Mapper

An object that sets up a communication between two independent objects.

• Overview
• How It Works
• When to Use It

Overview

Sometimes you need to set up communications between two subsystems that still need to stay ignorant of each other. This may be because you can’t modify them or you can but you don’t want to create dependencies between them.

How It Works

• Mapper controls the details of the communication between them without either subsystem being aware of it.

• The complicated part is deciding how to invoke it, since it can’t be directly invoked by either of the subsystems that it’s mapping between. Something a third subsystem derives the mapping and invokes the mapper as well. An alternative is to make the mapper an Observer (Gang of Four) of one or the other subsystem.

• How a mapper works depend on the kind of layer it’s mapping. The most common case is a Data Mapper.

When to Use It

Essentially a Mapper decoupels different parts of a system. When you want to do this you have a choie between Mapper and Gateway. Gateway is much simpler to use.

You should only use a Mapper when you need to ensure that neither subsystem has a dependency on this interaction. The only time this is really important is when the interaction between the subsystems is particularly complicated and somewhat independent to the main purpose of both subsystems.

Thus, in enterprise applications we mostly find Mapper used for interactions with a database, as in Data Mapper.

Mapper is similar to Mediator (Gang of Four) in that it’s used to separate different elements. However, the objects that use a Mediator are aware of it.