While working on AwesomeResource, I needed to implement functionality that would make the following test pass:
it "creates readers and writers for any attributes passed in during initialization" do
article_class = Class.new do
include AwesomeResource
end
article = article_class.new("title" => "Fun")
article.title.should == "Fun"
article.title = 'Fun Times!'
article.title.should == 'Fun Times!'
expect { article.unknown_attribute }.to raise_exception
end
Simple enough. If you initialize an instance of a class that includes AwesomeResource, then you get attribute readers and writers for any hash keys passed in during initialization.
My first attempt at implementation looked something like this:
module AwesomeResource
attr_reader :awesome_attributes
def initialize(attributes={})
@awesome_attributes = AwesomeResource::Attributes.new attributes
end
def method_missing(method_name, *args)
if method_name["="]
if awesome_attributes.has_key?(method_name[0...-1])
awesome_attributes[method_name[0…-1]] = args.first
else
super
end
else
if awesome_attributes.has_key?(method_name)
awesome_attributes[method_name]
else
super
end
end
end
end
Yuk. All those nested if/else’s didn’t sit well with me. Let me try to clean that up:
def method_missing(method_name, *args)
if method_name["="] && awesome_attributes.has_key?(method_name[0...-1])
awesome_attributes[method_name[0...-1]] = args.first
elsif awesome_attributes.has_key?(method_name)
awesome_attributes[method_name]
else
super
end
end
The best I can say for this code is that it’s more compact. But is it any more readable? Hardly. In fact it’s worse.
Let’s take another look at the first implementation. The nested if/else blocks look awfully similar. Perhaps there’s a polymorphic model lurking there? But what are we modeling? If we were to wrap a class around those blocks, we’d have to be modeling a method. What would that look like? Let’s extract some classes:
class AttributeWriter
attr_reader :attributes
def initialize(attributes)
@attributes = attributes
end
def call(attribute_name, attribute_value)
raise "Unknown Attribute" unless attributes.has_key?(attribute_name)
attributes[attribute_name] = attribute_value
end
end
class AttributeReader
attr_reader :attributes
def initialize(attributes)
@attributes = attributes
end
def call(attribute_name)
raise "Unknown Attribute" unless attributes.has_key?(attribute_name)
attributes[attribute_name]
end
end
def method_missing(method_name, *args)
if method_name["="]
AttributeWriter.new(awesome_attributes).call(method_name[0...-1], *args)
else
AttributeReader.new(awesome_attributes).call(method_name)
end
end
OK, I at least feel like I’m trying to write OO code at this point. There’s a bit of duplication between the AttributeReader and AttributeWriter classes. We could clean that up with template methods:
class AttributeAccessor
attr_reader :attributes, :attribute_name
def initialize(attributes, attribute_name)
@attribute_name = attribute_name
@attributes = attributes
end
def call(*args)
raise "Unknown Attribute" unless attributes.has_key?(attribute_name)
execute(*args)
end
def execute(*)
end
end
class AttributeWriter < AttributeAccessor
def attribute_name
super[0...-1]
end
def execute(attribute_value)
attributes[attribute_name] = attribute_value
end
end
class AttributeReader < AttributeAccessor
def execute
attributes[attribute_name]
end
end
def method_missing(method_name, *args)
if method_name["="]
AttributeWriter.new(awesome_attributes, method_name).call(*args)
else
AttributeReader.new(awesome_attributes, method_name).call
end
end
Notice that we've moved the responsibility of stripping off the "=" off the method_name from method_missing down into AttributeWriter.
Hmmm... The base class is pretty abstract. How easy it this code to understand now?
Also, the body of the method missing now looks a lot like a factory method. While in Rome...
class AttributeAccessor
attr_reader :attributes, :attribute_name
def self.from_method_name(attributes, method_name)
if method_name["="]
AttributeWriter.new(attributes, method_name)
else
AttributeReader.new(attributes, method_name)
end
end
def initialize(attributes, attribute_name)
@attribute_name = attribute_name
@attributes = attributes
end
def call(*args)
raise "Unknown Attribute" unless attributes.has_key?(attribute_name)
execute(*args)
end
def execute(*)
end
end
class AttributeWriter < AttributeAccessor
def attribute_name
super[0...-1]
end
def execute(attribute_value)
attributes[attribute_name] = attribute_value
end
end
class AttributeReader < AttributeAccessor
def execute
attributes[attribute_name]
end
end
def method_missing(method_name, *args)
AttributeAccessor.from_method_name(awesome_attributes, method_name).call(*args)
end
I'm now starting to question this code. Is there a good trade off here between indirection and maintainability? I don't think the number of accessors are likely to grow (accessors have consisted solely of getters and setters since the dawn of OO). Between all of the refactorings, I think the first (with its minor duplication) was the easiest to understand.
Yet something still doesn't feel right. Let's look back at the very first code snippet. Notice the `AwesomeResource::Attributes.new` in the `initialization` method? Those are our bag of attributes (they're basically a hash with indifferent access). We keep passing it around to all of our Attribute* classes… perhaps some of this code should have lived there in the first place?
module AwesomeResource
attr_reader :awesome_attributes
def initialize(attributes={})
@awesome_attributes = AwesomeResource::Attributes.new attributes
end
def method_missing(method_name, *args)
awesome_attributes.accessor_for_method_name(method_name).call(*args)
end
end
module AwesomeResource
class Attributes < SimpleDelegator
#...
def accessor_for_method_name(method_name)
if method_name["="]
->(attribute_value) { self[method_name[0...-1]] = attribute_value }
else
-> { self[method_name] }
end
end
def [](key)
validate_key_exists(key)
attributes[standardized_key(key)]
end
def []=(key, value)
validate_key_exists(key)
attributes[standardized_key(key)] = value
end
private
def validate_key_exists(key)
raise "Unknown attribute" unless has_key? key
end
#...
end
end
This feels right. We're now modeling methods with lambdas (Ruby's built in method objects). We've eliminated a tangle of nested if/else blocks without introducing too much indirection. We've maintained high-cohesion within the AwesomeResource::Attributes class despite the new methods.