etl/docs/Messaging/message-bus.md

title	weight
message_bus	5

{{< callout type="info">}} Header: message_broker.h
From: 20.33.0 {{< /callout >}}

Message Bus

This page documents version 20.0.0 and above.

A variant of the observer pattern in that message routers and derived types are be able to subscribe to messages on a bus. The messages can be either broadcast, to be automatically picked up by any router that has a handler, or addressed to a particular router or router id. Message buses may be nested by setting a successor.

imessage_bus

Derived from imessage_router.

The base for all message buses.
Inherits publicly from etl::imessage_router.
Message buses are therefore also a type of router.
Objects of type etl::imessage_bus cannot be directly constructed.

Member functions

bool subscribe(etl::imessage_router& router)

Subscribes an etl::imessage_router derived class to the bus.
Returns true on success.

---

void unsubscribe(etl::imessage_router& router)

Unsubscribes the specified etl::imessage_router derived class from the bus.
Does not unsubscribe from nested buses.

---

void unsubscribe(etl::message_router_id_t id)

Unsubscribes routers with the specified id from the bus.
Does not unsubscribe from nested buses.

etl::imessage::MESSAGE_BUS will unsubscribe all message buses.
etl::imessage::ALL_MESSAGE_ROUTERS will unsubscribe all routers and buses. Equivalent to calling clear().

---

void receive(const etl::imessage& message)
void receive(etl::shared_message message)

Receives a message and distributes it to all subscribers.
Forwards the message to any successor.

The routers are called first, in order of ascending router id.
Routers with the duplicate ids will be called in subscribe order.
Any nested message buses are called in subscribe order.

---

void receive(etl::message_router_id_t destination_router_id, 
             const etl::imessage&     message);
void receive(etl::message_router_id_t destination_router_id, 
             etl::shared_message message)

Receives a message and distributes it to all subscribers that have the specified router id.
Forwards the message to any successor.

Routers with the duplicate ids will be called in subscribe order.
Any nested message buses are called in subscribe order.

Override this in a derived class if you wish to capture messages sent to the bus.
Call the base receive function from here to allow normal operation to continue.

---

bool accepts(etl::message_id_t id) const

Always returns true.

---

size_t size() const

Returns the number of subscribers.

---

void clear()

Clears the bus of all subscribers.

Message buses inherit all of the public functions of etl::imessage_router.

Errors

message_bus_exception

Base error class for etl::message_bus. Inherits from etl::exception.

---

message_bus_too_many_subscribers

Emitted when the number of subscribers exceeds the capacity. Inherits from etl::message_bus_exception.

message_bus

Derived from imessage_bus.

template <const uint_least8_t MAX_ROUTERS>
class message_bus

MAX_ROUTERS The maximum number of routers that can be subscribed.

Member functions

message_bus()

Constructs a message bus.
Message buses always have a router id of etl::imessage::MESSAGE_BUS.

message_bus(etl::imessage_router& successor)

Constructs a message bus and sets the successor.
Message buses always have a router id of etl::imessage::MESSAGE_BUS.

Example

// Some router ids.
enum
{
  ROUTER_ID_1,
  ROUTER_ID_2,
  ROUTER_ID_3
};

// Instances of messages.
MessageA messageA;

// Instances of message routers.
RouterA routerA(ROUTER_ID_1);
RouterB routerB(ROUTER_ID_1);
RouterC routerC(ROUTER_ID_2);
RouterD routerD(ROUTER_ID_3);
RouterE routerE(ROUTER_ID_1);

// Instances of message buses.
etl::message_bus<4> bus1;
etl::message_bus<2> bus2;
etl::message_bus<1> bus3;

// Subscribe bus2 & bus3 to bus1.
bus1.subscribe(bus3);
bus1.subscribe(bus2);

// Subscribe routerB & routerA to bus1.
bus1.subscribe(routerB);
bus1.subscribe(routerA);

// Subscribe routerD & routerC to bus2.
bus2.subscribe(routerD);
bus2.subscribe(routerC);

// Subscribe routerE to bus3.
bus3.subscibe(routerE);

// Assume all routers accept the same messages.

// Broadcast messageA to everyone.
bus1.receive(messageA);

// The call order will be...
// routerB
// routerA
// routerE
// routerC
// routerD

// Address messageA to routers with id ROUTER_ID_1.
bus1.receive(ROUTER_ID_1, messageA);

// The call order will be...
// routerB
// routerA
// routerE

// Address messageA to routers with id ROUTER_ID_3.
bus1.receive(ROUTER_ID_3, messageA);

// The call order will be...
// routerD