Refactoring (#75)


#1

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  1. Please do not post any solutions or spoiler discussion for this quiz
    until
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by Pat Eyler

This week’s quiz is a bit of a departure from the normal. Instead of
submitting
different implementations of the same code, we’d like you to submit
different
implementations of the same process – Refactoring.

Refactoring is the art of improving the design of existing code, without
changing it’s functional behaviour. It is well documented in the book
‘Refactoring’ by Martin F., and on his website:

http://www.refactoring.com

The quiz this week is to submit refactorings of code you use – whether
your own
code, a library or application for RubyForge, or even something from the
Standard Library. Any submission should implement a refactoring from:

http://www.refactoring.com/catalog/index.html

or other citable sources, e.g.:

http://kallokain.blogspot.com/2006/01/refactoring-extract-mixin.html

Each Submission should follow this outline:

Refactoring name (and citation if needed)
Original code
Explanation of the purpose and mechanics of the refactoring
New code
(optionally, unit tests created/used to verify the code)

Submissions will be combined into an online catalog of Ruby
Refactorings,
probably on the RubyGarden wiki.


#2

On 4/14/06, Ruby Q. removed_email_address@domain.invalid wrote:

Submissions will be combined into an online catalog of Ruby Refactorings,
probably on the RubyGarden wiki.

I’ve created a page on the wiki to hold refactorings as they are
submitted.

http://www.rubygarden.org/ruby?RubyRefactoringCatalog


#3

On Apr 14, 2006, at 7:46 AM, Ruby Q. wrote:

by Pat Eyler

This week’s quiz is a bit of a departure from the normal. Instead
of submitting
different implementations of the same code, we’d like you to submit
different
implementations of the same process – Refactoring.

I’ll start us off this week. (Thanks for the easy but educational
quiz Pat!)

James Edward G. II

Replace Conditional with Polymorphism

This is a traditional refactoring pattern located on page 255 of my
copy of the book this quiz is named after. It can also be found on
the [Refactoring Catalog site](http://www.refactoring.com/catalog/
replaceConditionalWithPolymorphism.html).

Smelly Code

This is actual code from a Rails application we are building at
work. I noticed the problematic method when bug hunting in the
system the same day I had been reading in Refactoring. (This method
is not related to the bug I was after, it works fine.)

Here are the relevant pieces of the three classes involved:

 class TimePeriod < ActiveRecord::Base
   #------------------
   # VALIDATIONS
   #------------------
   validates_presence_of :name, :start_date, :end_date

   #------------------
   # INSTANCE METHODS
   #------------------
   #
   # Returns +true+ if the passed +date+ is in the TimePeriod,

but not in any
# attached closed day ranges.
#
def include?( date )
case self
when ClosedPeriod
(start_date…end_date).include?(date)
else
(start_date…end_date).include?(date) and
not closed_periods.any? { |closed| closed.include?(date) }
end
end
end

 class SchedulePeriod < TimePeriod
   #------------------
   # ASSOCIATIONS
   #------------------
   has_many :closed_periods, :dependent => :destroy
 end

 class ClosedPeriod < TimePeriod

 end

These classes are used to represent simple time spans in our
application. A SchedulePeriod might represent a semester at a
university, for example, which could have a related ClosedPeriod for
Spring Break. (These are stored in the database in the time_periods
table, making use of ActiveRecord’s Single Table Inheritance.)

What’s Wrong Here and Why “Fix” It?

Obviously, the method I intend to refactor is the only one shown
TimePeriod#include?. This method displays the code smell of a case
statement used to determine action based on object types. Message
dispatch like this is Ruby’s domain and not suited for case
statements. I will use Replace Conditional with Polymorphism to
eliminate this.

It might be interesting to note that this method got to the current
state through a very natural process of code decay. When the
database was first being established, we threw together a quick
TimePeriod class that looked something like:

 class TimePeriod < ActiveRecord::Base
   #------------------
   # ASSOCIATIONS
   #------------------
   has_many :closed_periods, :class_name  => "TimePeriod",
                             :foriegn_key => "closed_period_id",
                             :dependent   => :destroy

   #------------------
   # VALIDATIONS
   #------------------
   validates_presence_of :name, :start_date, :end_date

   #------------------
   # INSTANCE METHODS
   #------------------
   #
   # Returns +true+ if the passed +date+ is in the TimePeriod,

but not in any
# attached closed day ranges.
#
def include?( date )
(start_date…end_date).include?(date) and
not closed_periods.any? { |closed| closed.include?(date) }
end
end

This is the same thing, but tracked using only one object that can
have “closed_periods” of its own type. This worked just like the
current version does. However, when we began working through the
controllers to create, find, and relate these objects, it became
apparent that it would be easier on us programmers to keep everything
straight if we broke them down into separate objects. Thus the
change and the introduction of the smelly method.

