[ViewVC] Contents of: cvs/AnyEvent/README

NAME
    AnyEvent - provide framework for multiple event loops

    Event, Coro, 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

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::Event, Event, Glib, Tk. The first one found is
    used. If none is 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" teh callback
    to invoke everytime the filehandle becomes ready.

    Only one io watcher per "fh" and "poll" combination is allowed (i.e. on
    a socket you can have one r + one w, not any more (limitation comes from
    Tk - if you are sure you are not using Tk this limitation is gone).

    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;
       });

  TIMER WATCHERS
    You can create a timer 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.

    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.

        Not all event models support a blocking wait - some die in that
        case, 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 blockign
        waits if the caller so desires).

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

    $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;

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 prove 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 appliation, 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 parallelizing:

       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 support Event module:

       use Event;

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

       Event::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::Event, Coro, Event, Glib::Event, Glib.

    Implementations: AnyEvent::Impl::Coro, AnyEvent::Impl::Event,
    AnyEvent::Impl::Glib, AnyEvent::Impl::Tk.

    Nontrivial usage example: Net::FCP.


Revision:	1.6
Committed:	Wed Nov 1 01:22:19 2006 UTC (17 years, 6 months ago) by root
Branch:	MAIN
CVS Tags:	rel-2_0
Changes since 1.5:	+164 -36 lines
Log Message:	* empty log message *
#	Content
1	NAME
2	AnyEvent - provide framework for multiple event loops
3
4	Event, Coro, Glib, Tk, Perl - various supported event loops
5
6	SYNOPSIS
7	use AnyEvent;
8
9	my $w = AnyEvent->io (fh => $fh, poll => "r\|w", cb => sub {
10	...
11	});
12
13	my $w = AnyEvent->timer (after => $seconds, cb => sub {
14	...
15	});
16
17	my $w = AnyEvent->condvar; # stores wether a condition was flagged
18	$w->wait; # enters "main loop" till $condvar gets ->broadcast
19	$w->broadcast; # wake up current and all future wait's
20
21	DESCRIPTION
22	AnyEvent provides an identical interface to multiple event loops. This
23	allows module authors to utilise an event loop without forcing module
24	users to use the same event loop (as only a single event loop can
25	coexist peacefully at any one time).
26
27	The interface itself is vaguely similar but not identical to the Event
28	module.
29
30	On the first call of any method, the module tries to detect the
31	currently loaded event loop by probing wether any of the following
32	modules is loaded: Coro::Event, Event, Glib, Tk. The first one found is
33	used. If none is found, the module tries to load these modules in the
34	order given. The first one that could be successfully loaded will be
35	used. If still none could be found, AnyEvent will fall back to a
36	pure-perl event loop, which is also not very efficient.
37
38	Because AnyEvent first checks for modules that are already loaded,
39	loading an Event model explicitly before first using AnyEvent will
40	likely make that model the default. For example:
41
42	use Tk;
43	use AnyEvent;
44
45	# .. AnyEvent will likely default to Tk
46
47	The pure-perl implementation of AnyEvent is called
48	"AnyEvent::Impl::Perl". Like other event modules you can load it
49	explicitly.
50
51	WATCHERS
52	AnyEvent has the central concept of a watcher, which is an object that
53	stores relevant data for each kind of event you are waiting for, such as
54	the callback to call, the filehandle to watch, etc.
55
56	These watchers are normal Perl objects with normal Perl lifetime. After
57	creating a watcher it will immediately "watch" for events and invoke the
58	callback. To disable the watcher you have to destroy it (e.g. by setting
59	the variable that stores it to "undef" or otherwise deleting all
60	references to it).
61
62	All watchers are created by calling a method on the "AnyEvent" class.
63
64	IO WATCHERS
65	You can create I/O watcher by calling the "AnyEvent->io" method with the
66	following mandatory arguments:
67
68	"fh" the Perl filehandle (not filedescriptor) to watch for events.
69	"poll" must be a string that is either "r" or "w", that creates a
70	watcher waiting for "r"eadable or "w"ritable events. "cb" teh callback
71	to invoke everytime the filehandle becomes ready.
72
73	Only one io watcher per "fh" and "poll" combination is allowed (i.e. on
74	a socket you can have one r + one w, not any more (limitation comes from
75	Tk - if you are sure you are not using Tk this limitation is gone).
76
77	Filehandles will be kept alive, so as long as the watcher exists, the
78	filehandle exists, too.
79
80	Example:
81
82	# wait for readability of STDIN, then read a line and disable the watcher
83	my $w; $w = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
84	chomp (my $input = <STDIN>);
