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=head1 Introduction to AnyEvent |
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This is a tutorial that will introduce you to the features of AnyEvent. |
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The first part introduces the core AnyEvent module (after swamping you a |
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bit in evangelism), which might already provide all you ever need. |
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1.1 |
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The second part focuses on network programming using sockets, for which |
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AnyEvent offers a lot of support you can use. |
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=head1 What is AnyEvent? |
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Skip this section if you want to see code, now! |
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AnyEvent is first of all just a framework to do event-based |
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programming. Typically such frameworks are an all-or-nothing thing: If you |
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use one such framework, you can't (easily, or even at all) use another in |
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the same program. |
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AnyEvent is different - it is a thin abstraction layer above all kinds |
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of event loops. Its main purpose is to move the choice of the underlying |
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framework (the event loop) from the module author to the program author |
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using the module. |
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That means you can write code that uses events to control what it |
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does, without forcing other code in the same program to use the same |
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underlying framework as you do - i.e. you can create a Perl module |
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that is event-based using AnyEvent, and users of that module can still |
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choose between using L<Gtk2>, L<Tk>, L<Event> or no event loop at |
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all: AnyEvent comes with its own event loop implementation, so your |
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code works regardless of other modules that might or might not be |
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installed. The latter is important, as AnyEvent does not have any |
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dependencies to other modules, which makes it easy to install, for |
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example, when you lack a C compiler. |
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A typical problem with Perl modules such as L<Net::IRC> is that they |
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come with their own event loop: In L<Net::IRC>, the program who uses it |
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needs to start the event loop of L<Net::IRC>. That means that one cannot |
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integrate this module into a L<Gtk2> GUI for instance, as that module, |
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too, enforces the use of its own event loop (namely L<Glib>). |
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1.1 |
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Another example is L<LWP>: it provides no event interface at all. It's a |
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pure blocking HTTP (and FTP etc.) client library, which usually means that |
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you either have to start a thread or have to fork for a HTTP request, or |
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use L<Coro::LWP>, if you want to do something else while waiting for the |
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request to finish. |
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The motivation behind these designs is often that a module doesn't want to |
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depend on some complicated XS-module (Net::IRC), or that it doesn't want |
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to force the user to use some specific event loop at all (LWP). |
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L<AnyEvent> solves this dilemma, by B<not> forcing module authors to either |
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=over 4 |
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=item write their own event loop (because guarantees to offer one |
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everywhere - even on windows). |
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1.1 |
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=item choose one fixed event loop (because AnyEvent works with all |
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important event loops available for Perl, and adding others is trivial). |
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1.1 |
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=back |
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If the module author uses L<AnyEvent> for all his event needs (IO events, |
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timers, signals, ...) then all other modules can just use his module and |
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don't have to choose an event loop or adapt to his event loop. The choice |
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of the event loop is ultimately made by the program author who uses all |
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the modules and writes the main program. And even there he doesn't have to |
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choose, he can just let L<AnyEvent> choose the best available event loop |
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for him. |
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Read more about this in the main documentation of the L<AnyEvent> module. |
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1.2 |
=head1 Introduction to Event-Based Programming |
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So what exactly is programming using events? It quite simply means that |
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instead of your code actively waiting for something, such as the user |
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entering something on STDIN: |
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$| = 1; print "enter your name> "; |
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my $name = <STDIN>; |
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You instead tell your event framework to notify you in the event of some |
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data being available on STDIN, by using a callback mechanism: |
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use AnyEvent; |
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$| = 1; print "enter your name> "; |
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my $name; |
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my $wait_for_input = AnyEvent->io ( |
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fh => \*STDIN, # which file handle to check |
