Event Watcher Constructors¶

All watchers are constructed in one of the following ways:

  $w = Event->flavor( [attr1 => $value,]... );
 
  $w = Event::flavor($Class, [attr1 => $value,]...);
  $w = Event::flavor->new([attr1 => $value,]...);

Where flavor is substituted with the kind of watcher. Built-in types include idle, io, signal, timer, and var.

New watchers (hopefully) have reasonable defaults and can also be customized by passing extra attributes to the constructor. When created, watcher objects are "started" and are waiting for events (see "$event->start" below).

NetServer::Portal can display watchers in real-time, formatted similarly to the popular "top" program. You may find this a useful aide for debugging.

Shared Watcher Attributes¶

Watchers are configured with attributes (also known as properties). For example:

   $watcher->cb(\&some_code);   # set callback
   warn $event->w->desc.": ".$event->hits." events happened; Wow!";

All watchers support the following attributes: cb, cbtime, debug, desc, prio, max_cb_tm, reentrant, and repeat. Watcher constructors accept the preceding and additionally: async and nice. Moreover, watchers also offer extra attributes according to their specialty.

Shared Watcher Methods¶

The following methods are available for all watchers:

$watcher->start

Activate the watcher. Watchers refuse to start() without sufficient configuration information to generate events. Constructors always invoke start() unless the "parked=>1" option is requested. You will need to set the parked option if you preallocate unconfigured watchers.

Note: If there are any unreified asynchronous events that are of interest to the watcher, it will see these events even though they happened before it was started. This affects signal watchers, but there will only be existing unreified signal events if Event was already handling the signal, which it would only do if there were another active watcher for the same signal. If this situation might occur, and it would be a problem for the new watcher to see older events, call queue_pending() immediately before starting the new watcher in order to reify any outstanding events. This explanation may be more clear if read along with "demo/queue_pending.t".

$watcher->again

This is the same as the "start" except if a watcher has special repeat behavior. For example, repeating timers recalculate their alarm time using the "interval" parameter.

$watcher->now

Cause the watcher to generate an event, even if it is stopped. The callback may or may not run immediately depending upon the event's priority. If you must unconditionally invoke the callback, consider something like

  $w->cb->($w);
    

$watcher->stop

Don't look for events any more. Running events are allowed to complete but pending events are cancelled. Note that a stopped watcher can be reactivated by calling the "start" or "again" methods.

Watchers are stopped implicitly if their new configuration deprives them of the ability to generate events. For instance:

  my $io_watcher = Event->io(timeout => 1);  # started
  $io_watcher->timeout(undef);               # stopped implicitly
  $io_watcher->timeout(1);                   # still stopped
  $io_watcher->start;                        # restarted
    

$watcher->cancel

Stop and destroy $watcher. Running events are allowed to complete but pending events are cancelled. Cancelled watchers are no longer valid except for read-only operations. For example, prio() can return the watcher's priority, but start() will fail.

$watcher->is_cancelled

Reports whether the $watcher has been cancelled.

$watcher->is_active

Reports whether the $watcher has been started. The return value is not affected by suspend.

$watcher->is_running

Zero if the callback is not running. Otherwise, the number of levels that the callback has been entered. This can be greater than one if a "reentrant" callback invokes "loop" (or "sweep", with lesser probability).

$watcher->is_suspended

Reports whether the $watcher is suspended. Suspension is a debugging feature; see the discussion of the "suspend" attribute below.

$watcher->pending

In scalar context, returns a boolean indicating whether this watcher has any events pending in the event queue. In array context, returns a list of all the watcher's pending events.

Note that this does not check for unreified asynchronous events. Call queue_pending() first if you want to see signals received since the last operation of the event loop.

Watcher Types¶

idle

Extra attributes: min => $seconds, max => $seconds

Watches for the Event system to be idle, i.e., to have no events pending. If the system is never idle, an event will be generated at least every "max" seconds. While Event is idle, events will be generated not more often than "min" seconds.

If neither "min" nor "max" is specified, the watcher defaults to one-shot behaviour ("repeat" false), otherwise it defaults to repeating. In either case, the default can be overridden by specifying a "repeat" attribute. "min" defaults to 0.01, and "max" defaults to infinity.

var

Extra attributes: var => \$var, poll => 'rw'

Var watchers generate events when the given variable is read from or written to, as specified by "poll". "poll" defaults to "w".

As perl is a concise language, it is often difficult to predict when a variable will be read. For this reason, variable watchers should poll only for writes unless you know what you are doing.

timer

Extra attributes: at => $time, after => $sec, interval => $sec, hard => $bool

Generate events at particular times. The $time and $sec are in seconds. Fractional seconds may be used if Time::HiRes is available. "at" and "after" are mutually exclusive.

"at" or "after" specify the initial time that the event will trigger. Subsequent timer events occur at intervals specified by "interval" or "after" (in that order of preference) if either was supplied. The timer defaults to one-shot behaviour if "interval" was not specified, or repeating behaviour if "interval" was specified; in either case this can be overridden by providing "repeat" explicitly.

You need to know the time at the start of today if you are trying to set timers to trigger at day relative times. You can find it with:

  use Time::Local;
  my $TodaySeconds = int timelocal(0,0,0,(localtime)[3,4,5]);
    

This calculation may seem a little heavy weight. If you want to use UTC rather than local time then you can use this instead:

  my $TodaySeconds = time - time % 86400;
    

Beware that, due to lags in the event loop, the "interval" timeout may already be in the past. If the "hard" flag is set, the event will be queued for execution relative to the last time the callback was invoked. However, if "hard" is false the new timeout will be calculated relative to the current time. "hard" defaults to false.

io

Extra attributes: fd => $fd, poll => 'rwe' [timeout => $seconds, hard => $bool, timeout_cb => \&code]

The callback is invoked when the file descriptor, "fd", has data to be read, written, or pending exceptions. "fd" can be a GLOB, an IO::Handle object, or a file number (file descriptor). "poll" defaults to "r".

Note that it is your option whether to have multiple watchers per file handle or to use a single watcher for all event conditions.

If "timeout" is set, events are also generated regularly if no actual I/O event occurs. If "timeout_cb" is set then timeouts use this alternate callback instead of the main callback.

signal

Extra attribute: signal => $str

Generates events based on signal arrival. The events are not actually generated immediately when the signal arrives: signals received are counted and reified by queue_pending() or implicitly by one_event(). Several signals of the same type may be merged into a single event. In such cases, the number of signals represented by a single event is stored in the "hits" attribute.

PRIORITY¶

Priority is used to sort the event queue. Meaningful priorities range from -1 to 6 inclusive. Lower numbers mean higher priority (-1 is the highest priority and 6 is the lowest). If multiple events get queued, the ones with the highest priority are serviced first. Events with equal priority are serviced in first-in-first-out order.

  use Event qw(PRIO_HIGH PRIO_NORMAL);   # some constants
  LEVELS: -1      0      1      2      3      4      5      6
          ----------------------+-------------+---------------
                            PRIO_HIGH     PRIO_NORMAL

A negative priority causes the callback to be invoked immediately upon event occurrence. Use this with caution. While it may seem advantageous to use negative priorities, they bypass the whole point of having an event queue.

Each watcher has a default priority, assigned by its constructor:

  io       PRIO_NORMAL
  signal   PRIO_HIGH
  timer    PRIO_NORMAL
  var      PRIO_NORMAL

Default priorities are stored in ${"Event::${type}::DefaultPriority"}. If the default priority is not satisfactory for your purposes, the constructor options "nice", "async", or "prio" can be used to adjust it. "nice" specifies an offset from the default priority; "async" forces the priority to -1; and "prio" assigns a given priority of your choice. If more than one of these options are given then "prio" overrides "async" overrides "nice".

WATCHER CONSTRUCTOR ATTRIBUTES¶

These options are only supported as constructor arguments.

after => $seconds

See the discussion of the timer watcher.

async => $bool

If $bool then the watcher priority is set to -1.

nice => $offset

Offset from the default priority.

parked => $yes

By default, watcher constructors automatically invoke the start() method. If you don't want the watcher started then request "parked=>1".

WATCHER ATTRIBUTES¶

at => $time

The expiration time in the same units as the system clock. For a timer, "at" will usually be in the future.

cb => \&code

cb => [$class_or_object, $method_name]

The function or method to call when an event is dispatched. The callback is invoked with $event as its only argument.

Perhaps you are wondering what happens if something goes wrong and an untrapped "die" occurs within your callback? $Event::DIED is just for this purpose. See the full description of "DIED" below.

cbtime => $time

When the callback was invoked most recently.

data => $anything

The data() method associates arbitrary data with a watcher.

This method is not intended for implementers of watchers. If you are subclassing or implementing a watcher, consider the private() method.

debug => $bool

Debugging can be activated globally or per watcher. When debugging is enabled for a particular watcher, $Event::DebugLevel is treated as two levels higher. Levels of 1, 2, 3, or 4 give progressively more diagnostics on STDERR.

desc => $string

An identifying name. If this is not passed explicitly to the constructor, it will be initialized with a string that attempts to identify the location in the source code where the watcher was constructed.

fd => $filehandle

This attribute can accept either a perl-esque filehandle or a system call derived file descriptor number.

hard => $bool

Determines how repeating timers (or timeouts) are recalculated. The timer is restarted either before or after the callback depending on whether it is true or false, respectively. In long-running callbacks this can make a significant difference.

