Message Broker

A variant of the observer pattern in that message routers and derived types are be able to subscribe to selected sets of

messages. The message_broker is similar to the message_bus, but it provides more control over the routing of messages.

While the message_bus simply broadcasts every message to all subscribers, the message_broker allows you to specify

which subscribers should receive each message.

Derived from imessage_router

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Types

message_id_span_t etl::span<const etl::message_id_t>

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subscription

A nested class of etl::message_broker

The base for broker subscription information.

Derive from this to define your subscription class.

See Example.

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subscription(etl::imessage_router& router)

Constructor.

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virtual message_id_span_t message_id_list() const = 0;

Override this to return a span of message ids.

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message_broker

message_broker()

The broker is constructed with an id of etl::imessage_router::MESSAGE_BROKER.

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message_broker(etl::imessage_router& successor)

The broker is constructed with an id of etl::imessage_router::MESSAGE_BROKER.

Sets the successor.

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message_broker(etl::message_router_id_t id)

The broker is constructed with the specified id.

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message_broker(etl::message_router_id_t id, etl::imessage_router& successor)

The broker is constructed with the specified id.

Sets the successor.

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void subscribe(etl::imessage_router& router)

Subscribes an etl::imessage_router derived class to the broker.

A subscription object must have a lifetime of at least the same as the broker.

A subscription cannot be shared with another broker.

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void unsubscribe(etl::imessage_router& router)

Unsubscribes the specified etl::imessage_router derived class from the bus.

Does not unsubscribe from nested buses.

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void receive(const etl::imessage& message)

void receive(etl::shared_message message)

Receives a message and distributes it to all subscribers that have registered to receive the message type.

Forwards the message to any successor.

Override this in a derived class if you wish to capture messages sent to the broker.

Call the base receive function from here to allow normal operation to continue.

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bool accepts(etl::message_id_t id) const

Always returns true.

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void clear()

Clears the broker of all subscribers.

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ETL_DEPRECATED bool is_null_router() const ETL_OVERRIDE

Always returns false.

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bool is_producer() const ETL_OVERRIDE

Always returns true.

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bool is_consumer() const ETL_OVERRIDE

Always returns true.

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bool empty() const

Returns true is the are are no subscribers.

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Example

// Some router ids.

enum

{

ROUTER_ID_1,

ROUTER_ID_2,

};

// Custom subscription type.

class Subscription : public etl::message_broker::subscription

{

public:

Subscription(etl::imessage_router& router, std::initializer_list<etl::message_id_t> id_list_)

: etl::message_broker::subscription(router)

, id_list(id_list_)

{

}

etl::message_broker::message_id_span_t message_id_list() const override

{

return etl::message_broker::message_id_span_t(id_list.begin(), id_list.end());

}

std::vector<etl::message_id_t> id_list;

};

// Instances of messages.

Message1 message1;

Message2 message2;

Message3 message3;

Message4 message4;

// Custom broker.

class Broker : public etl::message_broker

{

public:

using etl::message_broker::receive;

// Hook incoming messages and translate Message4 to Message3.

void receive(const etl::imessage& msg) override

{

if (msg.get_message_id() == Message4::ID)

{

etl::message_broker::receive(Message3());

}

else

{

etl::message_broker::receive(msg);

}

};

// Instances of message routers.

Router1 router1;

Router2 router2;

// The subscriptions.

Subscription subscription1{ router1, { Message1::ID, Message2::ID } };

Subscription subscription2{ router2, { Message2::ID, Message3::ID } };

// Instance of message broker.

etl::message_broker broker;

// Subscribe router1 and router1 to the broker.

broker.subscribe(subscription1);

broker.subscribe(subscription2);

broker.receive(message1); // Received by router1

broker.receive(message2); // Received by router1 and router2

broker.receive(message3); // Received by router2

broker.receive(message4); // Received by router2 as a Message3

message_broker.h