code}}
  This command works like |\pgfkeysdef|, but it uses |\edef| rather
  than |\def| when defining the key macro. If you do not know the
  difference between the two, then you will not need this command;
  and if you know the difference, then you will know when you need this
  command.
\end{command}

\begin{command}{\pgfkeysdeargs\marg{key}\marg{argument pattern}\marg{code}}
  This command works like |\pgfkeysdef|, but it allows you to provide
  an arbitrary \meta{argument pattern} rather than just the simple
  single argument of |\pgfkeysdef|. 

\begin{codeexample}[]
\pgfkeysdefargs{/my key}{#1+#2}{\def\a{#1}\def\b{#2}}
\pgfkeys{/my key=hello+world}

|\a| is \a, |\b| is \b.
\end{codeexample}
\end{command}

\begin{command}{\pgfkeysedefargs\marg{key}\marg{argument pattern}\marg{code}}
  The |\edef| version of |\pgfkeysdefargs|.
\end{command}


\subsubsection{Keys That Store Values}

Let us continue with what happens when |\pgfkeys| processes the
current key and  the subkey \meta{key}|/.@cmd| is not defined. Then
it is checked whether the \meta{key} itself exists (has been
previously assigned a value using, for instance,
|\pgfkeyssetvalue|). In this case, the tokens stored in \meta{key} are
replaced by \meta{value} and |\pgfkeys| proceeds with the next key in
the \meta{key list}. 


\subsubsection{Keys That Are Handled}
\label{section-key-handlers}

If neither the \meta{key} itself nor the subkey \meta{key}|/.@cmd| are
defined, then the \meta{key} cannot be processed ``all by itself.''
Rather, a \meta{handler} is needed for this key. Most of the power of
|pgfkeys| comes from the proper use of such handlers.

Recall that the \meta{key} is always a full key (if it was not
originally, it has already been upgraded at this point to a full
key). It decomposed into  two parts:

\begin{enumerate}
\item The \meta{path} of \meta{key} (everything
  before the last slash) is stored in the macro |\pgfkeyscurrentpath|.
\item The \meta{name} of \meta{key} (everything
  after the last slash) is stored in the macro |\pgfkeyscurrentname|.

  It is recommended (but not necessary) that the name of a handler
  starts with a dot (but not with |.@|), so that they are easy to
  detect for the reader.  
\end{enumerate}

(For efficiency reasons, these two macros are only setup at this point;
so when code is executed for a key in the ``usual'' manner then these
macros are not setup.)

The |\pgfkeys| command now checks whether the key
|/handlers/|\meta{name}|/.@cmd| exists. If so, it should store a command
and this command is executed exactly in the same manner as described
in Section~\ref{section-key-code}.
Thus, this code gets the \meta{value} that was originally intended for
\meta{key} as its argument, followed by |\pgfeov|.
It is the job of the handlers to so something useful with the
\meta{value}.

For an example, let us write a handler that will output the value
stored in a key to the log file. We call this handler
|.print to log|. The idea is that when someone tries to use the key
|/my key/.print to log|, then this key will not be defined and the
handler gets executed. The handler will then have access to the
path-part of the key, which is |/my key|, via the macro
|\pgfkeyscurrentpath|. It can then lookup which value is stored in
this key and print it.

\begin{codeexample}[code only]
\pgfkeysdef{/handlers/.print to log}
{%
  \pgfkeysgetvalue{\pgfkeyscurrentpath}{\temp}
  \writetolog{\temp}
}
\pgfkeyssetvalue{/my key}{Hi!}
...
\pgfkeys{/my key/.print to log}
\end{codeexample}
The above code will print |Hi!| in the log, provided the macro
|\writetolog| is setup appropriately.

