[ViewVC] Contents of: cvs/AnyEvent/README

NAME
    AnyEvent - provide framework for multiple event loops

    EV, Event, Coro::EV, Coro::Event, Glib, Tk, Perl - various supported
    event loops

SYNOPSIS
       use AnyEvent;

       my $w = AnyEvent->io (fh => $fh, poll => "r|w", cb => sub {
          ...
       });

       my $w = AnyEvent->timer (after => $seconds, cb => sub {
          ...
       });

       my $w = AnyEvent->condvar; # stores wether a condition was flagged
       $w->wait; # enters "main loop" till $condvar gets ->broadcast
       $w->broadcast; # wake up current and all future wait's

WHY YOU SHOULD USE THIS MODULE (OR NOT)
    Glib, POE, IO::Async, Event... CPAN offers event models by the dozen
    nowadays. So what is different about AnyEvent?

    Executive Summary: AnyEvent is *compatible*, AnyEvent is *free of
    policy* and AnyEvent is *small and efficient*.

    First and foremost, *AnyEvent is not an event model* itself, it only
    interfaces to whatever event model the main program happens to use in a
    pragmatic way. For event models and certain classes of immortals alike,
    the statement "there can only be one" is a bitter reality, and AnyEvent
    helps hiding the differences.

    The goal of AnyEvent is to offer module authors the ability to do event
    programming (waiting for I/O or timer events) without subscribing to a
    religion, a way of living, and most importantly: without forcing your
    module users into the same thing by forcing them to use the same event
    model you use.

    For modules like POE or IO::Async (which is actually doing all I/O
    *synchronously*...), using them in your module is like joining a cult:
    After you joined, you are dependent on them and you cannot use anything
    else, as it is simply incompatible to everything that isn't itself.

    AnyEvent + POE works fine. AnyEvent + Glib works fine. AnyEvent + Tk
    works fine etc. etc. but none of these work together with the rest: POE
    + IO::Async? no go. Tk + Event? no go. If your module uses one of those,
    every user of your module has to use it, too. If your module uses
    AnyEvent, it works transparently with all event models it supports
    (including stuff like POE and IO::Async).

    In addition of being free of having to use *the one and only true event
    model*, AnyEvent also is free of bloat and policy: with POE or similar
    modules, you get an enourmous amount of code and strict rules you have
    to follow. AnyEvent, on the other hand, is lean and to the point by only
    offering the functionality that is useful, in as thin as a wrapper as
    technically possible.

    Of course, if you want lots of policy (this can arguably be somewhat
    useful) and you want to force your users to use the one and only event
    model, you should *not* use this module.

DESCRIPTION
    AnyEvent provides an identical interface to multiple event loops. This
    allows module authors to utilise an event loop without forcing module
    users to use the same event loop (as only a single event loop can
    coexist peacefully at any one time).

    The interface itself is vaguely similar but not identical to the Event
    module.

    On the first call of any method, the module tries to detect the
    currently loaded event loop by probing wether any of the following
    modules is loaded: Coro::EV, Coro::Event, EV, Event, Glib, Tk. The first
    one found is used. If none are found, the module tries to load these
    modules in the order given. The first one that could be successfully
    loaded will be used. If still none could be found, AnyEvent will fall
    back to a pure-perl event loop, which is also not very efficient.

    Because AnyEvent first checks for modules that are already loaded,
    loading an Event model explicitly before first using AnyEvent will
    likely make that model the default. For example:

       use Tk;
       use AnyEvent;

       # .. AnyEvent will likely default to Tk

    The pure-perl implementation of AnyEvent is called
    "AnyEvent::Impl::Perl". Like other event modules you can load it
    explicitly.

WATCHERS
    AnyEvent has the central concept of a *watcher*, which is an object that
    stores relevant data for each kind of event you are waiting for, such as
    the callback to call, the filehandle to watch, etc.

    These watchers are normal Perl objects with normal Perl lifetime. After
    creating a watcher it will immediately "watch" for events and invoke the
    callback. To disable the watcher you have to destroy it (e.g. by setting
    the variable that stores it to "undef" or otherwise deleting all
    references to it).

    All watchers are created by calling a method on the "AnyEvent" class.

  IO WATCHERS
    You can create I/O watcher by calling the "AnyEvent->io" method with the
    following mandatory arguments:

    "fh" the Perl *filehandle* (not filedescriptor) to watch for events.
    "poll" must be a string that is either "r" or "w", that creates a
    watcher waiting for "r"eadable or "w"ritable events. "cb" the callback
    to invoke everytime the filehandle becomes ready.