85	warn "read: $input\n";
86	undef $w;
87	});
88
89	TIMER WATCHERS
90	You can create a timer watcher by calling the "AnyEvent->timer" method
91	with the following mandatory arguments:
92
93	"after" after how many seconds (fractions are supported) should the
94	timer activate. "cb" the callback to invoke.
95
96	The timer callback will be invoked at most once: if you want a repeating
97	timer you have to create a new watcher (this is a limitation by both Tk
98	and Glib).
99
100	Example:
101
102	# fire an event after 7.7 seconds
103	my $w = AnyEvent->timer (after => 7.7, cb => sub {
104	warn "timeout\n";
105	});
106
107	# to cancel the timer:
108	undef $w
109
110	CONDITION WATCHERS
111	Condition watchers can be created by calling the "AnyEvent->condvar"
112	method without any arguments.
113
114	A condition watcher watches for a condition - precisely that the
115	"->broadcast" method has been called.
116
117	The watcher has only two methods:
118
119	$cv->wait
120	Wait (blocking if necessary) until the "->broadcast" method has been
121	called on c<$cv>, while servicing other watchers normally.
122
123	Not all event models support a blocking wait - some die in that
124	case, so if you are using this from a module, never require a
125	blocking wait, but let the caller decide wether the call will block
126	or not (for example, by coupling condition variables with some kind
127	of request results and supporting callbacks so the caller knows that
128	getting the result will not block, while still suppporting blockign
129	waits if the caller so desires).
130
131	You can only wait once on a condition - additional calls will return
132	immediately.
133
134	$cv->broadcast
135	Flag the condition as ready - a running "->wait" and all further
136	calls to "wait" will return after this method has been called. If
137	nobody is waiting the broadcast will be remembered..
138
139	Example:
140
141	# wait till the result is ready
142	my $result_ready = AnyEvent->condvar;
143
144	# do something such as adding a timer
145	# or socket watcher the calls $result_ready->broadcast
146	# when the "result" is ready.
147
148	$result_ready->wait;
149
150	WHAT TO DO IN A MODULE
151	As a module author, you should "use AnyEvent" and call AnyEvent methods
152	freely, but you should not load a specific event module or rely on it.
153
154	Be careful when you create watchers in the module body - Anyevent will
155	decide which event module to use as soon as the first method is called,
156	so by calling AnyEvent in your module body you force the user of your
157	module to load the event module first.
158
159	WHAT TO DO IN THE MAIN PROGRAM
160	There will always be a single main program - the only place that should
161	dictate which event model to use.
162
163	If it doesn't care, it can just "use AnyEvent" and use it itself, or not
164	do anything special and let AnyEvent decide which implementation to
165	chose.
166
167	If the main program relies on a specific event model (for example, in
168	Gtk2 programs you have to rely on either Glib or Glib::Event), you
169	should load it before loading AnyEvent or any module that uses it,
170	generally, as early as possible. The reason is that modules might create
171	watchers when they are loaded, and AnyEvent will decide on the event
172	model to use as soon as it creates watchers, and it might chose the
173	wrong one unless you load the correct one yourself.
174
175	You can chose to use a rather inefficient pure-perl implementation by
176	loading the "AnyEvent::Impl::Perl" module, but letting AnyEvent chose is
177	generally better.
178
179	SUPPLYING YOUR OWN EVENT MODEL INTERFACE
180	If you need to support another event library which isn't directly
181	supported by AnyEvent, you can supply your own interface to it by
182	pushing, before the first watcher gets created, the package name of the
183	event module and the package name of the interface to use onto
184	@AnyEvent::REGISTRY. You can do that before and even without loading
185	AnyEvent.
186
187	Example:
188
189	push @AnyEvent::REGISTRY, [urxvt => urxvt::anyevent::];
190
191	This tells AnyEvent to (literally) use the "urxvt::anyevent::"
192	package/class when it finds the "urxvt" package/module is loaded. When
193	AnyEvent is loaded and asked to find a suitable event model, it will
194	first check for the presence of urxvt.
195
196	The class should prove implementations for all watcher types (see
197	AnyEvent::Impl::Event (source code), AnyEvent::Impl::Glib (Source code)
198	and so on for actual examples, use "perldoc -m AnyEvent::Impl::Glib" to
199	see the sources).
200
201	The above isn't fictitious, the rxvt-unicode (a.k.a. urxvt) uses the
202	above line as-is. An interface isn't included in AnyEvent because it
203	doesn't make sense outside the embedded interpreter inside
204	rxvt-unicode, and it is updated and maintained as part of the
205	rxvt-unicode distribution.
206
207	rxvt-unicode also cheats a bit by not providing blocking access to
208	condition variables: code blocking while waiting for a condition will
209	"die". This still works with most modules/usages, and blocking calls
210	must not be in an interactive appliation, so it makes sense.
211
212	ENVIRONMENT VARIABLES
213	The following environment variables are used by this module:
