Facade

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Facade

• Eric Goodnough

Software Design Documentation
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About Facade

• Structural Pattern
• Provides a unified interface to a set of
  interfaces in a subsystem
• Defines a higher-level interface that makes the
  subsystem easier to use

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Structural Diagram
Client Classes
Facade
Subsystem Classes
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Participants

• Facade
• Knows which parts of the subsystem are
  responsible for what
• Delegates client requests to the appropriate
  objects in the subsystem
• Subsystem classes
• Implement subsystem functionality
• Handle work assigned by the facade
• Have no knowledge of the facade, since they keep
  no reference to it

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Applicability

• Use the facade structural pattern when
• You want to provide a simple interface for a
  complex subsystem
• There are many dependencies between the client
  and implementation classes
• You want to layer your subsystems

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Benefits

• Reduces the number of objects the clients have to
  deal with
• Promotes weak coupling between the subsystem and
  the clients
• Doesnt prevent applications from using subsystem
  classes

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Compiler Example
Compiler

• Compiler Subsystem
• Scanner
• Parser
• ProgramNode
• ProgramNode Builder
• CodeGenerator

Parser
Scanner
ProgramNodeBuilder
CodeGenerator
ProgramNode
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Example Code (pg. 189-191)

• Class Scanner //.
• Class Parser //.
• Class ProgramNodeBuilder //.
• Class ProgramNode //.
• Class CodeGenerator //.
• Class Compiler //facade class
• public
• Compiler()
• virtual void Compile(istream, BytecodeStream)
  

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Example Code (cont.)

• Void CompilerCompile(istream input,
  BytecodeStream output)
• Scanner scanner(input)
• ProgramNodeBuilder builder
• Parser parser
• parser.Parse(scanner, builder)
• RISCCodeGenerator generator(output)
• ProgramNode parseTree builder.GetRootNode()
• parseTree-gtTraverse(generator)

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• Any Questions?