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poll => "r", # which event to wait for ("r"ead data) |
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cb => sub { # what callback to execute |
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$name = <STDIN>; # read it |
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} |
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); |
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# do something else here |
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Looks more complicated, and surely is, but the advantage of using events |
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is that your program can do something else instead of waiting for |
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input. Waiting as in the first example is also called "blocking" because |
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you "block" your process from executing anything else while you do so. |
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The second example avoids blocking, by only registering interest in a read |
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event, which is fast and doesn't block your process. Only when read data |
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is available will the callback be called, which can then proceed to read |
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the data. |
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The "interest" is represented by an object returned by C<< AnyEvent->io |
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>> called a "watcher" object - called like that because it "watches" your |
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file handle (or other event sources) for the event you are interested in. |
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In the example above, we create an I/O watcher by calling the C<< |
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AnyEvent->io >> method. Disinterest in some event is simply expressed by |
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forgetting about the watcher, for example, by C<undef>'ing the variable it |
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is stored in. AnyEvent will automatically clean up the watcher if it is no |
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longer used, much like Perl closes your file handles if you no longer use |
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them anywhere. |
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=head2 Condition Variables |
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However, the above is not a fullly working program, and will not work |
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as-is. The reason is that your callback will not be invoked out of the |
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blue, you have to run the event loop. Also, event-based programs sometimes |
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have to block, too, as when there simply is nothing else to do and |
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everything waits for some events, it needs to block the process as well. |
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In AnyEvent, this is done using condition variables. Condition variables |
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are named "condition variables" because they represent a condition that is |
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initially false and needs to be fulfilled. |
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You can also call them mergepoints, syncpoints, rendezvous ports or even |
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callbacks and many other things (and they are often called like this in |
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other frameworks). The important point is that you can create them freely |
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and later wait for them to become true. |
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Condition variables have two sides - one side is the "producer" of the |
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condition (whatever code detects the condition), the other side is the |
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"consumer" (the code that waits for that condition). |
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In our example in the previous section, the producer is the event callback |
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and there is no consumer yet - let's change that now: |
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use AnyEvent; |
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$| = 1; print "enter your name> "; |
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my $name; |
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my $name_ready = AnyEvent->condvar; |
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my $wait_for_input = AnyEvent->io ( |
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fh => \*STDIN, |
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poll => "r", |
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cb => sub { |
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$name = <STDIN>; |
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$name_ready->send; |
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} |
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); |
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# do something else here |
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# now wait until the name is available: |
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$name_ready->recv; |
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undef $wait_for_input; # watche rno longer needed |
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print "your name is $name\n"; |
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This program creates an AnyEvent condvar by calling the C<< |
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AnyEvent->condvar >> method. It then creates a watcher as usual, but |
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inside the callback it C<send>'s the C<$name_ready> condition variable, |
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which causes anybody waiting on it to continue. |
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The "anybody" in this case is the code that follows, which calls C<< |
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$name_ready->recv >>: The producer calls C<send>, the consumer calls |
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C<recv>. |
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If there is no C<$name> available yet, then the call to C<< |
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$name_ready->recv >> will halt your program until the condition becomes |
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true. |
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As the names C<send> and C<recv> imply, you can actually send and receive |
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data using this, for example, the above code could also be written like |
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this, without an extra variable to store the name in: |
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use AnyEvent; |
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$| = 1; print "enter your name> "; |
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my $name_ready = AnyEvent->condvar; |
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my $wait_for_input = AnyEvent->io ( |
