Periodic watcher operation modes

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- Periodic watcher operation modes

# Periodic watcher operation modes

EvPeriodic watcher works in different modes depending on the `offset` , `interval` and `reschedule_cb` parameters.

1. Absolute timer . In this mode `interval` = `0` , `reschedule_cb` = `null`. This time simply fires at the wallclock time `offset` and doesn't repeat. It will not adjust when a time jump occurs, that is, if it is to be run at January 1st 2014 then it will run when the system time reaches or surpasses this time.

2. Repeating interval timer . In this mode `interval` > `0` , `reschedule_cb` = `null`; the watcher will always be scheduled to timeout at the next `offset` + `N` * `interval` time(for some integer `N` ) and then repeat, regardless of any time jumps.

   This can be used to create timers that do not drift with respect to system time:

       <?php
       $hourly = EvPeriodic(0, 3600, NULL, function () {
         echo "once per hour\n";
       });
       ?>

   That doesn't mean there will always be `3600` seconds in between triggers, but only that the callback will be called when the system time shows a full hour( UTC ).

   EvPeriodic will try to run the callback in this mode at the next possible time where time = `offset` ( `mod` `interval` ), regardless of any time jumps.

3. Manual reschedule mode . In this mode `reschedule_cb` is a callable .

   `interval` and `offset` are both being ignored. Instead, each time the periodic watcher gets scheduled, the reschedule callback ( `reschedule_cb` ) will be called with the watcher as first, and the current time as second argument.

   This callback must not stop or destroy this or any other periodic watchers, ever, and must not call any event loop functions or methods. To stop it return `1e30` and stop it afterwards. An EvPrepare watcher may be used for this task.

   It must return the next time to trigger, based on the passed time value (that is, the lowest time value larger than or equal to the second argument). It will usually be called just before the callback will be triggered, but might be called at other times, too.

   Example #1 Using reschedule callback

       <?php
       // Tick each 10.5 seconds

       function reschedule_cb ($watcher, $now) {
          return $now + (10.5. - fmod($now, 10.5));
       }

       $w = new EvPeriodic(0., 0., "reschedule_cb", function ($w, $revents) {
          echo time(), PHP_EOL;
       });

       Ev::run();
       ?>