NAME

       UR::Observer - bind callbacks to object changes

SYNOPSIS

           $rocket = Acme::Rocket->create(
               fuel_level => 100
           );

           $observer = $rocket->add_observer(
               aspect => 'fuel_level',
               callback =>
                   sub {
                       print "fuel level is: " . shift->fuel_level . "\n"
                   },
               priority => 2,
           );

           $observer2 = UR::Observer->create(
               subject_class_name => 'Acme::Rocket',
               subject_id    => $rocket->id,
               aspect => 'fuel_level',
               callback =>
                   sub {
                       my($self,$changed_aspect,$old_value,$new_value) = @_;
                       if ($new_value == 0) {
                           print "Bail out!\n";
                       }
                   },
               priority => 0
           );

           for (3 .. 0) {
               $rocket->fuel_level($_);
           }
           # fuel level is: 3
           # fuel level is: 2
           # fuel level is: 1
           # Bail out!
           # fuel level is: 0

           $observer->delete;

DESCRIPTION

       UR::Observer implements the observer pattern for UR objects.  These observers can be
       attached to individual object instances, or to whole classes.  They can send notifications
       for changes to object attributes, or to other state changes such as when an object is
       loaded from its datasource or deleted.

CONSTRUCTOR

       Observers can be created either by using the method "add_observer()" on another class, or
       by calling "create()" on the UR::Observer class.

         my $o1 = Some::Other::Class->add_observer(...);
         my $o2 = $object_instance->add_observer(...);
         my $o3 = UR::Observer->create(...);

       The constructor accepts these parameters:

       subject_class_name
         The name of the class the observer is watching.  If this observer is being created via
         "add_observer()", then it figures out the subject_class_name from the class or object it
         is being called on.

       subject_id
         The ID of the object the observer is watching.  If this observer is being created via
         "add_observer()", then it figures out the subject_id from the object it was called on.
         If "add_observer()" was called as a class method, then subject_id is omitted, and means
         that the observer should fire for changes on any instance of the class or sub-class.

       priority
         A numeric value used to determine the order the callbacks are fired.  Lower numbers are
         higher priority, and are run before callbacks with a numerically higher priority.  The
         default priority is 1.  Negative numbers are ok.

       aspect
         The attribute the observer is watching for changes on.  The aspect is commonly one of
         the properties of the class.  In this case, the callback is fired after the property's
         value changes.  aspect can be omitted, which means the observer should fire for any
         change in the object state.  If both subject_id and aspect are omitted, then the
         observer will fire for any change to any instance of the class.

         There are other, system-level aspects that can be watched for that correspond to other
         types of state change:

         create
           After a new object instance is created

         delete
           After an n object instance is deleted

         load
           After an object instance is loaded from its data source

         commit
           After an object instance has changes saved to its data source

       callback
         A coderef that is called after the observer's event happens.  The coderef is passed four
         parameters: $self, $aspect, $old_value, $new_value.  In this case, $self is the object
         that is changing, not the UR::Observer instance (unless, of course, you have created an
         observer on UR::Observer).  The return value of the callback is ignored.

       once
         If the 'once' attribute is true, the observer is deleted immediately after the callback
         is run.  This has the effect of running the callback only once, no matter how many times
         the observer condition is triggered.

       note
         A text string that is ignored by the system

   Custom aspects
       You can create an observer for an aspect that is neither a property nor one of the system
       aspects by listing the aspect names in the metadata for the class.

           class My::Class {
               has => [ 'prop_a', 'another_prop' ],
               valid_signals => ['custom', 'pow' ],
           };

           my $o = My::Class->add_observer(
                       aspect => 'pow',
                       callback => sub { print "POW!\n" },
                   );
           My::Class->__signal_observers__('pow');  # POW!

           my $obj = My::Class->create(prop_a => 1);
           $obj->__signal_observers__('custom');  # not an error

       To help catch typos, creating an observer for a non-standard aspect throws an exception
       unless the named aspect is in the list of 'valid_signals' in the class metadata.  Nothing
       in the system will trigger these observers, but they can be triggered in your own code
       using the "__signal_observers()__" class or object method.  Sending a signal for an aspect
       that no observers are watching for is not an error.