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fh => \*STDIN, poll => "r", |
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cb => sub { $name_ready->send (scalar = <STDIN>) } |
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); |
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# do something else here |
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# now wait and fetch the name |
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my $name = $name_ready->recv; |
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undef $wait_for_input; # watche rno longer needed |
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print "your name is $name\n"; |
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You can pass any number of arguments to C<send>, and everybody call to |
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C<recv> will return them. |
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=head2 The "main loop" |
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Most event-based frameworks have something called a "main loop" or "event |
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loop run function" or something similar. |
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Just like in C<recv> AnyEvent, these functions need to be called |
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eventually so that your event loop has a chance of actually looking for |
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those events you are interested in. |
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For example, in a L<Gtk2> program, the above example could also be written |
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like this: |
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use Gtk2 -init; |
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use AnyEvent; |
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############################################ |
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# create a window and some label |
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my $window = new Gtk2::Window "toplevel"; |
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$window->add (my $label = new Gtk2::Label "soon replaced by name"); |
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$window->show_all; |
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############################################ |
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# do our AnyEvent stuff |
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$| = 1; print "enter your name> "; |
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my $name_ready = AnyEvent->condvar; |
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my $wait_for_input = AnyEvent->io ( |
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fh => \*STDIN, poll => "r", |
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cb => sub { |
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# set the label |
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$label->set_text (scalar <STDIN>); |
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print "enter another name> "; |
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} |
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); |
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############################################ |
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# Now enter Gtk2's event loop |
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main Gtk2; |
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No condition variable anywhere in sight - instead, we just read a line |
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from STDIN and replace the text in the label. In fact, since nobody |
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C<undef>'s C<$wait_for_input> you can enter multiple lines. |
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Instead of waiting for a condition variable, the program enters the Gtk2 |
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main loop by calling C<< Gtk2->main >>, which will block the program and |
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wait for events to arrive. |
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This also shows that AnyEvent is quite flexible - you didn't have anything |
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to do to make the AnyEvent watcher use Gtk2 (actually Glib) - it just |
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worked. |
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Admittedly, the example is a bit silly - who would want to read names |
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form standard input in a Gtk+ application. But imagine that instead of |
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doing that, you would make a HTTP request in the background and display |
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it's results. In fact, with event-based programming you can make many |
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http-requests in parallel in your program and still provide feedback to |
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the user and stay interactive. |
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In the next part you will see how to do just that - by implementing an |
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HTTP request, on our own, with the utility modules AnyEvent comes with. |
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Before that, however, lets briefly look at how you would write your |
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program with using only AnyEvent, without ever calling some other event |
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loop's run function. |
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In the example using condition variables, we used that, and in fact, this |
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is the solution: |
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my $quit_program = AnyEvent->condvar; |
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# create AnyEvent watchers (or not) here |
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$quit_program->recv; |
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If any of your watcher callbacks decide to quit, they can simply call |
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C<< $quit_program->send >>. Of course, they could also decide not to and |
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simply call C<exit> instead, or they could decide not to quit, ever (e.g. |
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in a long-running daemon program). |
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In that case, you can simply use: |
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AnyEvent->condvar->recv; |
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And this is, in fact, closest to the idea of a main loop run function that |
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AnyEvent offers. |
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=head2 Timers and other event sources |
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So far, we have only used I/O watchers. These are useful mainly to find |
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out wether a Socket has data to read, or space to write more data. On sane |
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operating systems this also works for console windows/terminals (typically |
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on standard input), serial lines, all sorts of other devices, basically |