    Filehandles will be kept alive, so as long as the watcher exists, the
    filehandle exists, too.

    Example:

       # wait for readability of STDIN, then read a line and disable the watcher
       my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
          chomp (my $input = <STDIN>);
          warn "read: $input\n";
          undef $w;
       });

  TIME WATCHERS
    You can create a time watcher by calling the "AnyEvent->timer" method
    with the following mandatory arguments:

    "after" after how many seconds (fractions are supported) should the
    timer activate. "cb" the callback to invoke.

    The timer callback will be invoked at most once: if you want a repeating
    timer you have to create a new watcher (this is a limitation by both Tk
    and Glib).

    Example:

       # fire an event after 7.7 seconds
       my $w = AnyEvent->timer (after => 7.7, cb => sub {
          warn "timeout\n";
       });

       # to cancel the timer:
       undef $w;

  CONDITION WATCHERS
    Condition watchers can be created by calling the "AnyEvent->condvar"
    method without any arguments.

    A condition watcher watches for a condition - precisely that the
    "->broadcast" method has been called.

    Note that condition watchers recurse into the event loop - if you have
    two watchers that call "->wait" in a round-robbin fashion, you lose.
    Therefore, condition watchers are good to export to your caller, but you
    should avoid making a blocking wait, at least in callbacks, as this
    usually asks for trouble.

    The watcher has only two methods:

    $cv->wait
        Wait (blocking if necessary) until the "->broadcast" method has been
        called on c<$cv>, while servicing other watchers normally.

        You can only wait once on a condition - additional calls will return
        immediately.

        Not all event models support a blocking wait - some die in that case
        (programs might want to do that so they stay interactive), so *if
        you are using this from a module, never require a blocking wait*,
        but let the caller decide wether the call will block or not (for
        example, by coupling condition variables with some kind of request
        results and supporting callbacks so the caller knows that getting
        the result will not block, while still suppporting blocking waits if
        the caller so desires).

        Another reason *never* to "->wait" in a module is that you cannot
        sensibly have two "->wait"'s in parallel, as that would require
        multiple interpreters or coroutines/threads, none of which
        "AnyEvent" can supply (the coroutine-aware backends "Coro::EV" and
        "Coro::Event" explicitly support concurrent "->wait"'s from
        different coroutines, however).

    $cv->broadcast
        Flag the condition as ready - a running "->wait" and all further
        calls to "wait" will return after this method has been called. If
        nobody is waiting the broadcast will be remembered..

        Example:

           # wait till the result is ready
           my $result_ready = AnyEvent->condvar;

           # do something such as adding a timer
           # or socket watcher the calls $result_ready->broadcast
           # when the "result" is ready.

           $result_ready->wait;

  SIGNAL WATCHERS
    You can listen for signals using a signal watcher, "signal" is the
    signal *name* without any "SIG" prefix. Multiple signals events can be
    clumped together into one callback invocation, and callback invocation
    might or might not be asynchronous.

    These watchers might use %SIG, so programs overwriting those signals
    directly will likely not work correctly.

    Example: exit on SIGINT

       my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });

  CHILD PROCESS WATCHERS
    You can also listen for the status of a child process specified by the
    "pid" argument (or any child if the pid argument is 0). The watcher will
    trigger as often as status change for the child are received. This works
    by installing a signal handler for "SIGCHLD". The callback will be
    called with the pid and exit status (as returned by waitpid).

    Example: wait for pid 1333

      my $w = AnyEvent->child (pid => 1333, cb => sub { warn "exit status $?" });

GLOBALS
    $AnyEvent::MODEL
        Contains "undef" until the first watcher is being created. Then it
        contains the event model that is being used, which is the name of
        the Perl class implementing the model. This class is usually one of
        the "AnyEvent::Impl:xxx" modules, but can be any other class in the
        case AnyEvent has been extended at runtime (e.g. in *rxvt-unicode*).

        The known classes so far are:

           AnyEvent::Impl::CoroEV    based on Coro::EV, best choice.
           AnyEvent::Impl::CoroEvent based on Coro::Event, second best choice.
           AnyEvent::Impl::EV        based on EV (an interface to libev, also best choice).
           AnyEvent::Impl::Event     based on Event, also second best choice :)
           AnyEvent::Impl::Glib      based on Glib, third-best choice.
           AnyEvent::Impl::Tk        based on Tk, very bad choice.
           AnyEvent::Impl::Perl      pure-perl implementation, inefficient but portable.