For a more interesting handler, let us program a handler that will
setup a key so that when the key is used some code is executed. This
code is given as \meta{value}. All the handler must do is to call
|\pgfkeysdef| for the path of the key (which misses the handler's
name) and assign the parameter value to it.
\begin{codeexample}[]
\pgfkeysdef{/handlers/.my code}{\pgfkeysdef{\pgfkeyscurrentpath}{#1}}
\pgfkeys{/my key/.my code=(#1)}
\pgfkeys{/my key=hallo}
\end{codeexample}


\subsubsection{Keys That Are Unknown}

For some keys, neither the key is defined nor its |.@cmd| subkey nor
is a handler defined for this key. In this case, it is checked whether
the key \meta{current path}|/.unknown/.@cmd| exists. Thus, when you try to
use the key |/tikz/strange|, then it is checked whether
|/tikz/.unknown/.@cmd| exists. If this key exists (which it does), it is
executed. This code can then try to make sense of the key. For
instance, the handler for \tikzname\ will try to interpret the key's
name as a color or as an arrow specification or as a \pgfname\
option.

You can setup unknown key handlers for your own keys by simply setting
the code of the key \meta{my path prefix}|/.unknown|. This also allows
you to setup ``search paths.'' The idea is that you would like keys to
be searched not only in a single default path, but in
several. Suppose, for instance, that you would like keys to be
searched 
for in |/a|, |/b|, and |/b/c|. We setup a key |/my search path| for
this:
\begin{codeexample}[code only]
\pgfkeys{/my search path/.unknown/.code=
  {%
    \let\searchname=\pgfkeyscurrentname%
    \pgfkeysalso{%
      /a/\searchname/.try=#1,
      /b/\searchname/.retry=#1,
      /b/c/\searchname/.retry=#1%
    }%
  }%
}
\pgfkeys{/my search path/.cd,foo,bar}
\end{codeexample}
In the above code, |foo| and |bar| will be searched for in the three
directories  |/a|, |/b|, and |/b/c|. 

If the key \meta{current path}|/.unknown/.@cmd| does not exist, the
handler |/handlers/.unknown| is invoked instead, which is always
defined and which prints an error message by default.

\subsection{Key Handlers}

We now describe which key handlers are defined by default. You can
also define new ones as described in Section~\ref{section-key-handlers}.


\subsubsection{Handlers for Path Management}

\begin{handler}{{.cd}}
  This handler causes the default path to be set to \meta{key}. Note that
  the default path is reset at the beginning of each call to
  |\pgfkeys| to be equal to~|/|.

  \example |\pgfkeys{/tikz/.cd,...}|
\end{handler}

\begin{handler}{{.is family}}
  This handler sets up things such that when \meta{key} is executed, then
  the current path is set to \meta{key}. A typical use is the following:
\begin{codeexample}[code only]
\pgfkeys{/tikz/.is family}
\pgfkeys{tikz,line width=1cm}  
\end{codeexample}
  The effect of this handler is the same as if you had written
  \meta{key}|/.style=|\meta{key}|/.cd|, only the code produced by the
  |.is family| handler is quicker.
\end{handler}


\subsubsection{Setting Defaults}
\label{section-default-handlers}

\begin{handler}{{.default}|=|\meta{value}}
  Sets the default value of \meta{key} to \meta{value}. This means
  that whenever no value is provided in a call to |\pgfkeys|, then
  this \meta{value} will be used instead.

  \example |\pgfkeys{/width/.default=1cm}|
\end{handler}

\begin{handler}{{.value required}}
  This handler causes the error message key |/erros/value required| to
  be issued whenever the \meta{key} is used without a value.

  \example |\pgfkeys{/width/.value required}|
\end{handler}

\begin{handler}{{.value forbidden}}
  This handler causes the error message key |/erros/value forbidden|
  to be issued whenever the \meta{key} is used with a value.

