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Outlet Connections

Part of the task of designing an object-oriented program is to arrange the object network. The network doesn’t have to be static; it can change dynamically as the program runs. Relationships between objects can be improvised as needed, and the cast of objects that play assigned roles can change from time to time. But there has to be a script.

Some connections can be entirely transitory. A message might contain an argument identifying an object, perhaps the sender of the message, that the receiver can communicate with. As it responds to the message, the receiver can send messages to that object, perhaps identifying itself or still another object that the object can in turn communicate with. Such connections are fleeting; they last only as long as the chain of messages.

But not all connections between objects can be handled on the fly. Some need to be recorded in program data structures. There are various ways to do this. A table might be kept of object connections, or there might be a service that identifies objects by name. However, the simplest way is for each object to have instance variables that keep track of the other objects it must communicate with. These instance variables—termed outlets because they record the outlets for messages—define the principal connections between objects in the program network.

Although the names of outlet instance variables are arbitrary, they generally reflect the roles that outlet objects play. Figure 4-1 illustrates an object with four outlets—an “agent,” a “friend,” a “neighbor,” and a “boss.” The objects that play these parts may change every now and then, but the roles remain the same.

Some outlets are set when the object is first initialized and may never change. Others might be set automatically as the consequence of other actions. Still others can be set freely, using methods provided just for that purpose.

However they’re set, outlet instance variables reveal the structure of the application. They link objects into a communicating network, much as the components of a water system are linked by their physical connections or as individuals are linked by their patterns of social relations.

In this section:

Extrinsic and Intrinsic Connections
Activating the Object Network

Extrinsic and Intrinsic Connections

Outlet connections can capture many different kinds of relationships between objects. Sometimes the connection is between objects that communicate more or less as equal partners in an application, each with its own role to play and neither dominating the other. For example, an Appliance object might have an outlet instance variable to keep track of the Valve it’s connected to.

Sometimes one object should be seen as being part of another. For example, a Faucet might use a Meter object to measure the amount of water being released. The Meter would serve no other object and would act only under orders from the Faucet. It would be an intrinsic part of the Faucet, in contrast to an Appliance’s extrinsic connection to a Valve.

Similarly, an object that oversees other objects might keep a list of its charges. A Building object, for example, might have a list of all the Pipes in the program. The Pipes would be considered an intrinsic part of the Building and belong to it. Pipes, on the other hand, would maintain extrinsic connections to each other.

Intrinsic outlets behave differently from extrinsic ones. When an object is freed or archived in a file on disk, the objects that its intrinsic outlets point to must be freed or archived with it. For example, when a Faucet is freed, its Meter is rendered useless and therefore should be freed as well. A Faucet archived without its Meter would be of little use when it’s unarchived (unless it could create a new Meter for itself).

Extrinsic outlets , on the other hand, capture the organization of the program at a higher level. They record connections between relatively independent program subcomponents. When an Appliance is freed, the Valve it was connected to still is of use and remains in place. When an Appliance is unarchived, it can be connected to another Valve and resume playing the same sort of role it played before.

Activating the Object Network

The object network is set into motion by an external stimulus. If you’re writing an interactive application with a user interface, it will respond to user actions on the keyboard and mouse. A program that tries to factor very large numbers might start when you pass it a target number on the command line. Other programs might respond to data received over a phone line, information obtained from a database, or information about the state of a mechanical process the program monitors.

Programs often are activated by a flow of events , reports of external activity of some sort. Applications that display a user interface are driven by events from the keyboard and mouse. Every press of a key or click of the mouse generates events that the application receives and responds to. An object-oriented program structure (a network of objects that’s prepared to respond to an external stimulus) is ideally suited for this kind of user-driven application.

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