Design Patterns in Ruby

updated to work with ruby 2.2.0

Examples from the book Design Patterns in Ruby by Russ Olsen

This book covers 14 of the original 23 GoF design patterns.

Template Method
Strategy
Observer
Composite
Iterator
Commands
Adapter
Proxy
Decorator
Singleton
Factory
Builder
Interpreter

Five Main Points

Separate the things that change from the things that stay the same.
- promotes DRY code
- improves maintainability
Program to an interface, not an implementation.

Code should belong to the most general object possible, and specific classes should inherit from the general class.
- increases modularity
Prefer composition over inheritance

Avoid saying that an object is a kind of something and instead say that it has something. In other words, it's often better to reference other classes or modules than to put functionality in superclasses.
- increases encapsulation
- increases modularity, implementations can be easily swapped
delegate, delegate, delegate Let other classes handle functionality within their domain
You ain't gonna need it (Russ Olsen's addition)

Don't implement features or design in flexibility that you don't immediately need because you will probably never need it.

Design Patterns

TEMPLATE METHOD

Create a skeletal class with methods that are common between algorithms.
Create a subclass for each algorithm and override the common methods from the skeletal class.

Disadvantages:

no runtime flexibility

STRATEGY

The basic idea is to delegate tasks to encapsulated algorithms which are interchangable at runtime.

In the Strategy pattern we have an object (the context) that is trying to get something done. But to get that thing done, we need to supply the context with a second object, called the strategy, that helps get the thing done.

Define a family of objects which all do the same thing (ex: format output, generate graphics etc.).
Ensure the family of objects share the same interface so that they are interchangable.

There are two strategies to passing data from the context object to the strategy object. We can pass the data as paramaters when the strategy is called, or we can pass the context object as the single parameter.

If the strategies are very simple and have only one method, we can even use code blocks for our algorithms and simply use block.call. However, if multiple methods are needed, the strategies must be structured as separate classes.

Advantages:

algorithms are interchangable at runtime
promotes modularity

OBSERVER

The observer pattern is for event driven programming.

An object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

The Ruby Standard Library includes an Observable module that implements this pattern.

It's also possible to use code blocks as observers. This isn't supported by the Observable module in the Ruby Standard Library, but it's easy to implement.

Additional Concerns

There are a few additional concerns that should be addressed when working with the Observable pattern.

Push vs Pull

In the default implementation, the notification sent to the observer doesn't specify which of the Subjects attributes has changed. To find out which attribute has changed, the Observer has to check the Subjects attributes, this is the pull method.

Another approach would be the push method where the notification includes other attributes which provide the Observer with additional information like the examples below.

observer.update(self, :salary_changed)
observer.update(self, :salary_changed, old_salary, new_salary)

An observer may only need to know when a specific attribute of the subject changes. The simple implementation would notify the observer when any attribute changes.

Atomic Event Notifications

If multiple attributes of a Subject are being updated and the updates are not independent, notifying Observers before all the updates are executed could cause an inconsistent state.

fred = Employee.new('Fred', 'Crane Operator', 30000)
fred.salary = 1000000
# Warning! Inconsistent state here!
fred.title = 'Vice President of Sales'

What to do When an Observer Raises an Exception

It's also important to address what should happen when a notification causes an Observer to raise an exception. The correct way to handle exceptions will vary from case to case.

COMPOSITE

The composite design pattern is a structural pattern used to represent objects that have a hierarchical tree structure. It allows for the uniform treatment of both individual leaf nodes and of branches composed of many nodes.

The implementation in the book is inflexible and doesn't allow Tasks to be dynamically created and doesn't allow dynamically splitting tasks into subtasks. To subdivide a task into multiple subtasks, the class of the leaf Task must be changed to a CompositeTask before children can be added. A better solution would be to use a single Node class for both leaves and internal nodes. With this implementation, leaf nodes can have children added without the need to change it's class.

