Groovy for Domain-specific Languages - Second Edition

Chapter 8

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A state machine AST transformation

Unlike the previous examples, we won't need an annotation class for a global AST
transformation. AST transformation would get quite unwieldy if we tried to build
all the logic into the AST transformation class, so we separate out the responsibilities
into four classes:

• StateMachineASTTransformation: This is the GroovyASTTransfromation
  class similar to the ones we already encountered in our annotation-based
  examples.


• StateMachineParser: This class implements the GroovyClassVisitor
  interface. This class uses a visitor pattern to allow us to work on any part
  of the tree of AST nodes in our compiled source. The main responsibility of
  this class is to produce a simple data model to store the key elements of the
  state machine.


• StateMachineModel: This class stores the data model of the parsed state
  machine. It describes the possible states, the initial state, the available events,
  and the allowed transitions for each event.


• StateMachineBuilder: This class builds the new AST nodes we need to
  implement the state machine pattern.

@GroovyASTTransformation (phase = CompilePhase.SEMANTIC_ANALYSIS)

class StateMachineASTTransformation implements ASTTransformation {

def parser

def builder

void visit(ASTNode[] nodes, SourceUnit source) {

if (!nodes) return

if (!(nodes[0] instanceof ModuleNode)) return

if (!source?.name?.endsWith('State.groovy')) {

return

}

def model = new StateMachineModel()

parser = new StatePatternParser(model)

builder = new StatePatternBuilder(nodes, model)

for (ClassNode classNode : nodes[0].classes) {

classNode.visitContents(parser)

}

builder.buildStatePattern()

}

}