interval => $seconds

How long between repeating timeouts. The "at" attribute is recalculated using "interval" upon callback return.

max => $seconds

The maximum number of seconds to wait before triggering the callback. Similar to a "timeout".

max_cb_tm => $seconds

The maximum number of seconds to spend in a callback. If a callback uses more time then it is aborted. Defaults to 1 sec. This feature is normally disabled. See Event::Stats.

min => $seconds

Enforce a minimum number of seconds between triggering events.

poll => $bits

Determines which kinds of events are of interest. This attribute can be set with either strings or bit constants. The bit constants are available via 'use Event::Watcher qw(R W E T);'.

  string constant description
  ------ -------- ---------------
   'r'     R      read
   'w'     W      write
   'e'     E      exception
   't'     T      timeout
    

Thus, both of these statements enable interest in read:

  $w->poll($w->poll . 'r');
  $w->poll($w->poll | R);
    

A given type of watcher may support all or a subset of the available events.

prio => $level

Changes the watcher's priority to the given level. Events generated by a watcher usually inherit the priority of the watcher.

private => $anything

Use the private() method to associate arbitrary data with a watcher. This method is intended for implementers of watchers or watcher subclasses. Each caller's package accesses its own private attribute.

reentrant => $bool

By default, callbacks are allowed to invoke "sweep" or "loop" which in turn may invoke the same callback again recursively. This can be useful but can also be confusing. Moreover, if you keep reentering callbacks you will quickly run out of stack space. Disable this feature per watcher by setting reentrant to false. This will cause the watcher to be suspended during recursive calls to "sweep" or "loop".

repeat => $bool

The repeat flag controls whether the callback should either be one-shot or continue waiting for new events. The default setting depends on the type of watcher. io, signal, and var default to true.

signal => $str

The callback is invoked after the specified signal is received. The $str string should be something like 'INT' or 'QUIT'. Also see the documentation for %SIG.

A given signal can be handled by %SIG or Event, but not both at the same time. Event handles the signal as long as there is at least one active watcher. If all watchers for the signal are cancelled or stopped then Event sets the signal handler to SIG_DFL.

suspend => $bool

Stop looking for events. Running events are allowed to complete, but queued events are cancelled.

Suspend is for debugging. If you suspend all watchers in an application then you can examine the complete state unchanged for as long as you like without worrying about timer expirations. If you actually wish to stop a watcher then use the stop() method.

timeout => $seconds

The number of seconds before a watcher times out.

timeout_cb => \&code

timeout_cb => [$class_or_object, $method_name]

This is an optional attribute for use when it is desired that timeouts be serviced in a separate code path than normal events. When this attribute is unset, timeouts are serviced by "cb".

var => $ref

A reference to the variable being watched.

EVENT ATTRIBUTES¶

got => $bits

"got" is available in the callback of watchers with "poll". "got" is in the same format as "poll" except that it gives what kind of event actually happened. In contrast, "poll" is just an indication of interest.

hits => $int

Signals in quick succession can be clumped into a single event. The number of signals clumped together is indicated by this attribute. This is always one for event types which don't clump.

prio => $level

Be aware that this priority can differ from the watcher's priority. For instance, the watcher's priority may have changed since the event was generated. Moreover, the C extension API offers the freedom to queue events of arbitrary priority.

w => $watcher

This method return the event's watcher. It is read-only.

Customization and Exceptions¶

• $Event::DebugLevel

  Enables progressively more debugging output. Meaningful levels range from 1 (least output) to 5 (most output). Also see "debug".

• $Event::DIED

  When "loop" or "sweep" is called, an exception context is established for the duration of event processing. If an exception is detected then $Event::DIED is invoked. The default hook uses "warn" to output the exception. After the DIED handler completes, event processing continues as if nothing happened.

  If you'd like more detailed output you can install the verbose handler:

    $Event::DIED = \&Event::verbose_exception_handler;
      

  Or you can write your own. The handler is invoked like this:

    $Event::DIED->($event, $@);
      

  If you do not want to continue looping after an error, you can do something like this:

    $Event::DIED = sub {
      Event::verbose_exception_handler(@_);
      Event::unloop_all();
    };
      

• Event->add_hooks(key => sub { ... }, ...);

  The bulk of Event's implementation is in C for maximum performance. The "add_hooks" method allows insertion of perl code at key points in the optimized event processing core. While flexible, this can hurt performance *significantly*. If you want customization *and* performance, please see the C API.

  Currently support hooks are detailed as follows:

    hook          purpose
    ------------- ----------------------------------------------
    prepare       returns minimum time to block (timeable)
    check         assess state after normal return from select/poll
    asynccheck    check for signals, etc
    callback      invoked before each event callback