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Design pattern: Proxy

Description: "Provide a surrogate or placeholder for another object to control access to it."

This pattern can be used with both classes and prototypes.

Proxy objects are simple to implement. We can have the proxy object implementing a protocol, shared with the real object, that specifies only the predicates that the proxy object redefines. This allows the using the forward/1 handler to delegate all other messages to the real object. When the proxy object is expected to implement the full protocol of the real object, an alternative solution is for the proxy object to inherit privately from the real object and overriding (or specializing) only those predicates that motivated the use of a proxy.

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):

%versions

Start by loading the design pattern sample implementation:

logtalk_load(design_patterns('structural/proxy/loader')).

First, lets create a old enough driver, no other than Mr. Magoo itself (pedestrians be aware!):

driver::new(magoo, 'Mr. Magoo', 87).

<!-- true. -->

In some of the cartoons, Mr. Magoo appears to be driving a 1929 Packard Model 645 touring car; we can define the car as a static object in the source file or create it dynamically:

create_object(packard_645, [extends(car)], [], []).

<!-- true. -->

Then we check, using the proxy object, that the driver can actually drive the car:

car_proxy(packard_645, magoo)::drive.

<!-- Car has been driven!

true. -->

Now, lets create a driver that is just too young, one of the children that Mr. Magoo sometimes babysits:

driver::new(wheeler, 'Wheeler', 7).

<!-- true. -->

Then we check, using the proxy object, that the child is effectively not allowed to drive the car:

car_proxy(packard_645, wheeler)::drive.

<!-- Sorry, Wheeler is too young to drive!

true. -->