For a specific implementation, you can simply inherit from the Node class and extend it with any additional functions you may need.

ITERATOR

The iterator design pattern provides sequential access to elements within a container without exposing how the container actually represents the elements. The iterator can be thought of as a moveable pointer that allows access to elements encapsulated within a container.

external iterator

The iteration logic is contained in a separate class. The iteration class can be generalized to handle multiple object types as long as they allow indexing.

External iterator require the additional class to do the actual iterating, but they do allow for greater flexibility because you can control the iteration, which elements are iterated over and in what order.

internal iterator

All the iterating logic occurs inside the aggregate object. Use a code block to pass your logic into the aggregate which then calls the block for each of it's elements.

colors = ['red', 'green', 'blue']
colors.each { |color| puts color }

Enumerable Module

Ruby includes an Enumerator module

The Enumerable mixin provides collection classes with several traversal and searching methods, and with the ability to sort. The class must provide a method each, which yields successive members of the collection. If Enumerable#max, #min, or #sort is used, the objects in the collection must also implement a meaningful <=> operator, as these methods rely on an ordering between members of the collection.

With custom iterator implementations, if the original collection class changes while you're iterating through it's elements, it can create unexpected results. To remedy this, you can have the iterator operate on a copy of the original collection.

class ArrayIterator
  def initialize(array)
  	@array = Array.new(array)
  	@index = 0
  end
  …

COMMANDS

The command pattern is a behavior design pattern used to store the information necessary to call methods at a future time.

Uses

GUI Elements

For many GUIs, you may have a generic button class that you want to use in many different situations. In each situation, the button may need to do different things. One approach would be to create a subclass for each button your interface requires, however this would lead to a excessive number of button classes. A better approach would be to create separate classes for the code executed when the button is clicked. This action class can then be passed to the button telling it what the button should do when clicked.

separate the thing that changes, in this case the action, from the part that stays the same, the generic button object. One advantage of this approach is that since the action is passed to the button, it can be changed at runtime.

The command is merely a set of actions wrapped in an object. With ruby, we can use Procs to do the same thing without the need to create a separate object. This is a good option when the action is simple and doesn't require saving state information, otherwise, a command class is the better option.

Macro Recording

Many modern applications have the ability to undo actions, including word processors, spreadsheets and databases. This undo feature can be implemented by using the command design pattern by keeping track what code is executed.

Another great example of macro recording using the command design pattern is the handling of migrations in Ruby on Rails.

To undo actions we need to store some state information, so we must use a command class rather than a simple Proc.

Queuing Commands

Commands allow us to queue commands making applications more convenient for the user and for the system.

Installation Wizards

When installing applications, many will prompt the user with a number of installation options before starting the actual installations. The user's choices determines which method calls are added to the installer's to-do list. Without commands, the installation would have to stop periodically to query the user for their preferences.

Fixed Overhead

In some situations there is a fixed overhead to executing a certain type of command. Queuing up multiple commands and executing them together reduces the number of times we have to run the overhead code. Database operations are an example of this. If there isn't a persistent database connection, we have to create one each time we run database operations. Since there is a cost to connecting to the database, a good approach may be to queue up the database operations and execute them in a batch. The same logic holds for web applications when you need to make API calls to external applications.

ADAPTER

Convert the interface of a class into another interface clients expect. An adapter lets classes work together that could not otherwise because of incompatible interfaces.

http://en.wikipedia.org/wiki/Design_pattern_(computer_science)

Ruby allows for classes to be modified at runtime. Consequently, rather than create an adapter to modify a classes API, we can simply modify the class at runtime to add or alter methods.

Alternatively, Ruby also allows the runtime modification of individual instances.

Modifying instances or classes at run-time is advisable only when:

The modifications are simple
You understand the class you're modifying well and are sure your changes wont break things.

If either of these points aren't true, it's probably better to create a separate adapter.

