private final ArtifactVisitor visitor;
            private final List<ResolvedArtifactSet.Artifacts> results = new ArrayList<>();
            private BuildOperationQueue<RunnableBuildOperation> queue;

            StartVisitAction(ArtifactVisitor visitor) {
                this.visitor = visitor;
            }

            @Override

        final FileVisitor visitor = Mock()

        when:
        tree.visit(visitor)

        then:
        1 * source1.visit(visitor)
        1 * source2.visit(visitor)
    }

    def visitsEachTreeWithClosure() {
        final Closure visitor = TestUtil.TEST_CLOSURE
        final FileVisitor closureAsVisitor = DefaultGroovyMethods.asType(visitor, FileVisitor.class)

        when:

    def "walking type hierarchy happens breadth-first"() {
        def visitor = Mock(Types.TypeVisitor)
        when:
        Types.walkTypeHierarchy(Child, [Object, GroovyObject], visitor)

        then: 1 * visitor.visitType(Child) >> Types.TypeVisitResult.CONTINUE
        then: 1 * visitor.visitType(Base) >> Types.TypeVisitResult.CONTINUE
        then: 1 * visitor.visitType(Serializable) >> Types.TypeVisitResult.CONTINUE

        return transformedArtifacts;
    }

    @Override
    public void visit(Visitor visitor) {
        FileCollectionStructureVisitor.VisitType visitType = visitor.prepareForVisit(this);
        if (visitType == FileCollectionStructureVisitor.VisitType.NoContents) {
            visitor.visitArtifacts(new EndCollection(this));
        } else {
            visitor.visitArtifacts(this);
        }
    }

    @Override

    def patternSetFactory = Mock(Factory)
    def visitor = new UnpackingVisitor(context, resolver, TestFiles.taskDependencyFactory(), patternSetFactory)

    def "resolves null"() {
        when:
        visitor.add(null)

        then:
        0 * context._
    }

    def "resolves String"() {
        def file = new File('some-file')

        when:
        visitor.add('path')

        then:

        def visitor = new CollectingValueVisitor()
        hierarchy.visitValues("", visitor)
        return visitor.allValues
    }

    static List<Integer> getValuesFor(ValuedVfsHierarchy<Integer> hierarchy, String location) {
        def visitor = new CollectingValueVisitor()
        hierarchy.visitValues(location, visitor)
        return visitor.allValues
    }

    def "visitor visits location"() {
        ExceptionContextVisitor visitor = Mock()
        def cause = new RuntimeException()
        def e = new LocationAwareException(cause, "location", 42)

        when:
        e.accept(visitor)

        then:
        1 * visitor.visitCause(cause)
        1 * visitor.endVisiting()
        1 * visitor.visitLocation("Location line: 42")

                }

            default:
                if (!previous.isEmpty()) {
                    return delegate.visitChangesSince(previous, current, propertyTitle, visitor);
                }
                return reportAllAdded(visitor, current, propertyTitle);
        }
    }

    private boolean reportAllAdded(ChangeVisitor visitor, Map<String, S> current, String propertyTitle) {

        for (DependencyArtifactsVisitor visitor : visitors) {
            visitor.visitNode(node);
        }
    }

    @Override
    public void visitArtifacts(DependencyGraphNode from, LocalFileDependencyMetadata fileDependency, int artifactSetId, ArtifactSet artifactSet) {
        for (DependencyArtifactsVisitor visitor : visitors) {
            visitor.visitArtifacts(from, fileDependency, artifactSetId, artifactSet);

        def visitor = Mock(DiagnosticsVisitor)

        given:
        def availability = new ToolChainAvailability()
        availability.unavailable("some reason")

        when:
        availability.explain(visitor)

        then:
        1 * visitor.node("some reason")
    }

    def "visits message when unsupported"() {
        def visitor = Mock(DiagnosticsVisitor)

        given: