This gives several examples of using Symfony decorators in Drupal 8.
See the Phase2 blog for details.
To play with this, first enable the test_decorator_core module, which
provides the core service being overridden.
Then, enable different combination of the other decorator modules and run
the drush php-eval "test_decorator_core()" command to see which order the
different services run.
For example, enable test_decorator_a and run drush php-eval "test_decorator_core()" and you should see:
Decorator A Public
Core Protected
Core Public
which shows that the test_decorator_a ran first followed by the normal core service.
A simple decorator that extends the core service.
Implements the core service interface rather than subclassing.
A subclass decorator with a lower priority than decorator_a.
A subclass decorator with a higher priorty than decorator_a.
Yet another subclass decorator with the same priority as decorator_a.
A subclass decorator that overrides a protected method.