  This handler works be adding code to the code of the key. This means
  that you have to define the key first before you can use this
  handler. 
\begin{codeexample}[code only]
\pgfkeys{/my key/.code=I do not want an argument!}
\pgfkeys{/my key/.value forbidden}

\pgfkeys{/my key}     % Ok
\pgfkeys{/my key=foo} % Error
\end{codeexample}
\end{handler}


\subsubsection{Defining Key Codes}
\label{section-code-handlers}

A number of handlers exist for defining the code of keys.

\begin{handler}{{.code}|=|\meta{code}}
  This handler executes |\pgfkeysdef| with the parameters \meta{key}
  and \meta{code}. This means that, afterwards, whenever the
  \meta{key} is used, the \meta{code} gets executed. More precisely,
  when \meta{key}|=|\meta{value} is encountered in a key list,
  \meta{code} is executed with any occurrence of |#1| replaced by
  \meta{value}. As always, if no \meta{value} is given, the default
  value is used, if defined, or the special value |\pgfkeysnovalue|.

  It is permissible that \meta{code} calls the command |\pgfkeys|. It
  is also permissible the \meta{code} calls the command
  |\pgfkeysalso|, which is useful for styles, see below.

\begin{codeexample}[code only]
\pgfkeys{/par indent/.code={\parindent=#1},/par indent/.default=2em}
\pgfkeys{/par indent=1cm}
...
\pgfkeys{/par indent}
\end{codeexample}
\end{handler}

\begin{handler}{{.ecode}|=|\meta{code}}
  This handler works like |.code|, only the command |\pgfkeysedef| is
  used. 
\end{handler}


\begin{handler}{{.code 2 args}|=|\meta{code}}
  This handler works like |.code|, only two arguments rather than one
  are expected when the \meta{code} is executed. This means that when
  \meta{key}|=|\meta{value} is encountered in a key list, the
  \meta{value} should consist of two arguments. For instance,
  \meta{value} could be |{first}{second}|. Then \meta{code} is
  executed with any occurrence of |#1| replaced |first| and any
  occurrence of |#2| replaced by |second|.

  Because of the special way the \meta{value} is parsed, if you set
  \meta{value} to, for instance, |first| (without any braces), then
  |#1| will be set to |f| and |#2| will be set to |irst|. 

\begin{codeexample}[code only]
\pgfkeys{/page size/.code 2 args={\paperheight=#2\paperwidth=#1}}
\pgfkeys{/page size={30cm}{20cm}}
\end{codeexample}
\end{handler}

\begin{handler}{{.ecode 2 args}|=|\meta{code}}
  This handler works like |.code 2 args|, only an |\edef| is used
  rather than a |\def| to define the macro.
\end{handler}



\begin{handler}{{.code args}|=|\marg{argument pattern}\marg{code}}
  This handler also works like |.code|, but you can now specify an
  arbitrary \meta{argument pattern}. Such a pattern is a usual \TeX\
  macro pattern. For instance, suppose \meta{argument pattern} is
  |(#1/#2)| and \meta{key}|=|\meta{value} is encountered in a
  key list with \meta{value} being |(first/second)|. Then \meta{code}
  is executed with any occurrence of |#1| replaced |first| and any
  occurrence of |#2| replaced by |second|. So, the actual \meta{value}
  is matched against the \meta{argument pattern} in the standard \TeX\
  way. 

\begin{codeexample}[code only]
\pgfkeys{/page size/.code args={#1 and #2}{\paperheight=#2\paperwidth=#1}}
\pgfkeys{/page size=30cm and 20cm}
\end{codeexample}
\end{handler}

\begin{handler}{{.ecode args}|=|\marg{argument pattern}\marg{code}}
  This handler works like |.code args|, only an |\edef| is used
  rather than a |\def| to define the macro.
\end{handler}


There are also handlers for modifying existing keys.

\begin{handler}{{.add code}|=|\marg{prefix code}\marg{append code}}
  This handler adds code to an existing key. The \meta{prefix code} is
  added to the code stored in \meta{key}|/.@cmd| at the beginning, the
  \meta{append code} is added to this code at the end. Either can be
  empty. The argument list of \meta{code} cannot be changed using this
  handler. Note that both \meta{prefix code} and \meta{append code}
  may contain parameters like |#2|. 
  
\begin{codeexample}[code only]
\pgfkeys{/par indent/.code={\parindent=#1}}
\newdimen\myparindent  
\pgfkeys{/par indent/.add code={}{\myparindent=#1}}
...
\pgfkeys{/par indent=1cm} % This will set both \parindent and
                          % \myparindent to 1cm
\end{codeexample}
\end{handler}

\begin{handler}{{.prefix code}|=|\meta{prefix code}}
  This handler is a shortcut for \meta{key}|/.add code={|\meta{prefix
      code}|}{}|. That is, this handler adds the \meta{prefix code} at
  the beginning of the code stored in \meta{key}|/.@cmd|.
\end{handler}

\begin{handler}{{.append code}|=|\meta{append code}}
  This handler is a shortcut for \meta{key}|/.add code={}{|\meta{append
      code}|}{}|.
\end{handler}


\subsubsection{Defining Styles}

The following handlers allow you to define \emph{styles}. A style is a
key list that is processed whenever the style is given as a key in a
key list. Thus, a style ``stands for'' a certain key value
list. Styles can be parametrized just like normal code.

\begin{handler}{{.style}|=|\meta{key list}}
  This handler set things up so that whenever \meta{key}|=|\meta{value} is
  encountered in a key list, then the \meta{key list}, with every
  occurrence of |#1| replaced by \meta{value}, is processed
  instead. As always, if no \meta{value} is given, the default
  value is used, if defined, or the special value |\pgfkeysnovalue|.

  You can achieve the same effect by writing
  \meta{key}|/.code=\pgfkeysalso{|\meta{key list}|}|. This means, in
  particular, that the code of a key could also first execute some
  normal code and only then process some further keys. 

\begin{codeexample}[code only]
\pgfkeys{/par indent/.code={\parindent=#1}}
\pgfkeys{/no indent/.style={/par indent=0pt}}
\pgfkeys{/normal indent/.style={/par indent=2em}}
\pgfkeys{/no indent}
...
\pgfkeys{/normal indent}
\end{codeexample}
  The following example shows a parametrized style ``in action''.
\begin{codeexample}[]
\begin{tikzpicture}[outline/.style={draw=#1,fill=#1!20}]
  \node [outline=red]            {red box};
  \node [outline=blue] at (0,-1) {blue box};
\end{tikzpicture}
\end{codeexample}
\end{handler}

\begin{handler}{{.estyle}|=|\meta{key list}}
  This handler works like |.style|, only the \meta{code} is set using
  |\edef| rather than |\def|. Thus, all macros in the \meta{code} are
  expanded prior to saving the style.
\end{handler}

For styles the corresponding handlers as for normal code exist:

\begin{handler}{{.style 2 args}|=|\meta{key list}}
  This handler works like |.code 2 args|, only for styles. Thus, the
  \meta{key list} may contain occurrences of both |#1| and |#2| and
  when the style is used, two parameters must be given as
  \meta{value}. 
\begin{codeexample}[code only]
\pgfkeys{/paper height/.code={\paperheight=#1},/paper width/.code={\paperwidth=#1}}
\pgfkeys{/page size/.style 2 args={/paper height=#1,/paper width=#2}}
\pgfkeys{/page size={30cm}{20cm}}
\end{codeexample}
\end{handler}

\begin{handler}{{.estyle 2 args}|=|\meta{key list}}
  This handler works like |.style 2 args|, only an |\edef| is used
  rather than a |\def| to define the macro.
\end{handler}

\begin{handler}{{.style args}|=|\marg{argument pattern}\marg{key list}}
  This handler works like |.code args|, only for styles.
\end{handler}

\begin{handler}{{.estyle args}|=|\marg{argument pattern}\marg{code}}
  This handler works like |.ecode args|, only for styles.
\end{handler}

\begin{handler}{{.add style}|=|\marg{prefix key list}\marg{append key list}}
  This handler works like |.add code|, only for styles. However, it is
  permissible to add styles to keys that have previously been set
  using  |.code|. (It is also permissible to add normal \meta{code} to
  a key that has previously been set using |.style|). When you add a
  style to a key that was previously set using |.code|, the following
  happens: When \meta{key} is processed, the \meta{prefix key list}
  will be processed first, then the \meta{code} that was previously
  stored in \meta{key}|/.@cmd|, and then the keys in \meta{append key
    list} are processed.
\begin{codeexample}[code only]
\pgfkeys{/par indent/.code={\parindent=#1}}
\pgfkeys{/par indent/.add style={}{/my key=#1}}
...
\pgfkeys{/par indent=1cm} % This will set \parindent and
                          % then execute /my key=#1
\end{codeexample}
\end{handler}

\begin{handler}{{.prefix style}|=|\meta{prefix key list}}
  Works like |.add style|, but only for the prefix key list.
\end{handler}

\begin{handler}{{.append style}|=|\meta{append key list}}
  Works like |.add style|, but only for the append key list.
\end{handler}


\subsubsection{Defining Value-, Macro-, If- and Choice-Keys}

For some keys, the code that should be executed for them is rather
``specialized.'' For instance, it happens often that the code for a
key just sets a certain \TeX-if to true or false. For these case
predefine handlers make it easier to install the necessary code.

However, we start with some handlers that are used to manage the value
that is directly stored in a key.

\begin{handler}{{.initial}|=|\meta{value}}
  This handler sets the value of \meta{key} to \meta{value}. Note that
  no subkeys are involved. After this handler has been used, by the
  rules governing keys, you can subsequently change the value of the
  \meta{key} by just writing \meta{key}|=|\meta{value}. Thus, this
  handler is used to set the initial value of key.

\begin{codeexample}[code only]
\pgfkeys{/my key/.initial=red}
% "/my key" now stores the value "red"
\pgfkeys{/my key=blue}
% "/my key" now stores the value "blue"
\end{codeexample}

  Note that in the after the example, writing |\pgfkeys{/my key}| will not
  have the effect you might expect (namely that |blue| is inserted
  into the main text). Rather, |/my key| will be promoted to
  |/my key=\pgfkeysnovalue| and, thus, |\pgfkeysnovalue| will be
  stored in |/my key|.

  To retrieve the value stored in a key, the handler |.get| is used.
\end{handler}

\begin{handler}{{.get}|=|\meta{macro}}
  Executes a |\let| command so that \meta{macro} contains the contents
  stored in \meta{key}.  

\begin{codeexample}[]
\pgfkeys{/my key/.initial=red}
\pgfkeys{/my key=blue}
\pgfkeys{/my key/.get=\mymacro}
\mymacro
\end{codeexample}
\end{handler}

\begin{handler}{{.add}|=|\marg{prefix value}\marg{append value}}
  Adds the \meta{prefix value} and the beginning and the \meta{append
    value} at the end of the value stored in \meta{key}.
\end{handler}

The next handler is useful for the common situation where
\meta{key}|=|\meta{value} should cause the \meta{value} to be stored
in some macro. Note that, typically, you could just as well store the
value in the key itself.

\begin{handler}{{.store in}|=|\meta{macro}}
  This handler has the following effect: When you write
  \meta{key}|=|\meta{value}, the code
  |\def|\meta{macro}|{|\meta{value}|}| is executed. Thus, the given
  value is ``stored'' in the \meta{macro}.  
\begin{codeexample}[]
\pgfkeys{/text/.store in=\mytext}
\def\a{world} 
\pgfkeys{/text=Hello \a!}
\def\a{Gruffalo}
\mytext
\end{codeexample}
\end{handler}

\begin{handler}{{.estore in}|=|\meta{macro}}
  This handler is similar to |.store in|, only the code 
  |\edef|\meta{macro}|{|\meta{value}|}| is used. Thus, the
  macro-expanded version of \meta{value} is stored in the
  \meta{macro}. 
\begin{codeexample}[]
\pgfkeys{/text/.estore in=\mytext}
\def\a{world} 
\pgfkeys{/text=Hello \a!}
\def\a{Gruffalo}
\mytext
\end{codeexample}
\end{handler}