    AnyEvent::detect
        Returns $AnyEvent::MODEL, forcing autodetection of the event model
        if necessary. You should only call this function right before you
        would have created an AnyEvent watcher anyway, that is, very late at
        runtime.

WHAT TO DO IN A MODULE
    As a module author, you should "use AnyEvent" and call AnyEvent methods
    freely, but you should not load a specific event module or rely on it.

    Be careful when you create watchers in the module body - Anyevent will
    decide which event module to use as soon as the first method is called,
    so by calling AnyEvent in your module body you force the user of your
    module to load the event module first.

WHAT TO DO IN THE MAIN PROGRAM
    There will always be a single main program - the only place that should
    dictate which event model to use.

    If it doesn't care, it can just "use AnyEvent" and use it itself, or not
    do anything special and let AnyEvent decide which implementation to
    chose.

    If the main program relies on a specific event model (for example, in
    Gtk2 programs you have to rely on either Glib or Glib::Event), you
    should load it before loading AnyEvent or any module that uses it,
    generally, as early as possible. The reason is that modules might create
    watchers when they are loaded, and AnyEvent will decide on the event
    model to use as soon as it creates watchers, and it might chose the
    wrong one unless you load the correct one yourself.

    You can chose to use a rather inefficient pure-perl implementation by
    loading the "AnyEvent::Impl::Perl" module, but letting AnyEvent chose is
    generally better.

SUPPLYING YOUR OWN EVENT MODEL INTERFACE
    If you need to support another event library which isn't directly
    supported by AnyEvent, you can supply your own interface to it by
    pushing, before the first watcher gets created, the package name of the
    event module and the package name of the interface to use onto
    @AnyEvent::REGISTRY. You can do that before and even without loading
    AnyEvent.

    Example:

       push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];

    This tells AnyEvent to (literally) use the "urxvt::anyevent::"
    package/class when it finds the "urxvt" package/module is loaded. When
    AnyEvent is loaded and asked to find a suitable event model, it will
    first check for the presence of urxvt.

    The class should provide implementations for all watcher types (see
    AnyEvent::Impl::Event (source code), AnyEvent::Impl::Glib (Source code)
    and so on for actual examples, use "perldoc -m AnyEvent::Impl::Glib" to
    see the sources).

    The above isn't fictitious, the *rxvt-unicode* (a.k.a. urxvt) uses the
    above line as-is. An interface isn't included in AnyEvent because it
    doesn't make sense outside the embedded interpreter inside
    *rxvt-unicode*, and it is updated and maintained as part of the
    *rxvt-unicode* distribution.

    *rxvt-unicode* also cheats a bit by not providing blocking access to
    condition variables: code blocking while waiting for a condition will
    "die". This still works with most modules/usages, and blocking calls
    must not be in an interactive application, so it makes sense.

ENVIRONMENT VARIABLES
    The following environment variables are used by this module:

    "PERL_ANYEVENT_VERBOSE" when set to 2 or higher, reports which event
    model gets used.

EXAMPLE
    The following program uses an io watcher to read data from stdin, a
    timer to display a message once per second, and a condvar to exit the
    program when the user enters quit:

       use AnyEvent;

       my $cv = AnyEvent->condvar;

       my $io_watcher = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
          warn "io event <$_[0]>\n";   # will always output <r>
          chomp (my $input = <STDIN>); # read a line
          warn "read: $input\n";       # output what has been read
          $cv->broadcast if $input =~ /^q/i; # quit program if /^q/i
       });

       my $time_watcher; # can only be used once

       sub new_timer {
          $timer = AnyEvent->timer (after => 1, cb => sub {
             warn "timeout\n"; # print 'timeout' about every second
             &new_timer; # and restart the time
          });
       }

       new_timer; # create first timer

       $cv->wait; # wait until user enters /^q/i

REAL-WORLD EXAMPLE
    Consider the Net::FCP module. It features (among others) the following
    API calls, which are to freenet what HTTP GET requests are to http:

       my $data = $fcp->client_get ($url); # blocks

       my $transaction = $fcp->txn_client_get ($url); # does not block
       $transaction->cb ( sub { ... } ); # set optional result callback
       my $data = $transaction->result; # possibly blocks

    The "client_get" method works like "LWP::Simple::get": it requests the
    given URL and waits till the data has arrived. It is defined to be:

       sub client_get { $_[0]->txn_client_get ($_[1])->result }

    And in fact is automatically generated. This is the blocking API of
    Net::FCP, and it works as simple as in any other, similar, module.

