trivialvendingmachine.cpp File Reference

Vending Machine with Named TopicStates and GraphAnalyzer. More...

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Detailed Description

Vending Machine with Named TopicStates and GraphAnalyzer.

Introduction

This example gives a single-detector solution for a hypothetical one-item, one-coin vending machine. It shows how a single detector can be used to synchronize combine signals - coins being inserted and the "Buy" button being clicked - to produce a third - SaleCompleted. It also introduces the concepts of Trivial TopicStates, Named TopicStates and using GraphAnalyzer.

Trivial TopicStates

A Trivial TopicState has no data fields:

struct CoinInserted : public DetectorGraph::TopicState
{
};

These are sometimes useful to represent parameter-less signals, timeouts etc.

Named TopicStates

Named TopicStates, as opposed to Anonymous ones, are TopicStates that override the TopicState::GetId method. This allows for code that deals with sequences of different TopicStates in a general way. This is used primarily outside (or at the boundary) of the DetectorGraph portion of the application in applications that necessitate any need of generic/transparent treatment of TopicStates. An example of this is GraphStateStore & StateSnapshot. They are often also called Public TopicStates as they're what's exposed to the outside world from the point of view of a DetectorGraph graph.

TopicState "names" come from an application-specific enumeration value for your named TopicStates - values must be cast-able to TopicStateIdType and be different than TopicState::kAnonymousTopicState (-1). An example would be:

enum class VendingMachineTopicStateIds{
    kSaleCompleted = 0,
    kBalance
};

(C++03 enums are fine too)

Which then can be used in the overriding of TopicState::GetId for the desired named TopicState:

struct SaleCompleted : public DetectorGraph::TopicState
{
    DetectorGraph::TopicStateIdType GetId() const
    {
        return static_cast<DetectorGraph::TopicStateIdType>(
            VendingMachineTopicStateIds::kSaleCompleted);
    }
};

That then enables things like:

    void ProcessOutput()
    {
        // Now we can iterate exclusively through newly updated TopicStates
        // instead of checking through every Topic of interest.
        for (const auto topicState : mGraph.GetOutputList())
        {
            switch(static_cast<VendingMachineTopicStateIds>(
                topicState->GetId()))
            {
                case VendingMachineTopicStateIds::kSaleCompleted:
                    cout << "Sale Made" << endl;
                break;

                case VendingMachineTopicStateIds::kBalance:
                {
                    const std::shared_ptr<const Balance> balance =
                        std::static_pointer_cast<const Balance>(topicState);

                    cout << "Balance: " << balance->numberOfCoins;
                    cout << " coins" << endl;
                }
                break;
            }
        }

        // Or update a GraphStateStore that can be used for persistence or for
        // a State querying API to be used from outside of the graph.
        mStateStore.TakeNewSnapshot(mGraph.GetOutputList());
    }

GraphAnalyzer

Finally, this example also shows how to use DetectorGraph::GraphAnalyzer to generate a GraphViz-compatible .dot representation of the graph:

    DetectorGraph::GraphAnalyzer analyzer(vendingMachine.mGraph);
    analyzer.GenerateDotFile("trivial_vending_machine.dot");

That .dot file when rendered generates the following graph:

Trivial Vending Machine

Note that the Named TopicStates are represented by outlined rectangles.

For a more sophisticated solution to a similar problem, see fancyvendingmachine.cpp .

Definition in file trivialvendingmachine.cpp.