In another common situation a key is used to set a \TeX-if to true or
false. 

\begin{handler}{{.is if}|=|\meta{\TeX-if name}}
  This handler has the following effect: When you write
  \meta{key}|=|\meta{value}, it is first checked that \meta{value} is
  |true| or |false| (the default is |true| if no \meta{value} is
  given). If this is not the case, the error key
  |/errors/boolean expected| is executed. Otherwise, 
  the code |\|\meta{\TeX-if name}\meta{value} is executed, which sets
  the \TeX-if accordingly.
\begin{codeexample}[]
\newif\iftheworldisflat    
\pgfkeys{/flat world/.is if=theworldisflat}
\pgfkeys{/flat world=false}
\iftheworldisflat
  Flat
\else
  Round?
\fi
\end{codeexample}
\end{handler}

The next handler deals with the problem when a
\meta{key}|=|\meta{value} makes sense only for a small set of possible
\meta{value}s. For instance, the line cap can only be |rounded| or
|rect| or |butt|, but nothing else. For this situation the following
handler is useful.

\begin{handler}{{.is choice}}
  This handler set things up so that writing \meta{key}|=|\meta{value}
  will cause the subkey \meta{key}|/|\meta{value} to be executed. So,
  each of the different possible choices should be given by a subkey
  of \meta{key}.
\begin{codeexample}[code only]
\pgfkeys{/line cap/.is choice}
\pgfkeys{/line cap/round/.style={\pgfsetbuttcap}}
\pgfkeys{/line cap/butt/.style={\pgfsetroundcap}}
\pgfkeys{/line cap/rect/.style={\pgfsetrectcap}}
\pgfkeys{/line cap/rectangle/.style={/line cap=rect}}
...
\draw [/line cap=butt] ...
\end{codeexample}
  If the subkey \meta{key}|/|\meta{value} does not exist, the error
  key |/errors/unknown choice value| is executed.
\end{handler}


\subsubsection{Expanding Values}

When you write \meta{key}|=|\meta{value}, you usually wish to use the
\meta{value} ``as is.'' Indeed, great care is taken to ensure that you
can even use things like |#1| or unbalanced \TeX-ifs inside
\meta{value}. However, sometimes you want the \meta{value} to be
expanded before it is used. For instance, \meta{value} might be a
macro name like |\mymacro| and you do not want |\mymacro| to be used
as the macro, but rather the \emph{contents} of |\mymacro|. Thus,
instead of using \meta{value} you wish to use whatever \meta{value}
expands to. Instead of using some fancy |\expandafter| hackery, you
can use the following handlers:

\begin{handler}{{.expand once}|=|\meta{value}}
  This handler expands \meta{value} once (more precisely, it executes
  an |\expandafter| command on the first token of \meta{value}) and
  then process the resulting \meta{result} as if you had written
  \meta{key}|=|\meta{result}. Note that if \meta{key} contains a
  handler itself, this handler will be called normally.
\begin{codeexample}[]
\def\a{bottom}
\def\b{\a}
\def\c{\b}

\pgfkeys{/key1/.initial=\c}
\pgfkeys{/key2/.initial/.expand once=\c}
\pgfkeys{/key3/.initial/.expand twice=\c}
\pgfkeys{/key4/.initial/.expanded=\c}

\def\a{{\ttfamily\string\a}}
\def\b{{\ttfamily\string\b}}
\def\c{{\ttfamily\string\c}}

\begin{tabular}{ll}
Key 1:& \pgfkeys{/key1} \\
Key 2:& \pgfkeys{/key2} \\
Key 3:& \pgfkeys{/key3} \\
Key 4:& \pgfkeys{/key4}
\end{tabular}
\end{codeexample}
\end{handler}

\begin{handler}{{.expand twice}|=|\meta{value}}
  This handler works like saying \meta{key}|/.expand once/.expand once=|\meta{value}.
\end{handler}

\begin{handler}{{.expanded}|=|\meta{value}}
  This handler will completely expand \meta{value} (using |\edef|)
  before processing \meta{key}|=|\meta{result}.
\end{handler}