    More complicated is "txn_client_get": It only creates a transaction
    (completion, result, ...) object and initiates the transaction.

       my $txn = bless { }, Net::FCP::Txn::;

    It also creates a condition variable that is used to signal the
    completion of the request:

       $txn->{finished} = AnyAvent->condvar;

    It then creates a socket in non-blocking mode.

       socket $txn->{fh}, ...;
       fcntl $txn->{fh}, F_SETFL, O_NONBLOCK;
       connect $txn->{fh}, ...
          and !$!{EWOULDBLOCK}
          and !$!{EINPROGRESS}
          and Carp::croak "unable to connect: $!\n";

    Then it creates a write-watcher which gets called whenever an error
    occurs or the connection succeeds:

       $txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'w', cb => sub { $txn->fh_ready_w });

    And returns this transaction object. The "fh_ready_w" callback gets
    called as soon as the event loop detects that the socket is ready for
    writing.

    The "fh_ready_w" method makes the socket blocking again, writes the
    request data and replaces the watcher by a read watcher (waiting for
    reply data). The actual code is more complicated, but that doesn't
    matter for this example:

       fcntl $txn->{fh}, F_SETFL, 0;
       syswrite $txn->{fh}, $txn->{request}
          or die "connection or write error";
       $txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });

    Again, "fh_ready_r" waits till all data has arrived, and then stores the
    result and signals any possible waiters that the request ahs finished:

       sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};

       if (end-of-file or data complete) {
         $txn->{result} = $txn->{buf};
         $txn->{finished}->broadcast;
         $txb->{cb}->($txn) of $txn->{cb}; # also call callback
       }

    The "result" method, finally, just waits for the finished signal (if the
    request was already finished, it doesn't wait, of course, and returns
    the data:

       $txn->{finished}->wait;
       return $txn->{result};

    The actual code goes further and collects all errors ("die"s,
    exceptions) that occured during request processing. The "result" method
    detects wether an exception as thrown (it is stored inside the $txn
    object) and just throws the exception, which means connection errors and
    other problems get reported tot he code that tries to use the result,
    not in a random callback.

    All of this enables the following usage styles:

    1. Blocking:

       my $data = $fcp->client_get ($url);

    2. Blocking, but running in parallel:

       my @datas = map $_->result,
                      map $fcp->txn_client_get ($_),
                         @urls;

    Both blocking examples work without the module user having to know
    anything about events.

    3a. Event-based in a main program, using any supported event module:

       use EV;

       $fcp->txn_client_get ($url)->cb (sub {
          my $txn = shift;
          my $data = $txn->result;
          ...
       });

       EV::loop;

    3b. The module user could use AnyEvent, too:

       use AnyEvent;

       my $quit = AnyEvent->condvar;

       $fcp->txn_client_get ($url)->cb (sub {
          ...
          $quit->broadcast;
       });

       $quit->wait;

SEE ALSO
    Event modules: Coro::EV, EV, EV::Glib, Glib::EV, Coro::Event, Event,
    Glib::Event, Glib, Coro, Tk.

    Implementations: AnyEvent::Impl::CoroEV, AnyEvent::Impl::EV,
    AnyEvent::Impl::CoroEvent, AnyEvent::Impl::Event, AnyEvent::Impl::Glib,
    AnyEvent::Impl::Tk, AnyEvent::Impl::Perl.

    Nontrivial usage examples: Net::FCP, Net::XMPP2.


