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Strategy in Ruby
Strategy is a behavioral design pattern that turns a set of behaviors into objects and makes them interchangeable inside original context object.
The original object, called context, holds a reference to a strategy object. The context delegates executing the behavior to the linked strategy object. In order to change the way the context performs its work, other objects may replace the currently linked strategy object with another one.
Complexity:
Popularity:
Usage examples: The Strategy pattern is very common in Ruby code. It’s often used in various frameworks to provide users a way to change the behavior of a class without extending it.
Identification: Strategy pattern can be recognized by a method that lets a nested object do the actual work, as well as a setter that allows replacing that object with a different one.
Conceptual Example
This example illustrates the structure of the Strategy design pattern. It focuses on answering these questions:
- What classes does it consist of?
- What roles do these classes play?
- In what way the elements of the pattern are related?
main.rb: Conceptual example
# The Context defines the interface of interest to clients.
class Context
# The Context maintains a reference to one of the Strategy objects. The
# Context does not know the concrete class of a strategy. It should work with
# all strategies via the Strategy interface.
attr_writer :strategy
# Usually, the Context accepts a strategy through the constructor, but also
# provides a setter to change it at runtime.
def initialize(strategy)
@strategy = strategy
end
# Usually, the Context allows replacing a Strategy object at runtime.
def strategy=(strategy)
@strategy = strategy
end
# The Context delegates some work to the Strategy object instead of
# implementing multiple versions of the algorithm on its own.
def do_some_business_logic
# ...
puts 'Context: Sorting data using the strategy (not sure how it\'ll do it)'
result = @strategy.do_algorithm(%w[a b c d e])
print result.join(',')
# ...
end
end
# The Strategy interface declares operations common to all supported versions of
# some algorithm.
#
# The Context uses this interface to call the algorithm defined by Concrete
# Strategies.
class Strategy
# @abstract
#
# @param [Array] data
def do_algorithm(_data)
raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
end
end
# Concrete Strategies implement the algorithm while following the base Strategy
# interface. The interface makes them interchangeable in the Context.
class ConcreteStrategyA < Strategy
# @param [Array] data
#
# @return [Array]
def do_algorithm(data)
data.sort
end
end
class ConcreteStrategyB < Strategy
# @param [Array] data
#
# @return [Array]
def do_algorithm(data)
data.sort.reverse
end
end
# The client code picks a concrete strategy and passes it to the context. The
# client should be aware of the differences between strategies in order to make
# the right choice.
context = Context.new(ConcreteStrategyA.new)
puts 'Client: Strategy is set to normal sorting.'
context.do_some_business_logic
puts "\n\n"
puts 'Client: Strategy is set to reverse sorting.'
context.strategy = ConcreteStrategyB.new
context.do_some_business_logic
output.txt: Execution result
Client: Strategy is set to normal sorting.
Context: Sorting data using the strategy (not sure how it'll do it)
a,b,c,d,e
Client: Strategy is set to reverse sorting.
Context: Sorting data using the strategy (not sure how it'll do it)
e,d,c,b,a
