[ViewVC] Annotation 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 *
#	User	Rev	Content
1	root	1.2	NAME
2			AnyEvent - provide framework for multiple event loops
3
4	root	1.6	Event, Coro, Glib, Tk, Perl - various supported event loops
5	root	1.2
6			SYNOPSIS
7	root	1.4	use AnyEvent;
8	root	1.2
9	root	1.6	my $w = AnyEvent->io (fh => $fh, poll => "r\|w", cb => sub {
10	root	1.2	...
11			});
12	root	1.3
13			my $w = AnyEvent->timer (after => $seconds, cb => sub {
14	root	1.2	...
15			});
16
17	root	1.6	my $w = AnyEvent->condvar; # stores wether a condition was flagged
18			$w->wait; # enters "main loop" till $condvar gets ->broadcast
19	root	1.3	$w->broadcast; # wake up current and all future wait's
20
21	root	1.2	DESCRIPTION
22			AnyEvent provides an identical interface to multiple event loops. This
23	root	1.6	allows module authors to utilise an event loop without forcing module
24	root	1.2	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	root	1.6	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	root	1.2
179	root	1.5	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	root	1.6	pushing, before the first watcher gets created, the package name of the
183	root	1.5	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	root	1.6	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	root	1.5
201			The above isn't fictitious, the rxvt-unicode (a.k.a. urxvt) uses the
202	root	1.6	above line as-is. An interface isn't included in AnyEvent because it
203	root	1.5	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	root	1.6	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	root	1.4	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	root	1.2	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	root	1.3	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	root	1.4	Then it creates a write-watcher which gets called whenever an error
285	root	1.3	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	root	1.4	$txb->{cb}->($txn) of $txn->{cb}; # also call callback
312	root	1.3	}
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	root	1.4	return $txn->{result};
320	root	1.3
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	root	1.2	SEE ALSO
369	root	1.3	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	root	1.2
376