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almost everything that has a file descriptor but isn't a file itself. (As |
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usual, "sane" excludes windows - on that platform you would need different |
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functions for all of these, complicating code immesely - think "socket |
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only" on windows). |
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However, I/O is not everything - the secondmost important event source is |
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the clock. For example when doing an HTTP request you might want to time |
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out when the server doesn't answre within some predefined amount of time. |
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In AnyEvent, timer event watchers are created by calling the C<< |
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AnyEvent->timer >> method: |
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use AnyEvent; |
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my $cv = AnyEvent->condvar; |
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my $wait_one_and_a_half_seconds = AnyEvent->timer ( |
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after => 1.5, # after how many seconds to invoke the cb? |
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cb => sub { # the callback to invoke |
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$cv->send; |
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}, |
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); |
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# can do somehting else here |
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# now wait till our time has come |
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$cv->recv; |
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Unlike I/O watchers, timers are only interested in the amount of seconds |
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they have to wait. When that amount of time has passed, AnyEvent will |
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invoke your callback. |
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Unlike I/O watchers, which will call your callback as many times as there |
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is data available, timers are one-shot: after they have "fired" once and |
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invoked your callback, they are dead and no longer do anything. |
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To get a repeating timer, such as a timer firing roughly once per second, |
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you have to recreate it: |
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use AnyEvent; |
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my $time_watcher; |
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sub once_per_second { |
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print "tick\n"; |
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# (re-)create the watcher |
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$time_watcher = AnyEvent->timer ( |
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after => 1, |
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cb => \&once_per_second, |
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); |
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} |
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# now start the timer |
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once_per_second; |
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Having to recreate your timer is a restriction put on AnyEvent that is |
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present in most event libraries it uses. It is so annoying that some |
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future version might worka round this limitation, but right now, it's the |
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only way to do repeating timers. |
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Fortunately most timers aren't really repeating but specify timeouts of |
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some sort. |
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=head3 More esoteric sources |
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AnyEvent also has some other, more esoteric event sources you can tap |
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into: signal and child watchers. |
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Signal watchers can be used to wait for "signal events", which simply |
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means your process got send a signal (Such as C<SIGTERM> or C<SIGUSR1>). |
384 |
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|
385 |
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Process watchers wait for a child process to exit. They are useful when |
386 |
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you fork a separate process and ened to know when it exits, but you do not |
387 |
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wait for that by blocking. |
388 |
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|
389 |
|
|
Both watcher types are described in detail in the main L<AnyEvent> manual |
390 |
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page. |
391 |
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392 |
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|
393 |
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=head1 Network programming and AnyEvent |
394 |
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|
395 |
|
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AnyEvent is not just a simple abstraction anymore. While the core |
396 |
root |
1.1 |
L<AnyEvent> module is still small and self-contained, the distribution |
397 |
|
|
comes with some very useful utility modules such as L<AnyEvent::Handle>, |
398 |
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L<AnyEvent::DNS> and L<AnyEvent::Socket>. These can make your life as |
399 |
|
|
non-blocking network programmer a lot easier. |
400 |
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|
401 |
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Here is an introduction into these three submodules: |
402 |
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|
403 |
root |
1.2 |
=head2 L<AnyEvent::Handle> |
404 |
root |
1.1 |
|
405 |
|
|
This module handles non-blocking IO on file handles in an event based |
406 |
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manner. It provides a wrapper object around your file handle that provides |
407 |
|
|
queueing and buffering of incoming and outgoing data for you. |
408 |
|
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|
409 |
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|
More about this later. |
410 |
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|
411 |
root |
1.2 |
=head2 L<AnyEvent::Socket> |
412 |
root |
1.1 |
|
413 |
|
|
This module provides you with functions that handle socket creation |
414 |
|
|
and IP address magic. The two main functions are C<tcp_connect> and |
415 |
|
|
C<tcp_server>. The former will connect a (streaming) socket to an internet |
416 |
|
|
host for you and the later will make a server socket for you, to accept |
417 |
|
|
connections. |
418 |
|
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|
419 |
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This module also comes with transparent IPv6 support, this means: If you |
