https://github.com/ruby/observer
The Observer pattern provides a simple mechanism for one object to inform a set of interested third-party objects when its state changes.
https://github.com/ruby/observer
Last synced: 3 months ago
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The Observer pattern provides a simple mechanism for one object to inform a set of interested third-party objects when its state changes.
- Host: GitHub
- URL: https://github.com/ruby/observer
- Owner: ruby
- License: bsd-2-clause
- Created: 2019-08-06T03:51:08.000Z (almost 6 years ago)
- Default Branch: master
- Last Pushed: 2023-11-07T03:42:57.000Z (over 1 year ago)
- Last Synced: 2025-03-29T13:09:13.547Z (3 months ago)
- Language: Ruby
- Homepage:
- Size: 50.8 KB
- Stars: 36
- Watchers: 32
- Forks: 9
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE.txt
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README
# Observer
The Observer pattern (also known as publish/subscribe) provides a simple
mechanism for one object to inform a set of interested third-party objects
when its state changes.
## Mechanism
The notifying class mixes in the +Observable+
module, which provides the methods for managing the associated observer
objects.
The observable object must:
* assert that it has +#changed+
* call +#notify_observers+
An observer subscribes to updates using Observable#add_observer, which also
specifies the method called via #notify_observers. The default method for
notify_observers is #update.
## Installation
Add this line to your application's Gemfile:
```ruby
gem 'observer'
```
And then execute:
$ bundle
Or install it yourself as:
$ gem install observer
## Usage
The following example demonstrates this nicely. A +Ticker+, when run,
continually receives the stock +Price+ for its @symbol. A +Warner+
is a general observer of the price, and two warners are demonstrated, a
+WarnLow+ and a +WarnHigh+, which print a warning if the price is below or
above their set limits, respectively.
The +update+ callback allows the warners to run without being explicitly
called. The system is set up with the +Ticker+ and several observers, and the
observers do their duty without the top-level code having to interfere.
Note that the contract between publisher and subscriber (observable and
observer) is not declared or enforced. The +Ticker+ publishes a time and a
price, and the warners receive that. But if you don't ensure that your
contracts are correct, nothing else can warn you.
```ruby
require "observer"
class Ticker ### Periodically fetch a stock price.
include Observable
def initialize(symbol)
@symbol = symbol
end
def run
last_price = nil
loop do
price = Price.fetch(@symbol)
print "Current price: #{price}\n"
if price != last_price
changed # notify observers
last_price = price
notify_observers(Time.now, price)
end
sleep 1
end
end
end
class Price ### A mock class to fetch a stock price (60 - 140).
def self.fetch(symbol)
60 + rand(80)
end
end
class Warner ### An abstract observer of Ticker objects.
def initialize(ticker, limit)
@limit = limit
ticker.add_observer(self)
end
end
class WarnLow < Warner
def update(time, price) # callback for observer
if price < @limit
print "--- #{time.to_s}: Price below #@limit: #{price}\n"
end
end
end
class WarnHigh < Warner
def update(time, price) # callback for observer
if price > @limit
print "+++ #{time.to_s}: Price above #@limit: #{price}\n"
end
end
end
ticker = Ticker.new("MSFT")
WarnLow.new(ticker, 80)
WarnHigh.new(ticker, 120)
ticker.run
```
Produces:
```
Current price: 83
Current price: 75
--- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 75
Current price: 90
Current price: 134
+++ Sun Jun 09 00:10:25 CDT 2002: Price above 120: 134
Current price: 134
Current price: 112
Current price: 79
--- Sun Jun 09 00:10:25 CDT 2002: Price below 80: 79
```
## Development
After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake test` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.
To install this gem onto your local machine, run `bundle exec rake install`. To release a new version, update the version number in `version.rb`, and then run `bundle exec rake release`, which will create a git tag for the version, push git commits and tags, and push the `.gem` file to [rubygems.org](https://rubygems.org).
## Contributing
Bug reports and pull requests are welcome on GitHub at https://github.com/ruby/observer.