SandthornEventFilter

SandthornEventFilter is a library for creating composable event filters. They are composable in the following senses:

• chained filters get boiled down to a single expression, meaning we can filter in just one pass
• filters are immutable and can be extended with further chaining

Installation

Add this line to your application's Gemfile:

gem 'sandthorn_event_filter'

And then execute:

$ bundle

Or install it yourself as:

$ gem install sandthorn_event_filter

Usage

The basic way to use a Filter is to wrap an array of Events:

filter = SandthornEventFilter::Filter.new(my_events)
# Or using shorthand
filter = SandthornEventFilter.filter(my_events)

You can always call .events to get the filtered events:

# This filter has no matchers, and will return the original events
filter.events
# => [...]

You can also enumerate on the filter directly:

filter.each { |event| puts event.inspect }

extract returns a new filter that extracts the chosen events:

new_events = filter.extract(events: "new", types: MyAggregate, after_sequence_number: 100)
new_events.events
# => events that match the criteria

remove returns a new filter which removes the chosen events:

except_new = filter.remove(events: "new")
except_new.events
# => events except those with event name "new"

select and reject behave as you would expect, except they return a new filter instance.

You can also create a filter chain without supplying initial events, and apply that chain to any array of events.

filter = SandthornEventFilter.filter.reject(events: "new")
filter.apply(my_array_of_events)
# => [ ... array ... ]