214
215	"PERL_ANYEVENT_VERBOSE" when set to 2 or higher, reports which event
216	model gets used.
217
218	EXAMPLE
219	The following program uses an io watcher to read data from stdin, a
220	timer to display a message once per second, and a condvar to exit the
221	program when the user enters quit:
222
223	use AnyEvent;
224
225	my $cv = AnyEvent->condvar;
226
227	my $io_watcher = AnyEvent->io (fh => \*STDIN, poll => 'r', cb => sub {
228	warn "io event <$_[0]>\n"; # will always output <r>
229	chomp (my $input = <STDIN>); # read a line
230	warn "read: $input\n"; # output what has been read
231	$cv->broadcast if $input =~ /^q/i; # quit program if /^q/i
232	});
233
234	my $time_watcher; # can only be used once
235
236	sub new_timer {
237	$timer = AnyEvent->timer (after => 1, cb => sub {
238	warn "timeout\n"; # print 'timeout' about every second
239	&new_timer; # and restart the time
240	});
241	}
242
243	new_timer; # create first timer
244
245	$cv->wait; # wait until user enters /^q/i
246
247	REAL-WORLD EXAMPLE
248	Consider the Net::FCP module. It features (among others) the following
249	API calls, which are to freenet what HTTP GET requests are to http:
250
251	my $data = $fcp->client_get ($url); # blocks
252
253	my $transaction = $fcp->txn_client_get ($url); # does not block
254	$transaction->cb ( sub { ... } ); # set optional result callback
255	my $data = $transaction->result; # possibly blocks
256
257	The "client_get" method works like "LWP::Simple::get": it requests the
258	given URL and waits till the data has arrived. It is defined to be:
259
260	sub client_get { $_[0]->txn_client_get ($_[1])->result }
261
262	And in fact is automatically generated. This is the blocking API of
263	Net::FCP, and it works as simple as in any other, similar, module.
264
265	More complicated is "txn_client_get": It only creates a transaction
266	(completion, result, ...) object and initiates the transaction.
267
268	my $txn = bless { }, Net::FCP::Txn::;
269
270	It also creates a condition variable that is used to signal the
271	completion of the request:
272
273	$txn->{finished} = AnyAvent->condvar;
274
275	It then creates a socket in non-blocking mode.
276
277	socket $txn->{fh}, ...;
278	fcntl $txn->{fh}, F_SETFL, O_NONBLOCK;
279	connect $txn->{fh}, ...
280	and !$!{EWOULDBLOCK}
281	and !$!{EINPROGRESS}
282	and Carp::croak "unable to connect: $!\n";
283
284	Then it creates a write-watcher which gets called whenever an error
285	occurs or the connection succeeds:
286
287	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'w', cb => sub { $txn->fh_ready_w });
288
289	And returns this transaction object. The "fh_ready_w" callback gets
290	called as soon as the event loop detects that the socket is ready for
291	writing.
292
293	The "fh_ready_w" method makes the socket blocking again, writes the
294	request data and replaces the watcher by a read watcher (waiting for
295	reply data). The actual code is more complicated, but that doesn't
296	matter for this example:
297
298	fcntl $txn->{fh}, F_SETFL, 0;
299	syswrite $txn->{fh}, $txn->{request}
300	or die "connection or write error";
301	$txn->{w} = AnyEvent->io (fh => $txn->{fh}, poll => 'r', cb => sub { $txn->fh_ready_r });
302
303	Again, "fh_ready_r" waits till all data has arrived, and then stores the
304	result and signals any possible waiters that the request ahs finished:
305
306	sysread $txn->{fh}, $txn->{buf}, length $txn->{$buf};
307
308	if (end-of-file or data complete) {
309	$txn->{result} = $txn->{buf};
310	$txn->{finished}->broadcast;
311	$txb->{cb}->($txn) of $txn->{cb}; # also call callback
312	}
313
314	The "result" method, finally, just waits for the finished signal (if the
315	request was already finished, it doesn't wait, of course, and returns
316	the data:
317
318	$txn->{finished}->wait;
319	return $txn->{result};
320
321	The actual code goes further and collects all errors ("die"s,
322	exceptions) that occured during request processing. The "result" method
323	detects wether an exception as thrown (it is stored inside the $txn
324	object) and just throws the exception, which means connection errors and
325	other problems get reported tot he code that tries to use the result,
326	not in a random callback.
327
328	All of this enables the following usage styles:
329
330	1. Blocking:
331
332	my $data = $fcp->client_get ($url);
333
334	2. Blocking, but parallelizing:
335
336	my @datas = map $_->result,
337	map $fcp->txn_client_get ($_),
338	@urls;
339
340	Both blocking examples work without the module user having to know
341	anything about events.
342
343	3a. Event-based in a main program, using any support Event module:
344
345	use Event;
346
347	$fcp->txn_client_get ($url)->cb (sub {
348	my $txn = shift;
349	my $data = $txn->result;
350	...
351	});
352
353	Event::loop;
354
355	3b. The module user could use AnyEvent, too:
356
357	use AnyEvent;
358
359	my $quit = AnyEvent->condvar;
360
361	$fcp->txn_client_get ($url)->cb (sub {
362	...
363	$quit->broadcast;
364	});
365
366	$quit->wait;
367
368	SEE ALSO
369	Event modules: Coro::Event, Coro, Event, Glib::Event, Glib.
370
371	Implementations: AnyEvent::Impl::Coro, AnyEvent::Impl::Event,
372	AnyEvent::Impl::Glib, AnyEvent::Impl::Tk.
373
374	Nontrivial usage example: Net::FCP.
375
376