Refactored Code

Looking at the refactoring check list, I see that I first need to
cover the method with tests to ensure that I won’t break anything.
Luckily, these were already in place for the code. Here is one such
test, just to give you an idea of how they look:

 spring_break = TimePeriod.find_by_name("Spring Break")
 current = Date.civil(2006, 3, 1)
 loop do
   # all days in March should validate, except for Spring Break
   assert_equal( !spring_break.include?(current),
                 time_periods(:march).include?(current) )

   current += 1
   break if current.month == 4
 end

Now according to the refactoring catalog, I need to do two things.
The first is: “Move each leg of the conditional to an overriding
method in a subclass.” That doesn’t sound too tough. Here’s the
first such move:

 class ClosedPeriod < TimePeriod
   #------------------
   # INSTANCE METHODS
   #------------------
   # Returns +true+ if the passed +date+ is in the ClosedPeriod.
   def include?( date )
     (start_date..end_date).include?(date)
   end
 end

I ran the tests and nothing broke. I’m now ready to move the second
leg:

 class SchedulePeriod < TimePeriod
   #------------------
   # ASSOCIATIONS
   #------------------
   has_many :closed_periods, :dependent => :destroy

   #------------------
   # INSTANCE METHODS
   #------------------
   #
   # Returns +true+ if the passed +date+ is in the

SchedulePeriod, but not in
# any attached closed day ranges.
#
def include?( date )
(start_date…end_date).include?(date) and
not closed_periods.any? { |closed| closed.include?(date) }
end
end

Again the tests gave the green light. All is good.

The second piece to this refactoring is to: “Make the original
method abstract.” I considered that for a brief instance, mentally
translating the Javaism into Ruby. My decision made, I highlighted
TimePeriod#include? and tapped the delete key on my keyboard. I
briefly considered having it raise an exception, but Ruby is going to
do that for me anyway and this was less work.

The tests still pass. Refactoring complete.

You may want to do another refactoring to move the duplicated date
range test ((start_date..end_date).include?(date)) out of the two
methods, but that’s a different pattern and probably not too critical
in this example.


#4

Hi Ruby Q.,

Sorry, this is likely to be rather uninteresting, and I’m not even quite
following the rules, I think, but here is a refactoring that’s not on
(Fowler’s) official lists, even though it applies to Java as much as to
Ruby, that I did the other day; let’s call them “Merge Redundant
Exception Handlers.” Also, there’s the more Ruby-exclusive “Remove
Unused Scope”

This code is from the Gem Server tutorial for win32/service on
RubyForge. It’s by Daniel B., and refactored publicly without his
permission; I hope he doesn’t mind.
http://rubyforge.org/docman/view.php/85/126/gemserver_tutorial.txt

def service_main
begin
@s.mount_proc("/yaml") { |req, res|
res[‘content-type’] = ‘text/plain’
res.body <<
Gem::Cache.from_installed_gems(File.join(Gem.dir,
“specifications”)).to_yaml
}
rescue Exception => e
File.open(@logfile,‘w+’){ |f| f.puts “Start failed: #{e}” }
service_stop
end

  begin
     @s.mount_proc("/") { |req, res|
        specs = []
        specifications_dir = File.join(Gem.dir, "specifications")
        Gem::Cache.from_installed_gems(specifications_dir).each do

|path, spec|
specs << {
“name” => spec.name,
“version” => spec.version,
“full_name” => spec.full_name,
“summary” => spec.summary,
“rdoc_installed” =>
Gem::DocManager.new(spec).rdoc_installed?,
“doc_path” => (’/doc_root/’ + spec.full_name +
‘/rdoc/index.html’)
}
end
specs = specs.sort_by { |spec| [spec[“name”].downcase,
spec[“version”]] }
template = TemplatePage.new(DOC_TEMPLATE)
res[‘content-type’] = ‘text/html’
template.write_html_on(res.body, {“specs” => specs})
}
rescue Exception => e
File.open(@logfile,‘w+’){ |f| f.puts “Start failed: #{e}” }
service_stop
end

  begin
     {"/gems" => "/cache/", "/doc_root" => "/doc/"}.each do

|mount_point, mount_dir|
@s.mount(mount_point, WEBrick::HTTPServlet::FileHandler,
File.join(Gem.dir, mount_dir), true)
end
rescue Exception => e
File.open(@logfile,‘w+’){ |f| f.puts “Start failed: #{e}” }
service_stop
end

  begin
     @s.start
  rescue Exception => e
     File.open(@logfile,'w+'){ |f| f.puts "Start failed: #{e}" }
     service_stop
  end

end

Refactoring 1: Merge Redundant Exception Handlers
/You have a series of exception handling blocks with identical
handlers./
Merge the code into a single exception handling block.

