![]() ![]() Here we'll look at three simple ways Bridge patterns can be built in FoxPro: First with member properties, second we'll do more flexible implementations using member arrays, and third using object composition within containers. Those descriptive names are good - the dual-object nature of Bridge patterns is well conveyed, as is the tight coupling usually found between the programming interface and implementation objects. A.K.A."Handle And Body" and "Envelope And Letter". The Bridge in all this is the relationship between the interface and the implementation objects that together form a self-supporting system. This allows the interface and implementation classes to vary independently, and the inherent substitutability of subclasses makes the structure intrinsically adaptable (and hence reusable). The Bridge pattern mitigates class sclerosis by decoupling interfaces from implementations, and putting the abstraction and implementation in separate class hierarchies. This will occasionally limit what can be reasonably done at a given level in a class hierarchy. In a giant class hierarchy, making a small change near the top can potentially be very expensive. The probability of change side-effects in clients increases with the number and variety of those clients, and correspondingly with the current state of the hierarchy. ![]() This means that interface and implementation aspects cannot be independently varied without extending the class hierarchy.īut class hierarchies naturally loses their inherent adaptability with size and as the number and variety of clients increases. Simple, single-object inheritance binds interface and implementation - you only have one package to transform into a subclass. ![]() And why not? In the early going, inheritance is always a big success.Īt the limit, however, inheritance leads to sclerosis - inheritance doesn't scale particularly well. After all, inheritance is usually the first mechanism most folks use to reuse their classes. The usual way to manage the requirement for different interface or implementation is by extending the class hierarchy using inheritance. A Bridge also serves as an atomic element player in other design patterns. Bridges are scale-independent structures that apply in many object oriented situations. Resulting ContextĪ Bridge is a decoupling of an abstraction (interface) from its implementation so the two can vary independently. The tight coupling is much easier to manage when all the implementation objects come from the same class, as illustrated in the Codebook example illustrated below. Tight coupling within the Bridge structure is both common and desirable. The workings among the players in a Bridge should be direct and simple: the interface object forwards client requests to the appropriate implementation object. This way each can be varied independently of the other. Separate a class's interface from its implementation by making each a separate but tightly coupled objects. We may need to adapt a class's interface, implementation, or both.ĭecouple an object's public interface ("form") from its implementation (its "function"), using two (or more) objects where otherwise one might less flexibly suffice.Reuse could mean reuse by other systems that may expect or require a different interface than the one currently provided by our existing class.The existing codebase may constrain what we can do with a class's implementation. Vfp define class how to#How to structure a class so that either (or both) can be easily modified? Forces To achieve reuse, some classes need to adapt either their implementations, their interfaces, or both. perhaps you'd like to choose a specific behavior at run-time. You anticipate future extensions to object's form or function and you want to avoid extending a class hierarchy for the sake of new implementations. You are designing a new class, or adapting an existing class to make it more reusable. ![]()
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