PROXY

Types of Proxies

Protection Proxy

A protection proxy protects an object from unauthorized access. To ensure methods can only be run by authorized users, we can run an authorization check before messages are passed to the underlying object.

Remote Proxy

Remote proxies provides access to objects which are running on remote machines.

A great example of a remote proxy is Distributed Ruby (DRb), which allows Ruby programs to communicate over a network. With DRb, the client machines runs a proxy which handles all of the network communications behind the scenes.

Virtual Proxy

Virtual proxies allow us to delay the creation of an object until it is absolutely necessary. This is useful when creating the object is computationaly expensive.

Using Message Passing to Simplify Proxies

When building a proxy, we could implement a method for each method in the underlying object. However, this leads to a lot of repeated code and tightly couples the proxy with the underlying object. A better alternative is to pass method calls direcly to the underlying object. Ruby includes a method that is perfect for this situation called method_missing.

In Ruby, when you call a method on an object, Ruby looks for the method in the initial object and it's modules and then works it's way up the stack to that objects superclass and then it's superclass and so on. If the method is not found, Ruby then looks for the method method_missing in the initial object, then it's parent, and it's parent… etc.

Rather than implement each of the underlying objects methods in the proxy, we can use method_missing to simply pass method calls to the unerlying object.

Again, this has to big advantages:

The proxy is greatly simplified by having far fewer methods
The proxy is not coupled to the underlying object and can therefore be reused for multiple object classes.

DECORATOR

The decorator design pattern allows for features to be added dynamically to an existing object.

Motivation via wikipedia

With Ruby, the easiest way to use the decorator pattern is to create a module for each decorator. The decorators that you want to use can be dynamically added to an instance using extend.

Ruby includes a forwardable module that provides an easy way to add delegation methods. forwardable may allow you to create cleaner more readable code, it really depends on the situation.

Dynamic Alternatives

Ruby allows for dynamically modifying instances. We can use to this feature to add decorators at run-time.

This is a quick and dirty approach to adding decorators to an instance. Create an alias for the original method, then add a module with the same name as the original method. This is shown below:

w = SimpleWriter.new('out')

class << w
  alias old_write_line write_line

  def write_line(line)
    old_write_line("#{Time.new}: #{line}")
  end
end

For the w instance of SimpleWriter, the original write_line method still exists and is pointed to by old_write_line. Now, when write_line is called, the new method is executed and it then executes old_write_line.

disadvantage of using extend with modules:

can't easily remove modules

SINGLETON

The singleton pattern is used to ensure that there is only one instance of a class and provides global access to that instance.

This pattern is useful when you want one instance of a class and many different objects need to access it, rather than pass the object around, we can make the object global.

GoF: Let the class of the singleton object manage the creation and access to its sole instance.

The Ruby standard library includes a Singleton module module.

The singleton module is thorough, it overrides multiple methods to prevent multiple instances of the singleton class. It also uses lazy instantiation, creating the instance only once the instance method is called.

Alternatives

Global Variables

As an alternative, it is possible to use global variables to store singletons. There can be only one instance of a global variable and by definition they are globally accessible.

Disadvantages:

Global variables can be reassigned at any time.
Lazy instantiation isn't possible.

Constants

Since we can use global variables to hold singletons, it follows that we can also constants.

Disadvantages:

Constant variables can be reassigned at any time. A warning is returned when they are changed but they are still changed.
Lazy instantiation isn't possible.
We can't prevent the creation of more than one instance of the singleton class.

Classes as Singletons

Disadvantages:

Using a class to hold all the variables and methods doesn't follow traditional coding conventions.

Advantages:

There can be only one class of a given name and therefore only one instance of the singleton.

Modules as Singletons

The code is exactly the same as using a class as a singleton, except we define a module instead of a class.

A module is a collection of methods and variables, but unlike classes, modules cannot be instantiated. Because of this, using a module for a singleton better adheres to coding conventions and makes for clearer code.

headshota/design-patterns-in-ruby

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