Factory Method Pattern

Problem

You don’t know what kind of object you will need until runtime.

Solution

Use the Factory Method pattern and choose the object to be generated dynamically.

Say that you need to load a file into an editor but you don’t know its format until the user chooses the file. A class using the Factory Method pattern can serve up different parsers depending on the file’s extension.

class HTMLParser
	constructor: ->
		@type = "HTML parser"
class MarkdownParser
	constructor: ->
		@type = "Markdown parser"
class JSONParser
	constructor: ->
		@type = "JSON parser"

class ParserFactory
	makeParser: (filename) ->
		matches = filename.match /\.(\w*)$/
		extension = matches[1]
		switch extension
			when "html" then new HTMLParser
			when "htm" then new HTMLParser
			when "markdown" then new MarkdownParser
			when "md" then new MarkdownParser
			when "json" then new JSONParser

factory = new ParserFactory

factory.makeParser("example.html").type # => "HTML parser"

factory.makeParser("example.md").type # => "Markdown parser"

factory.makeParser("example.json").type # => "JSON parser"

Discussion

In the example, you can ignore the specifics of the file’s format and focus on the parsed content. A more advanced Factory Method might, for instance, also search for versioning data within the file itself before returning a more precise parser (e.g. an HTML5 parser instead of an HTML v4 parser).