420 |
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|
write your programs with this module, you will be IPv6 ready without doing |
421 |
|
|
anything further. |
422 |
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|
423 |
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It also works around a lot of portability quirks (especially on the |
424 |
|
|
windows platform), which makes it even easier to write your programs in a |
425 |
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|
portable way. |
426 |
|
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|
427 |
root |
1.2 |
=head2 L<AnyEvent::DNS> |
428 |
root |
1.1 |
|
429 |
|
|
This module allows fully asynchronous DNS resolution. It is used mainly |
430 |
|
|
by L<AnyEvent::Socket> to resolve hostnames and service ports, but is a |
431 |
|
|
great way to do other DNS resolution tasks, such as reverse lookups of IP |
432 |
|
|
addresses for log files. |
433 |
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|
434 |
|
|
=head2 First experiments with AnyEvent::Handle |
435 |
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|
436 |
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Now let's start with something simple: a program that reads from standard |
437 |
|
|
input in a non-blocking way, that is, in a way that lets your program do |
438 |
|
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other things while it is waiting for input. |
439 |
|
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|
440 |
|
|
First, the full program listing: |
441 |
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|
442 |
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|
#!/usr/bin/perl |
443 |
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|
444 |
|
|
use AnyEvent; |
445 |
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use AnyEvent::Handle; |
446 |
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|
447 |
|
|
my $end_prog = AnyEvent->condvar; |
448 |
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|
449 |
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my $handle = |
450 |
|
|
AnyEvent::Handle->new ( |
451 |
|
|
fh => \*STDIN, |
452 |
|
|
on_eof => sub { |
453 |
|
|
print "received EOF, exiting...\n"; |
454 |
|
|
$end_prog->broadcast; |
455 |
|
|
}, |
456 |
|
|
on_error => sub { |
457 |
|
|
print "error while reading from STDIN: $!\n"; |
458 |
|
|
$end_prog->broadcast; |
459 |
|
|
} |
460 |
|
|
); |
461 |
|
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|
462 |
|
|
$handle->push_read (sub { |
463 |
|
|
my ($handle) = @_; |
464 |
|
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|
465 |
|
|
if ($handle->rbuf =~ s/^.*?\bend\b.*$//s) { |
466 |
|
|
print "got 'end', existing...\n"; |
467 |
|
|
$end_prog->broadcast; |
468 |
|
|
return 1 |
469 |
|
|
} |
470 |
|
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|
471 |
|
|
0 |
472 |
|
|
}); |
473 |
|
|
|
474 |
|
|
$end_prog->recv; |
475 |
|
|
|
476 |
|
|
That's a mouthful, so lets go through it step by step: |
477 |
|
|
|
478 |
|
|
#!/usr/bin/perl |
479 |
|
|
|
480 |
|
|
use AnyEvent; |
481 |
|
|
use AnyEvent::Handle; |
482 |
|
|
|
483 |
|
|
Nothing unexpected here, just load AnyEvent for the event functionality |
484 |
|
|
and AnyEvent::Handle for your file handling needs. |
485 |
|
|
|
486 |
|
|
my $end_prog = AnyEvent->condvar; |
487 |
|
|
|
488 |
|
|
Here the program creates a so-called 'condition variable': Condition |
489 |
|
|
variables are a great way to signal the completion of some event, or to |
490 |
|
|
state that some condition became true (thus the name). |
491 |
|
|
|
492 |
|
|
This condition variable represents the condition that the program wants to |
493 |
|
|
terminate. Later in the progra, we will 'recv' that condition (call the |
494 |
|
|
C<recv> method on it), which will wait until the condition gets signalled |
495 |
|
|
(which is done by calling the C<send> method on it). |
496 |
|
|
|
497 |
|
|
The next step is to create the handle object: |
498 |
|
|
|
499 |
|
|
my $handle = |
500 |
|
|
AnyEvent::Handle->new ( |
501 |
|
|
fh => \*STDIN, |
502 |
|
|
on_eof => sub { |
503 |
|
|
print "received EOF, exiting...\n"; |
504 |
|
|
$end_prog->broadcast; |
505 |
|
|
}, |
506 |
|
|
|
507 |
|
|
This handle object will read from standard input. Setting the C<on_eof> |
508 |
|
|
callback should be done for every file handle, as that is a condition that |
509 |
|
|
we always need to check for when working with file handles, to prevent |
510 |
|
|
reading or writing to a closed file handle, or getting stuck indefinitely |
511 |
|
|
in case of an error. |
512 |
|
|
|
513 |
|
|
Speaking of errors: |
514 |
|
|
|
515 |
|
|
on_error => sub { |
516 |
|
|
print "error while reading from STDIN: $!\n"; |
517 |
|
|
$end_prog->broadcast; |
518 |
|
|
} |
519 |
|
|
); |
520 |
|
|
|
521 |
|
|
The C<on_error> callback is also not required, but we set it here in case |
522 |
|
|
any error happens when we read from the file handle. It is usually a good |
523 |
|
|
idea to set this callback and at least print some diagnostic message: Even |
524 |
|
|
in our small example an error can happen. More on this later... |
525 |
|
|
|
526 |
|
|
$handle->push_read (sub { |
527 |
|
|
|
528 |
|
|
Next we push a general read callback on the read queue, which |
529 |
|
|
will wait until we have received all the data we wanted to |
530 |
|
|
receive. L<AnyEvent::Handle> has two queues per file handle, a read and a |
531 |
|
|
write queue. The write queue queues pending data that waits to be written |
532 |
|
|
to the file handle. And the read queue queues reading callbacks. For more |
533 |
|
|
details see the documentation L<AnyEvent::Handle> about the READ QUEUE and |
534 |
|
|
WRITE QUEUE. |
535 |
|
|
|
536 |
|
|
my ($handle) = @_; |
537 |
|
|
|
538 |
|
|
if ($handle->rbuf =~ s/^.*?\bend\b.*$//s) { |
539 |
|
|
print "got 'end', existing...\n"; |
540 |
|
|
$end_prog->broadcast; |
541 |
|
|
return 1 |
542 |
|
|
} |
543 |
|
|
|
544 |
|
|
0 |
545 |
|
|
}); |
546 |
|
|
|
547 |
|
|
The actual callback waits until the word 'end' has been seen in the data |
548 |
|
|
received on standard input. Once we encounter the stop word 'end' we |
549 |
|
|
remove everything from the read buffer and call the condition variable |
550 |
|
|
we setup earlier, that signals our 'end of program' condition. And the |
551 |
|
|
callback returns with a true value, that signals we are done with reading |
552 |
|
|
all the data we were interested in (all data until the word 'end' has been |
553 |
|
|
seen). |
554 |
|
|
|
555 |
|
|
In all other cases, when the stop word has not been seen yet, we just |
556 |
|
|
return a false value, to indicate that we are not finished yet. |
557 |
|
|
|
558 |
|
|
The C<rbuf> method returns our read buffer, that we can directly modify as |
559 |
|
|
lvalue. Alternatively we also could have written: |
560 |
|
|
|
561 |
|
|
if ($handle->{rbuf} =~ s/^.*?\bend\b.*$//s) { |
562 |
|
|
|
563 |
|
|
The last line will wait for the condition that our program wants to exit: |
564 |
|
|
|
565 |
|
|
$end_prog->recv; |
566 |
|
|
|
567 |
|
|
The call to C<recv> will setup an event loop for us and wait for IO, timer |
568 |
|
|
or signal events and will handle them until the condition gets sent (by |
569 |
|
|
calling its C<send> method). |
570 |
|
|
|
571 |
|
|
The key points to learn from this example are: |
572 |
|
|
|
573 |
|
|
=over 4 |
574 |
|
|
|
575 |
|
|
=item * Condition variables are used to start an event loop. |
576 |
|
|
|
577 |
|
|
=item * How to registering some basic callbacks on AnyEvent::Handle's. |
578 |
|
|
|
579 |
|
|
=item * How to process data in the read buffer. |
580 |
|
|
|
581 |
|
|
=back |
582 |
|
|
|