event_emitter

# Event Emitter

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EventEmitter provides an idiomatic asynchronous event-driven architecture by registering listener functions that are called by named event emits. This shard is heavily inspired by Node.js [events API](https://nodejs.org/api/events.html).

When the `emit` method is called, the listeners attached to it will be called (synchronously or asynchronously) with the possibility to pass arguments to it.

The following example shows a simple EventEmitter usage with a single listener.

```crystal
require "event_emitter"

class MyEmitter
  include EventEmitter::DSL

  on :connect, sync: true do |name|
    puts "Hello #{name}"
  end

  def connect(name)
    emit :connect, name
  end
end

emitter = MyEmitter.new
emitter.connect("Hugo")
```

Another approach is to inherit from `EventEmitter::Base` class, as the example above:

```crystal
class MyEmitter < EventEmitter::Base; end

my = MyEmitter.new
my.on :event do |name|
  puts "Hello #{name}"
end

my.emit :event, "hugo"
my.emit :event, "abonizio"
```

## Usage

### DSL

#### Synchronous and asynchronous

A listener can execute a block synchronously or asynchronously depending on the argument `sync` it is passed.

```crystal
# Asynchronous (executed in another fiber)
on :message, do |message|
  puts "ASYNC: Message: #{message}"
end

# Synchronous
on :connect, sync: true do |name|
  puts "SYNC: Hello #{name}"
end
```

#### Passing arguments to the listeners

```crystal
class MyEmitter
  include EventEmitter::DSL

  on :finish do |id, name|
    puts "Hello #{name} (#{id})"
  end

  def perform(name)
    id = User.create(name)
    emit :finish, id, name
  end
end
```

### Instance

The class `EventEmitter::Base` provides the methods `on`, `once` and `emit` to insert a new listener, insert a one-time listener and trigger an event, respectively. It also provides an `all` method for responding to all events, and a `remove_listener` method to delete a listener.

You can inherit from `EventEmitter::Base` class to add custom functionality (`class MyEmitter < EventEmitter::Base; end`) or simply create an instance of `EventEmitter::Base` as the following example.

```crystal
emitter = EventEmitter::Base.new
emitter.on :message do |body|
  puts "> #{body}"
end

delay 200.milliseconds do
  emitter.emit :message, "Hello, world!"
end
```

Handling events only once:

```crystal
emitter = EventEmitter::Base.new
flag = 1
emitter.once :trigger do
  flag = 2
end

emitter.emit :trigger
emitter.emit :trigger # Will execute only the first trigger
```

Listening to all events:

```crystal
emitter = EventEmitter::Base.new
flag = 0

emitter.all { flag += 1 }

emitter.emit(:foo)
emitter.emit(:bar)
emitter.emit(:foobar)

puts flag
# => 3
```

Removing an event listener:

```crystal
emitter = EventEmitter::Base.new
flag = 0

emitter.on(:foo) { flag += 1 }

emitter.emit(:foo)

emitter.remove_listener(:foo)

emitter.emit(:foo)

puts flag
# => 1
```

## Installation

Add this to your application's `shard.yml`:

```yaml
dependencies:
  event_emitter:
    github: hugoabonizio/event_emitter.cr
```


## Contributing

1. Fork it ( https://github.com/hugoabonizio/event_emitter.cr/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

## Contributors

- [hugoabonizio](https://github.com/hugoabonizio) Hugo Abonizio - creator, maintainer
- [watzon](https://github.com/watzon) Chris Watson - contributor