Composition or inheritance for delegating page methods?

引用链接：http://watirmelon.com/2011/01/24/composition-or-inheritance-for-delegating-page-methods/

Composition or inheritance for delegating page methods?

Posted on January 24, 2011

The thing I like to do when creating a page object pattern for automated web testing is delegating any methods that don’t belong to the Page object itself.

For example, a very simple page object model like this GoogleHomePage doesn’t delegate any methods to the Browser object.

require "rubygems"
require "watir-webdriver"

class GoogleHomePage
  def initialize(browser)
    @browser = browser
  end
  def visit
    @browser.goto "www.google.com"
  end
end

b = Watir::Browser.new :firefox
p = GoogleHomePage.new b
p.visit
puts p.title
p.close

So, the p.title and p.close statements both fail with an error: undefined method `goto' for # (NoMethodError).

One approach would be simply to write appropriate methods for what you would do on the Browser object. For example:

require "rubygems"
require "watir-webdriver"

class GoogleHomePage
  def initialize(browser)
    @browser = browser
  end
  def visit
    @browser.goto "www.google.com"
  end
  def title
    @browser.title
  end
  def close
    @browser.close
  end
end

b = Watir::Browser.new :firefox
p = GoogleHomePage.new b
p.visit
puts p.title
p.close

But this isn’t DRY. It means every method of Browser you access to needs to be rewritten. But I often see this happen.

What we should be doing is simply delegating any methods that don’t exist on the Page object to the Browser object which is passed in at initialization. There are two ways I know of to do this: inheritance delegation and composition.

Inheritance Delegation

Inheritance delegation means changing our class so it delegates appropriately using a DelegateClass. This means anything of class Browser is delegated.

For example:

require "rubygems"
require "watir-webdriver"

class GoogleHomePage < DelegateClass(Watir::Browser) 
  def initialize(browser)
    super(browser)
  end
  def visit
    self.goto "www.google.com"
  end
end

b = Watir::Browser.new :firefox
p = GoogleHomePage.new b
p.visit
puts p.title
p.close

From this point forward you don’t need to refer to @browser, instead you just refer to self in your class.

Composition

Composition is about composing the class of different elements, some of which are passed to the browser. This essentially involves creating a method_missing method and passing these methods to the instance variable @browser.

require "rubygems"
require "watir-webdriver"

class GoogleHomePage
  def initialize(browser)
    @browser = browser
  end
  def method_missing(sym, *args, &block)
    @browser.send sym, *args, &block
  end
  def visit
    @browser.goto "www.google.com"
  end
end

b = Watir::Browser.new :firefox
p = GoogleHomePage.new b
p.visit
puts p.title
p.close

This means that any reference to the Browser object still needs to refer to @browser throughout the class.

Inheritance or Composition?

You can see from the examples above, both approaches are very similar, but from researching these, it seems that most people prefer composition to inheritance in ruby, mainly due to maintainability of class chains. In our example, the inheritance chain is small and very simple, so I don’t think this poses a great maintainability issue.