Revision:	1.15
Committed:	Wed Apr 16 15:10:10 2008 UTC (16 years, 1 month ago) by root
Branch:	MAIN
CVS Tags:	rel-3_1
Changes since 1.14:	+36 -29 lines
Log Message:	* empty log message *
#	Content
1	NAME
2	AnyEvent - provide framework for multiple event loops
3
4	EV, Event, Coro::EV, Coro::Event, Glib, Tk, Perl - various supported
5	event loops
6
7	SYNOPSIS
8	use AnyEvent;
9
10	my $w = AnyEvent->io (fh => $fh, poll => "r\|w", cb => sub {
11	...
12	});
13
14	my $w = AnyEvent->timer (after => $seconds, cb => sub {
15	...
16	});
17
18	my $w = AnyEvent->condvar; # stores wether a condition was flagged
19	$w->wait; # enters "main loop" till $condvar gets ->broadcast
20	$w->broadcast; # wake up current and all future wait's
21
22	WHY YOU SHOULD USE THIS MODULE (OR NOT)
23	Glib, POE, IO::Async, Event... CPAN offers event models by the dozen
24	nowadays. So what is different about AnyEvent?
25
26	Executive Summary: AnyEvent is compatible, AnyEvent is *free of
27	policy* and AnyEvent is small and efficient.
28
29	First and foremost, AnyEvent is not an event model itself, it only
30	interfaces to whatever event model the main program happens to use in a
31	pragmatic way. For event models and certain classes of immortals alike,
32	the statement "there can only be one" is a bitter reality, and AnyEvent
33	helps hiding the differences.
34
35	The goal of AnyEvent is to offer module authors the ability to do event
36	programming (waiting for I/O or timer events) without subscribing to a
37	religion, a way of living, and most importantly: without forcing your
38	module users into the same thing by forcing them to use the same event
39	model you use.
40
41	For modules like POE or IO::Async (which is actually doing all I/O
42	synchronously...), using them in your module is like joining a cult:
43	After you joined, you are dependent on them and you cannot use anything
44	else, as it is simply incompatible to everything that isn't itself.
45
46	AnyEvent + POE works fine. AnyEvent + Glib works fine. AnyEvent + Tk
47	works fine etc. etc. but none of these work together with the rest: POE
48	+ IO::Async? no go. Tk + Event? no go. If your module uses one of those,
49	every user of your module has to use it, too. If your module uses
50	AnyEvent, it works transparently with all event models it supports
51	(including stuff like POE and IO::Async).
52
53	In addition of being free of having to use *the one and only true event
54	model*, AnyEvent also is free of bloat and policy: with POE or similar
55	modules, you get an enourmous amount of code and strict rules you have
56	to follow. AnyEvent, on the other hand, is lean and to the point by only
57	offering the functionality that is useful, in as thin as a wrapper as
58	technically possible.
59
60	Of course, if you want lots of policy (this can arguably be somewhat
61	useful) and you want to force your users to use the one and only event
62	model, you should not use this module.
63
64	DESCRIPTION
65	AnyEvent provides an identical interface to multiple event loops. This
66	allows module authors to utilise an event loop without forcing module
67	users to use the same event loop (as only a single event loop can
68	coexist peacefully at any one time).
69
70	The interface itself is vaguely similar but not identical to the Event
71	module.
72
73	On the first call of any method, the module tries to detect the
74	currently loaded event loop by probing wether any of the following
75	modules is loaded: Coro::EV, Coro::Event, EV, Event, Glib, Tk. The first
76	one found is used. If none are found, the module tries to load these
77	modules in the order given. The first one that could be successfully
78	loaded will be used. If still none could be found, AnyEvent will fall
79	back to a pure-perl event loop, which is also not very efficient.
80
81	Because AnyEvent first checks for modules that are already loaded,
82	loading an Event model explicitly before first using AnyEvent will
83	likely make that model the default. For example:
84
85	use Tk;
86	use AnyEvent;
87
88	# .. AnyEvent will likely default to Tk
89
90	The pure-perl implementation of AnyEvent is called
91	"AnyEvent::Impl::Perl". Like other event modules you can load it
92	explicitly.
93
94	WATCHERS
95	AnyEvent has the central concept of a watcher, which is an object that
96	stores relevant data for each kind of event you are waiting for, such as
97	the callback to call, the filehandle to watch, etc.
98
99	These watchers are normal Perl objects with normal Perl lifetime. After
100	creating a watcher it will immediately "watch" for events and invoke the
101	callback. To disable the watcher you have to destroy it (e.g. by setting
102	the variable that stores it to "undef" or otherwise deleting all
103	references to it).
104
