1. Software Design & Documentation
Facade
Eric Goodnough
Software Design & Documentation

2. 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

3. Structural Diagram
Client Classes
Facade
Subsystem Classes

4. Participants Facade Subsystem classes
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

5. 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

6. Benefits Reduces the number of objects the clients have to deal with
Promotes weak coupling between the subsystem and the clients
Doesn’t prevent applications from using subsystem classes

7. Compiler Example Compiler Subsystem: Scanner Parser ProgramNode
ProgramNode Builder
CodeGenerator
Parser
Scanner
ProgramNodeBuilder
CodeGenerator
ProgramNode

8. 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&); };

9. Example Code (cont.)
Void Compiler::Compile(istream& input, BytecodeStream& output) {
Scanner scanner(input);
ProgramNodeBuilder builder;
Parser parser;
parser.Parse(scanner, builder);
RISCCodeGenerator generator(output);
ProgramNode* parseTree = builder.GetRootNode();
parseTree->Traverse(generator); }

10. Any Questions?