\subsubsection{Handlers for Testing Keys}

\begin{handler}{{.try}|=|\meta{value}}
  This handler causes the same things to be done as if
  \meta{key}|=|\meta{value} had been written instead. However, if
  neither \meta{key}|/.@cmd| nor the key itself is defined, no
  handlers will be called. Instead, 
  the execution of the key just stops. Thus, this handler will ``try''
  to use the key, but no further action is taken when the key is not
  defined.

  The \TeX-if |\||ifpgfkeyssuccess| will be set according to whether
  the \meta{key} was successfully executed or not. 
\begin{codeexample}[]
\pgfkeys{/a/.code=(a:#1)}
\pgfkeys{/b/.code=(b:#1)}
\pgfkeys{/x/.try=hmm,/a/.try=hallo,/b/.try=welt}
\end{codeexample}
\end{handler}

\begin{handler}{{.retry}|=|\meta{value}}
  This handler works just like |.try|, only it will not do anything if
  |\||ifpgfkeyssuccess| is false. Thus, this handler will only retry
  to set a key if ``the last attempt failed''. 
\begin{codeexample}[]
\pgfkeys{/a/.code=(a:#1)}
\pgfkeys{/b/.code=(b:#1)}
\pgfkeys{/x/.try=hmm,/a/.retry=hallo,/b/.retry=welt}
\end{codeexample}
\end{handler}


\subsubsection{Handlers for Key Inspection}

\begin{handler}{{.show value}}
  This handler executes a |\show| command on the value stored in
  \meta{key}. This is useful mostly for debugging.

  \example |\pgfkeys{/my/obscure key/.show value}|
\end{handler}

\begin{handler}{{.show code}}
  This handler executes a |\show| command on the code stored in
  \meta{key}|/.@cmd|. This is useful mostly for debugging.

  \example |\pgfkeys{/my/obscure key/.show code}|
\end{handler}

The following key is not a handler, but it also commonly used for
inspecting things:
\begin{key}{/utils/exec=\meta{code}}
  This key will simply execute the given \meta{code}. 

  \example |\pgfkeys{some key=some value,/utils/exec=\show\hallo,obscure key=obscure}|
\end{key}


\subsection{Error Keys}

In certain situations errors can occur, like using an undefined
key. In these situations error keys are executed. They should store a
macro that gets two arguments: The first is the offending key
(possibly only after macro expansion), the second is the value that
was passed as a parameter (also possibly only after macro expansion).

Currently, error keys are simply executed. In the future it might be a
good idea to have different subkeys that are executed depending on the
language currently set so that users get a localized error message.

\begin{key}{/errors/value required=\marg{offending key}\marg{value}}
  This key is executed whenever an \meta{offending key} is used
  without a value when a value is actually required. 
\end{key}

\begin{key}{/errors/value forbidden=\marg{offending key}\marg{value}}
  This key is executed whenever a key is used with a value when a
  value is actually forbidden.
\end{key}

\begin{key}{/errors/boolean expected=\marg{offending key}\marg{value}}
  This key is executed whenever a key setup using |.is if| gets called
  with a \meta{value} other than |true| or |false|.
\end{key}

\begin{key}{/errors/unknown choice value=\marg{offending key}\marg{value}}
  This key is executed whenever a choice is used as a \meta{value} for
  a key setup using the |.is choice| handler that is not defined.
\end{key}

\begin{key}{/errors/unknown key=\marg{offending key}\marg{value}}
  This key is executed whenever a key is unknown and no specific
  |.unknown| handler is found.
\end{key}