Thus:

begin
foo
rescue Exception => e
barf e
end
begin
bar
rescue Exception => e
barf e
end

Becomes:

begin
foo
bar
rescue Exception => e
barf e
end

Refactoring 2: Remove Unused Scope
/You have a begin…end block that introduces a scope that is unused./
Remove the unused scope.

Thus:

def foo
begin
bar
rescue
baz
end
end

Becomes:

def foo
bar
rescue
baz
end

Here is the refactored code:

def service_main
@s.mount_proc("/yaml") { |req, res|
res[‘content-type’] = ‘text/plain’
res.body << Gem::Cache.from_installed_gems(File.join(Gem.dir,
“specifications”)).to_yaml
}

  @s.mount_proc("/") { |req, res|
     specs = []
     specifications_dir = File.join(Gem.dir, "specifications")
     Gem::Cache.from_installed_gems(specifications_dir).each do

|path, spec|
specs << {
“name” => spec.name,
“version” => spec.version,
“full_name” => spec.full_name,
“summary” => spec.summary,
“rdoc_installed” =>
Gem::DocManager.new(spec).rdoc_installed?,
“doc_path” => (’/doc_root/’ + spec.full_name +
‘/rdoc/index.html’)
}
end
specs = specs.sort_by { |spec| [spec[“name”].downcase,
spec[“version”]] }
template = TemplatePage.new(DOC_TEMPLATE)
res[‘content-type’] = ‘text/html’
template.write_html_on(res.body, {“specs” => specs})
}

  {"/gems" => "/cache/", "/doc_root" => "/doc/"}.each do

|mount_point, mount_dir|
@s.mount(mount_point, WEBrick::HTTPServlet::FileHandler,
File.join(Gem.dir, mount_dir), true)
end

  @s.start

rescue Exception => e
File.open(@logfile,‘w+’){ |f| f.puts “Start failed: #{e}” }
service_stop
end

Cheers,
Dave


#5

Dave B. wrote:

Hi Ruby Q.,

Sorry, this is likely to be rather uninteresting,

Actually, I find it very interesting … but then, you have to consider
my tastes. :slight_smile:

Refactoring 1: Merge Redundant Exception Handlers
/You have a series of exception handling blocks with identical
handlers./
Merge the code into a single exception handling block.

Thus:

begin
foo
rescue Exception => e
barf e
end
begin
bar
rescue Exception => e
barf e
end

Becomes:

begin
foo
bar
rescue Exception => e
barf e
end

You need to be careful here. If ‘barf’ does not thrown an exception,
but merely prints an error message of some kind, then the refactoring
changes the behavior (which technically, makes it not a refactoring).

For example, from some (simplified) gem code I was working on just last
week:

def generate_docs
begin
generate_ri_docs
rescue Exception => e
puts “Problems in RI docs”
end
begin
generate_rdocs
rescue Exception => e
puts “Problems in RDocs”
end
end

Merging the exception blocks in this case would mean that errors in
generating the RI documents would skip the generation of the RDocs.
Definitatly not a good thing.


– Jim W.

Refactoring 2: Remove Unused Scope
/You have a begin…end block that introduces a scope that is unused./
Remove the unused scope.

Thus:

def foo
begin
bar
rescue
baz
end
end

Becomes:

def foo
bar
rescue
baz
end

Here is the refactored code:

def service_main
@s.mount_proc("/yaml") { |req, res|
res[‘content-type’] = ‘text/plain’
res.body << Gem::Cache.from_installed_gems(File.join(Gem.dir,
“specifications”)).to_yaml
}

  @s.mount_proc("/") { |req, res|
     specs = []
     specifications_dir = File.join(Gem.dir, "specifications")
     Gem::Cache.from_installed_gems(specifications_dir).each do

|path, spec|
specs << {
“name” => spec.name,
“version” => spec.version,
“full_name” => spec.full_name,
“summary” => spec.summary,
“rdoc_installed” =>
Gem::DocManager.new(spec).rdoc_installed?,
“doc_path” => (’/doc_root/’ + spec.full_name +
‘/rdoc/index.html’)
}
end
specs = specs.sort_by { |spec| [spec[“name”].downcase,
spec[“version”]] }
template = TemplatePage.new(DOC_TEMPLATE)
res[‘content-type’] = ‘text/html’
template.write_html_on(res.body, {“specs” => specs})
}

  {"/gems" => "/cache/", "/doc_root" => "/doc/"}.each do

|mount_point, mount_dir|
@s.mount(mount_point, WEBrick::HTTPServlet::FileHandler,
File.join(Gem.dir, mount_dir), true)
end

  @s.start

rescue Exception => e
File.open(@logfile,‘w+’){ |f| f.puts “Start failed: #{e}” }
service_stop
end

Cheers,
Dave


#6

If ‘barf’ does not thrown an exception,
but merely prints an error message of some kind, then the refactoring
changes the behavior (which technically, makes it not a refactoring).
Not only printing, anything that causes a side-effect.