105	All watchers are created by calling a method on the "AnyEvent" class.
106
107	IO WATCHERS
108	You can create I/O watcher by calling the "AnyEvent->io" method with the
109	following mandatory arguments:
110
111	"fh" the Perl filehandle (not filedescriptor) to watch for events.
112	"poll" must be a string that is either "r" or "w", that creates a
113	watcher waiting for "r"eadable or "w"ritable events. "cb" the callback
114	to invoke everytime the filehandle becomes ready.
115
116	Filehandles will be kept alive, so as long as the watcher exists, the
117	filehandle exists, too.
118
119	Example:
120
121	# wait for readability of STDIN, then read a line and disable the watcher
122	my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
123	chomp (my $input = <STDIN>);
124	warn "read: $input\n";
125	undef $w;
126	});
127
128	TIME WATCHERS
129	You can create a time watcher by calling the "AnyEvent->timer" method
130	with the following mandatory arguments:
131
132	"after" after how many seconds (fractions are supported) should the
133	timer activate. "cb" the callback to invoke.
134
135	The timer callback will be invoked at most once: if you want a repeating
136	timer you have to create a new watcher (this is a limitation by both Tk
137	and Glib).
138
139	Example:
140
141	# fire an event after 7.7 seconds
142	my $w = AnyEvent->timer (after => 7.7, cb => sub {
143	warn "timeout\n";
144	});
145
146	# to cancel the timer:
147	undef $w;
148
149	CONDITION WATCHERS
150	Condition watchers can be created by calling the "AnyEvent->condvar"
151	method without any arguments.
152
153	A condition watcher watches for a condition - precisely that the
154	"->broadcast" method has been called.
155
156	Note that condition watchers recurse into the event loop - if you have
157	two watchers that call "->wait" in a round-robbin fashion, you lose.
158	Therefore, condition watchers are good to export to your caller, but you
159	should avoid making a blocking wait, at least in callbacks, as this
160	usually asks for trouble.
161
162	The watcher has only two methods:
163
164	$cv->wait
165	Wait (blocking if necessary) until the "->broadcast" method has been
166	called on c<$cv>, while servicing other watchers normally.
167
168	You can only wait once on a condition - additional calls will return
169	immediately.
170
171	Not all event models support a blocking wait - some die in that case
172	(programs might want to do that so they stay interactive), so *if
173	you are using this from a module, never require a blocking wait*,
174	but let the caller decide wether the call will block or not (for
175	example, by coupling condition variables with some kind of request
176	results and supporting callbacks so the caller knows that getting
177	the result will not block, while still suppporting blocking waits if
178	the caller so desires).
179
180	Another reason never to "->wait" in a module is that you cannot
181	sensibly have two "->wait"'s in parallel, as that would require
182	multiple interpreters or coroutines/threads, none of which
183	"AnyEvent" can supply (the coroutine-aware backends "Coro::EV" and
184	"Coro::Event" explicitly support concurrent "->wait"'s from
185	different coroutines, however).
186
187	$cv->broadcast
188	Flag the condition as ready - a running "->wait" and all further
189	calls to "wait" will return after this method has been called. If
190	nobody is waiting the broadcast will be remembered..
191
192	Example:
193
194	# wait till the result is ready
195	my $result_ready = AnyEvent->condvar;
196
197	# do something such as adding a timer
198	# or socket watcher the calls $result_ready->broadcast
199	# when the "result" is ready.
200
201	$result_ready->wait;
202
203	SIGNAL WATCHERS
204	You can listen for signals using a signal watcher, "signal" is the
205	signal name without any "SIG" prefix. Multiple signals events can be
206	clumped together into one callback invocation, and callback invocation
207	might or might not be asynchronous.
208
209	These watchers might use %SIG, so programs overwriting those signals
210	directly will likely not work correctly.
211
212	Example: exit on SIGINT
213
214	my $w = AnyEvent->signal (signal => "INT", cb => sub { exit 1 });
215
216	CHILD PROCESS WATCHERS
217	You can also listen for the status of a child process specified by the
218	"pid" argument (or any child if the pid argument is 0). The watcher will
219	trigger as often as status change for the child are received. This works
220	by installing a signal handler for "SIGCHLD". The callback will be
221	called with the pid and exit status (as returned by waitpid).
222
223	Example: wait for pid 1333
224
225	my $w = AnyEvent->child (pid => 1333, cb => sub { warn "exit status $?" });
226
227	GLOBALS
228	$AnyEvent::MODEL
229	Contains "undef" until the first watcher is being created. Then it
