Create some sort of mechanism for inter-mod communication. This has been requested a few times, but needs further research.

This was extensively discussed in the #modding channel in the Stardew Valley Discord. Here's what we came up with.

Intent

• We want to let mods send notifications, push messages, or exchange information with other mods.
• This should be easy to understand, use, and troubleshoot for mod authors.
• We do not want a message exchange with built-in support for every possible case. That would become far too complicated and difficult to understand. Instead we should make the common cases simple, and have a mechanism to support more complicated edge cases.

Goals

We aim to support three use cases:

• Publish/subscribe: mod publishes an event that any mod can listen for.
• Push: mod creates a channel that other mods can push to, and it'll listen for data it receives.
• Exchange: any number of mods exchange information on a common channel. This would mainly handle more complex cases.
  For example, mods using Advanced Location Loader might publish a Func<Vector2, bool> callback to an Entoarox.AdvancedLocationLoader.Intent channel, which they use to check whether another ALL mod has 'reserved' a tile for modification. Exchanging callbacks would allow them to perform this check synchronously, without waiting for an asynchronous message.

Message exchange API

Concept

SMAPI would expose a simple message exchange API that lets mods publish messages via a channel, which is a unique ID used to publish or subscribe. Here's the most basic exchange possible:

// mod #1 listens for messages on a channel
helper.MessageExchange.Subscribe("Namespace.UniqueChannelName", this.HandleMessage);

// mod #2 publishes a message that mod #1 receives
helper.MessageExchange.Publish("Namespace.UniqueChannelName", message);

Reserving a channel

Channels may optionally be reserved, which means one mod has ownership of that channel and can specify the following:

• A contract/datatype that is accepted. Any message published with a different contract is rejected.
• Whether other mods can publish messages to it.
• Whether other mods can listen to it.

Supported operations

This API would support three operations:

• Reserve a channel and specify its metadata.
• Publish a message to a channel ID. If the channel is reserved, its options are enforced; otherwise the message is published as-is to any listener.
• Listen for messages from a channel ID. If the channel is reserved, its options are enforced; otherwise the mod just receives any message sent to that channel ID as-is.

This is a stale proposal that's unlikely to be implemented in the foreseeable future. We can reopen or create a new issue when we're ready to re-examine it.