1. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 1 Mid-Level Design Patterns Categories

2. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2 Objectives  To propose a classification scheme to help remember patterns  To present the three categories in the scheme

3. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 3 Topics  The need for classification  Pattern categories

4. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 4 The Need for Classification  Since their introduction in the mid-1990s, hundreds of mid-level design patterns have been published.  How can designers keep them all in mind? Many are not that important or have narrow application. A pattern classification scheme can help designers remember many important patterns.

5. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 5 Pattern Categories  Broker patterns have a client that needs a service from a supplier, and a broker that mediates the interaction between client and supplier.  Generator patterns have a client who needs a new instance of a product, and a generator class that supplies the instance.  Reactor patterns have a client that needs to respond to an event in a target. The client delegates this responsibility to a reactor.

6. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 6 Category Analogies  Brokers are like stock brokers who mediate interactions between an investor (client) and the stock market (supplier).  Generators are like interior designers who obtain material from manufacturers (products) on behalf of their clients.  Reactors are like lawn service companies that respond to conditions in a lawn (target) on behalf of a homeowner (client).

7. © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 7 Summary  There are hundreds of design patterns and they are hard to remember.  One way to help remember them is to classify them.  Our pattern collection is classified into three categories: Broker patterns Generator patterns Reactor patterns