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Message Bus

The message bus is the core of the engine, it is used to manage sending messages between different parts of the engine/game.

Adding a Message to the Message Bus

A message can be added to the message bus by using the Publish method, this will not immediately dispatch the message, instead it will be stored until a Dispatch occurs.

Messages added to the message bus should be added as a unique pointer (std::unique_ptr) - this is because ownership of the memory that a message is sent with should be held by the message bus, helping to prevent memory leaks and memory management confusion. The message bus will take ownership of the pointer and will clean it up after it has been sent out.

An example of adding a message to the message bus looks like this (taken from the Game class):

this->m_messageBus->Publish(std::make_unique<JamJar::MessagePayload<float>>(
    JamJar::System::MESSAGE_UPDATE, float(TIME_STEP) / MICROSECOND_TO_SECOND_CONVERSION));

This dispatches a new update message, with the delta time as a payload.

Dispatching Messages

Every message in the message bus is dispatched - sent out to any subscribing part of the engine - when the Dispatch method of the message bus is called. This will iterate through every message, in the order that they are published to the message bus, and will fetch all subscribers to the specific message type, each subscriber will be sent the message by calling their OnMessage method.

The Dispatch function will keep calling until it is completely empty, meaning that you should be careful to not introduce any circular dependencies of messages, as this could cause an infinite loop. An example of a circular dependency that would cause this is:

  1. A sends out message X, B is a subscriber to X, so the message X is sent to B.
  2. Once B receives the message, it immediately sends out message Y.
  3. A is a subscriber to Y, so the message Y is sent to A.
  4. Once A receives the message, it immediately sends out message X.
  5. This loops on infinitely.

When Does Dispatching Happen?

Message dispatching should probably not be triggered by game code, instead it should be left to the engine to decide when to call it. The engine will dispatch messages in these three instances (all in the Game class):

  1. After every update message is added, in the main update loop.
  2. After the pre-rendering stage of the render pipeline.
  3. After the rendering and post-rendering stage of the render pipeline.

Subscribing to Message Types

A part of the game/engine can subscribe to certain message types in the message bus by using the Subscribe method, which takes a listener and a message type to filter for. This means that any message that has the type subscribed to will be sent on to the listener.

An example of a listener subscribing to a message is the System subscribing to update messages:

this->m_messageBus->Subscribe(this, JamJar::System::MESSAGE_UPDATE);

See the Listeners page for more details about listeners.