230	contains the event model that is being used, which is the name of
231	the Perl class implementing the model. This class is usually one of
232	the "AnyEvent::Impl:xxx" modules, but can be any other class in the
233	case AnyEvent has been extended at runtime (e.g. in rxvt-unicode).
234
235	The known classes so far are:
236
237	AnyEvent::Impl::CoroEV based on Coro::EV, best choice.
238	AnyEvent::Impl::CoroEvent based on Coro::Event, second best choice.
239	AnyEvent::Impl::EV based on EV (an interface to libev, also best choice).
240	AnyEvent::Impl::Event based on Event, also second best choice :)
241	AnyEvent::Impl::Glib based on Glib, third-best choice.
242	AnyEvent::Impl::Tk based on Tk, very bad choice.
243	AnyEvent::Impl::Perl pure-perl implementation, inefficient but portable.
244
245	AnyEvent::detect
246	Returns $AnyEvent::MODEL, forcing autodetection of the event model
247	if necessary. You should only call this function right before you
248	would have created an AnyEvent watcher anyway, that is, very late at
249	runtime.
250
251	WHAT TO DO IN A MODULE
252	As a module author, you should "use AnyEvent" and call AnyEvent methods
253	freely, but you should not load a specific event module or rely on it.
254
255	Be careful when you create watchers in the module body - Anyevent will
256	decide which event module to use as soon as the first method is called,
257	so by calling AnyEvent in your module body you force the user of your
258	module to load the event module first.
259
260	WHAT TO DO IN THE MAIN PROGRAM
261	There will always be a single main program - the only place that should
262	dictate which event model to use.
263
264	If it doesn't care, it can just "use AnyEvent" and use it itself, or not
265	do anything special and let AnyEvent decide which implementation to
266	chose.
267
268	If the main program relies on a specific event model (for example, in
269	Gtk2 programs you have to rely on either Glib or Glib::Event), you
270	should load it before loading AnyEvent or any module that uses it,
271	generally, as early as possible. The reason is that modules might create
272	watchers when they are loaded, and AnyEvent will decide on the event
273	model to use as soon as it creates watchers, and it might chose the
274	wrong one unless you load the correct one yourself.
275
276	You can chose to use a rather inefficient pure-perl implementation by
277	loading the "AnyEvent::Impl::Perl" module, but letting AnyEvent chose is
278	generally better.
279
280	SUPPLYING YOUR OWN EVENT MODEL INTERFACE
281	If you need to support another event library which isn't directly
282	supported by AnyEvent, you can supply your own interface to it by
283	pushing, before the first watcher gets created, the package name of the
284	event module and the package name of the interface to use onto
285	@AnyEvent::REGISTRY. You can do that before and even without loading
286	AnyEvent.
287
288	Example:
289
290	push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];
291
292	This tells AnyEvent to (literally) use the "urxvt::anyevent::"
293	package/class when it finds the "urxvt" package/module is loaded. When
294	AnyEvent is loaded and asked to find a suitable event model, it will
295	first check for the presence of urxvt.
296
297	The class should provide implementations for all watcher types (see
298	AnyEvent::Impl::Event (source code), AnyEvent::Impl::Glib (Source code)
299	and so on for actual examples, use "perldoc -m AnyEvent::Impl::Glib" to
300	see the sources).
301
302	The above isn't fictitious, the rxvt-unicode (a.k.a. urxvt) uses the
303	above line as-is. An interface isn't included in AnyEvent because it
304	doesn't make sense outside the embedded interpreter inside
305	rxvt-unicode, and it is updated and maintained as part of the
306	rxvt-unicode distribution.
307
308	rxvt-unicode also cheats a bit by not providing blocking access to
309	condition variables: code blocking while waiting for a condition will
310	"die". This still works with most modules/usages, and blocking calls
311	must not be in an interactive application, so it makes sense.
312
313	ENVIRONMENT VARIABLES
314	The following environment variables are used by this module:
315
316	"PERL_ANYEVENT_VERBOSE" when set to 2 or higher, reports which event
317	model gets used.
318
319	EXAMPLE
320	The following program uses an io watcher to read data from stdin, a
321	timer to display a message once per second, and a condvar to exit the
322	program when the user enters quit:
323
324	use AnyEvent;
325
326	my $cv = AnyEvent->condvar;
327
328	my $io_watcher = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
329	warn "io event <$_[0]>\n"; # will always output <r>
330	chomp (my $input = <STDIN>); # read a line
331	warn "read: $input\n"; # output what has been read
332	$cv->broadcast if $input =~ /^q/i; # quit program if /^q/i
333	});
334
335	my $time_watcher; # can only be used once
336
337	sub new_timer {
338	$timer = AnyEvent->timer (after => 1, cb => sub {
339	warn "timeout\n"; # print 'timeout' about every second
340	&new_timer; # and restart the time
341	});
342	}
343
344	new_timer; # create first timer
345
346	$cv->wait; # wait until user enters /^q/i
347
348	REAL-WORLD EXAMPLE
349	Consider the Net::FCP module. It features (among others) the following
350	API calls, which are to freenet what HTTP GET requests are to http:
351
352	my $data = $fcp->client_get ($url); # blocks
353
354	my $transaction = $fcp->txn_client_get ($url); # does not block
355	$transaction->cb ( sub { ... } ); # set optional result callback
356	my $data = $transaction->result; # possibly blocks
357
358	The "client_get" method works like "LWP::Simple::get": it requests the
359	given URL and waits till the data has arrived. It is defined to be:
360
361	sub client_get { $_[0]->txn_client_get ($_[1])->result }
362
363	And in fact is automatically generated. This is the blocking API of
364	Net::FCP, and it works as simple as in any other, similar, module.
365
366	More complicated is "txn_client_get": It only creates a transaction
367	(completion, result, ...) object and initiates the transaction.
368
369	my $txn = bless { }, Net::FCP::Txn::;
370
371	It also creates a condition variable that is used to signal the
372	completion of the request:
373
374	$txn->{finished} = AnyAvent->condvar;
375
376	It then creates a socket in non-blocking mode.
377
378	socket $txn->{fh}, ...;
379	fcntl $txn->{fh}, F_SETFL, O_NONBLOCK;
380	connect $txn->{fh}, ...
381	and !$!{EWOULDBLOCK}
382	and !$!{EINPROGRESS}
383	and Carp::croak "unable to connect: $!\n";
384
385	Then it creates a write-watcher which gets called whenever an error
386	occurs or the connection succeeds:
387
388	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'w', cb => sub { $txn->fh_ready_w });
389
390	And returns this transaction object. The "fh_ready_w" callback gets
391	called as soon as the event loop detects that the socket is ready for
392	writing.
393
394	The "fh_ready_w" method makes the socket blocking again, writes the
395	request data and replaces the watcher by a read watcher (waiting for
396	reply data). The actual code is more complicated, but that doesn't
397	matter for this example:
398
399	fcntl $txn->{fh}, F_SETFL, 0;
400	syswrite $txn->{fh}, $txn->{request}
401	or die "connection or write error";
402	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });
403
404	Again, "fh_ready_r" waits till all data has arrived, and then stores the
405	result and signals any possible waiters that the request ahs finished:
406
407	sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};
408
409	if (end-of-file or data complete) {
410	$txn->{result} = $txn->{buf};
411	$txn->{finished}->broadcast;
412	$txb->{cb}->($txn) of $txn->{cb}; # also call callback
413	}
414
415	The "result" method, finally, just waits for the finished signal (if the
416	request was already finished, it doesn't wait, of course, and returns
417	the data:
418
419	$txn->{finished}->wait;
420	return $txn->{result};
421
422	The actual code goes further and collects all errors ("die"s,
423	exceptions) that occured during request processing. The "result" method
424	detects wether an exception as thrown (it is stored inside the $txn
425	object) and just throws the exception, which means connection errors and
426	other problems get reported tot he code that tries to use the result,
427	not in a random callback.
428
429	All of this enables the following usage styles:
430
431	1. Blocking:
432
433	my $data = $fcp->client_get ($url);
434
435	2. Blocking, but running in parallel:
436
437	my @datas = map $_->result,
438	map $fcp->txn_client_get ($_),
439	@urls;
440
441	Both blocking examples work without the module user having to know
442	anything about events.
443
444	3a. Event-based in a main program, using any supported event module:
445
446	use EV;
447
448	$fcp->txn_client_get ($url)->cb (sub {
449	my $txn = shift;
450	my $data = $txn->result;
451	...
452	});
453
454	EV::loop;
455
456	3b. The module user could use AnyEvent, too:
457
458	use AnyEvent;
459
460	my $quit = AnyEvent->condvar;
461
462	$fcp->txn_client_get ($url)->cb (sub {
463	...
464	$quit->broadcast;
465	});
466
467	$quit->wait;
468
469	SEE ALSO
470	Event modules: Coro::EV, EV, EV::Glib, Glib::EV, Coro::Event, Event,
471	Glib::Event, Glib, Coro, Tk.
472
473	Implementations: AnyEvent::Impl::CoroEV, AnyEvent::Impl::EV,
474	AnyEvent::Impl::CoroEvent, AnyEvent::Impl::Event, AnyEvent::Impl::Glib,
475	AnyEvent::Impl::Tk, AnyEvent::Impl::Perl.
476
477	Nontrivial usage examples: Net::FCP, Net::XMPP2.
478
479