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Lua Event Dispatcher

This is an implementation of the Mediator pattern for Lua. It provides an event dispatcher and a generic event object, supports priorities, propagation stops and error safe executors.

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Installation

Install lua-event-dispatcher through luarocks.

$ luarocks install lua-event-dispatcher

Documentation

Usage examples can be found in the spec/ folder in form of tests.

Basic usage

A simple example of how to use this library is the following one.

local Dispatcher = require "event-dispatcher.Dispatcher"
local Event = require "event-dispatcher.Event"

-- Create an event dispatcher
local dispatcher = Dispatcher:new()

-- Create an event listener
local listener = function(event)
  event.data.meep = 2
end

-- Register this listener to a specific event
dispatcher:on("event-name", listener)

-- Create an event object with
local event = Event:new({
  meep = 1
})

dispatcher:dispatch("event-name", event)

Event object

The event object already contains a data table on the key data which can be used to store any arbitrary data. The same Event instance is passed to all the listeners. This allows for data to be changed along the way.

The data passed to the constructor of the event object is stored as the initial data for this event.

local event = Event:new({
    foo: true
    bar: false
})

print(event.data.foo) -- true
print(event.data.bar) -- false

You don't need to create and pass an event object yourself.

local dispatcher = Dispatcher:new()

dispatcher:dispatch("something-happened")

Still, your listeners will receive an event object, where they can stop the propagation for example. The data table on such an implicit created event object is empty.

Priority queues

Event listeners can be added with a specific priority.

dispatcher:on("event-name", listener, 32)
dispatcher:on("event-name", listener, 64)

-- You can add multiple listeners with the same priority
dispatcher:on("event-name", listener, 128)
dispatcher:on("event-name", listener, 128)

If no priority is given, an implicit priority of 0 will be used. Listeners with lower priorities will be executed first.

Stop propagation

If for some reason you want to stop the propagation of the event in a listener, call the stopPropagation method to guarantee that the current listener is the last one to run.

local listener = function(event)
  event:stopPropagation()
end

Executors

An executor is responsible for calling your listeners. This library provides two different implementations.

  • direct: Listeners will be called directly. This means that any error that is thrown inside of a listener immediately bubbles up and stops the execution of other registered listeners.
  • protected: Listeners will be called with pcall which means that all of the registered listeners will be called, even if a preceding one errors on the way.

By default, the Dispatcher uses the direct executor. You can easily provided your own executor as a callable to a new dispatcher object.

local Dispatcher = require "event-dispatcher.Dispatcher"

local directExecutor = require "event-dispatcher.Executor.direct"
local protectedExecutor = require "event-dispatcher.Executor.protected"

Dispatcher:new() -- Implicit use of a direct executor
Dispatcher:new(directExecutor) -- Explicit use of a direct executor
Dispatcher:new(protectedExecutor) -- Use a protected call

-- ... or create an executor yourself
Dispatcher:new(function(listener, event)
    -- do some other work
    listener(event)
end)

Remove event listeners

To remove event listeners from a given dispatcher, use one of the following methods.

dispatcher:removeListener("event", listener) -- Remove a specific listener from an event
dispatcher:removeListeners("event") -- Remove all listeners from an event
dispatcher:removeAllListeners() -- Clear the dispatcher from all registered events

Testing

This repository uses the busted framework to run its unit tests. Install it through luarocks.

$ luarocks install busted

Afterwards, you should be able to run busted from the project root.

License

This library is licensed under the MIT license. See the complete license text in the LICENSE file

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