–Meador


#7

Jim W. wrote:

handlers./
bar
barf e
end

You need to be careful here. If ‘barf’ does not thrown an exception,
but merely prints an error message of some kind, then the refactoring
changes the behavior (which technically, makes it not a refactoring).

What about using another method and yield here?

def barf_on_exception
yield
rescue Exception => e
barf e
end

barf_on_exception {foo}
barf_on_exception {bar}

Or even:

def on(ex, m)
yield
rescue ex => e
send m, e
end

on(Exception, :barf) {foo}
on(Exception, :barf) {bar}

But I’m not sure that it’s really better than the original exception
clause.


#8

Meador I. wrote:

If ‘barf’ does not thrown an exception,
but merely prints an error message of some kind, then the refactoring
changes the behavior (which technically, makes it not a refactoring).
Not only printing, anything that causes a side-effect.

It’s not about the side-effect at all (I think) it’s about not exiting
the overall scope. I should stop posting in the wee hours.

This might be a correction, but it’s 1:30am, so I can’t be too sure.

begin
foo
rescue Exception => e
handler
end
begin
bar
rescue Exception => e
handler
end

Becomes:

begin
foo
bar
rescue Exception => e
handler
end

ONLY IF handler doesn’t return or does throw an exception.

Does that hold? I’d like to get an actual re-factoring out of this :slight_smile:

Oh, by the way,
Jim said:

Actually, I find it very interesting … but then, you have to
consider my tastes. :slight_smile:

You’re tastes are indeed strange, Jim!

Cheers,
Dave


#9

On 4/14/06, Ruby Q. removed_email_address@domain.invalid wrote:

    http://www.refactoring.com/catalog/index.html
    New code
    (optionally, unit tests created/used to verify the code)

Submissions will be combined into an online catalog of Ruby Refactorings,
probably on the RubyGarden wiki.

Extract method calls (
http://www.theserverside.com/articles/article.tss?l=AspectOrientedRefactoringPart1
)

Consider a cache that needs to be kept up to date.
For example, primitives in a complex vector scene:
redrawing the scene takes a relatively long time,
degrading framerate, but not redrawing the scene
when something changes is even worse. So I need
to detect when the cached rendering is outdated.

To do this, I keep track of the time of the last
rendering, and compare the vector object mtimes
to it. If a mtime is more recent than the last render
time, the scene needs to be redrawn.

Original code:

class VectorObject
attr_reader :x, :y, :z, :path, :stroke, :fill, :mtime

def x= x
@x = x
@mtime = Time.now
end

def y= y
@y = y
@mtime = Time.now
end

def z= z
@z = z
@mtime = Time.now
end

def path= pt
case pt
when Path
@path = pt
when Array
@path = Path.new(pt)
end
@mtime = Time.now
end

… and so on for stroke and fill

end

Detecting repetition, it makes code fragile and a pain to edit.
First pass, refactor @mtime = Time.now out to a method:

class VectorObject
attr_reader :x, :y, :z, :path, :stroke, :fill, :mtime
def touch!
@mtime = Time.now
end

def x= x
@x = x
touch!
end

… similarly for other accessors
end

A bit better. Still quite unwieldy though.
But! Metaprogramming to the rescue!
Alias the original methods to _non_touching and
replace them with a version that calls the original
method, followed by touch!, and returns what the
original method returned:

class VectorObject
def self.touching! *mnames
mnames.each{|mn|
alias_method “#{mn}_non_touching”, mn
define_method(mn){|*args|
rv = send “#{mn}_non_touching”, *args
touch!
rv
}
}
end

attr_reader :mname
attr_accessor :x, :y, :z, :path, :stroke, :fill
touching! :x=, :y=, :z=, :path=, :stroke=, :fill=
end

Finally, move #touching! out from the class and into a module:

module Touchable
def touching! *mnames
mnames.each{|mn|
alias_method “#{mn}_non_touching”, mn
define_method(mn){|*args|
rv = send “#{mn}_non_touching”, *args
touch!
rv
}
end
end

class VectorObject
extend Touchable
attr_reader :mname
attr_accessor :x, :y, :z, :path, :stroke, :fill

def path= pt
case pt
when Path
@path = pt
when Array
@path = Path.new(pt)
end
end

touching! :x=, :y=, :z=, :path=, :stroke=, :fill=
end

Now we can use attr_accessor for creating
the simple methods, override #path= with
a magical setter, and not type touch! a lot.