In Object-Oriented Programming, one object will often depend on another object in order to function.
For example, if I create a simple class to run finance reports:
class FinanceReport def net_income FinanceApi.gross_income - FinanceApi.total_costs end end
We can say that
FinanceReport depends on
FinanceApi, which it uses to pull information from an external payment processor.
But what if we want to hit a different API at some point? Or, more likely, what if we want to test this class without hitting external resources? The most common answer is to use Dependency Injection.
With Dependency Injection, we don't explicitly refer to
FinanceReport. Instead, we pass it in as an argument. We inject it.
Using Dependency Injection, our class becomes:
class FinanceReport def net_income(financials) financials.gross_income - financials.total_costs end end
Now our class has no knowledge that the
FinanceApi object even exists! We can pass any object to it as long as it implements
This has a number of benefits:
- Our code is now less “coupled” to
- We're forced to use
FinanceApivia a public interface.
- We can now pass in a mock or stub object in our tests so that we don't have to hit the real API.
Most developers consider Dependency Injection to be a good thing in general (me too!). However, as with all techniques, there are trade-offs.
Our code is slightly more opaque now. When we explicitly used
FinanceApi, it was clear where our values were coming from. It's not quite as clear in the code incorporating Dependency Injection.
If the calls would otherwise have gone to
self, then we have made the code more verbose. Instead of using the Object-Oriented “send a message to an object and let it act” paradigm, we find ourselves moving to a more functional “inputs -> outputs” paradigm.
It is this last case (redirecting calls that would have gone to
self) that I want to look at today. I want to present a possible alternative to Dependency Injection for these situations: change the base class dynamically (kinda).
The Problem to Solve
Let’s back up a moment and start with the problem that led me down this path to start with: PDF reports.
My client requested the ability to generate various printable PDF reports — one report listing all expenses for an account, another listing revenue, another the forecasted profits for future years, etc.
We’re using the venerable
prawn gem to create these PDFs, with each report being its own Ruby object subclassed from
class CostReport < Prawn::Document def initialize(...) ... end def render text "Cost Report" move_down 20 ... end
So far, so good. But here’s the rub: the client wants an “Overview” report that includes portions from all these other reports.
Solution 1: Dependency Injection
As mentioned previously, one common solution to this kind of problem is to refactor the code to use Dependency Injection. That is, rather than having all these reports call methods on
self, we will instead pass in our PDF document as an argument.
This would give us something more like:
class CostReport < Prawn::Document ... def title(pdf = self) pdf.text "Cost Report" pdf.move_down 20 ... end end
This works, but there is some overhead here. For one thing, every single drawing method now has to take the
prawn now has to go through this
Dependency injection has some benefits: it pushes us toward decoupled components in our system and allows us to pass in mocks or stubs to make unit testing easier.
However, we are not reaping the rewards of these benefits in our scenario. We are already strongly coupled to the
prawn API, so changing to a different PDF library would almost certainly require an entire rewrite of the code.
Testing is also not a big concern here, because in our case testing generated PDF reports with automated tests is too cumbersome to be worthwhile.
So Dependency Injection gives us the behavior we want but also introduces additional overhead with minimal benefits for us. Let’s have a look at another option.
Solution 2: Delegation
Ruby’s standard library provides us
SimpleDelegator as an easy way to implement the decorator pattern. You pass in your object to the constructor, and then any method calls to the delegator are forwarded to your object.
SimpleDelegator, we can create a base report class that wraps around
class PrawnWrapper < SimpleDelegator def initialize(document: nil) document ||= Prawn::Document.new(...) super(document) end end
We can then update our reports to inherit from this class, and they will still function the same as before, using the default document created in our initializer. The magic happens when we use this in our overview report:
class OverviewReport < PrawnWrapper ... def render sales = SaleReport.new(..., document: self) sales.sales_table costs = CostReport.new(..., document: self) costs.costs_pie_chart ... end end
CostReport#costs_pie_chart remain unchanged, but their calls to
move_down 20, etc.) are now being forwarded to
OverviewReport via the
SimpleDelegator we created.
In terms of behavior, we have essentially made it as if
SalesReport is now a subclass of
OverviewReport. In our case, this means that all the calls to
prawn’s API now go
SalesReport -> OverviewReport -> Prawn::Document.
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How SimpleDelegator Works
SimpleDelegator works under the hood is basically to use Ruby's
method_missing functionality to forward method calls to another object.
SimpleDelegator (or a subclass of it) receives a method call. If it implements that method, great; it will execute it just as any other object would. However, it if does not have that method defined, then it will hit
method_missing will then attempt to
call that method on the object given to its constructor.
A simple example:
require 'simple_delegator' class Thing def one 'one' end def two 'two' end end class ThingDecorator < SimpleDelegator def two 'three!' end end ThingDecorator.new(Thing.new).one #=> "one" ThingDecorator.new(Thing.new).two #=> "three!"
SimpleDelegator with our own
ThingDecorator class here, we can overwrite some methods and let others fall through to the default
The trivial example above doesn’t really do
SimpleDelegator justice, though. You may look at this code and very well say to me, “Doesn’t subclassing
Thing give me the same outcome?”
Yes, yes it does. But here’s the key difference:
SimpleDelegator takes the object it will delegate to as an argument in its constructor. This means we can pass in different objects at runtime.
This is what allows use to redirect the calls to a
prawn object in Solution 2 above. If we call a single report, the
prawn calls go to a new document created in the constructor. The overview report, however, can change this so that calls to
prawn are forwarded to its document.
Dependency Injection is probably the best solution to most decoupling problems most of the time.
As with all techniques, however, there are trade-offs. In my case, I didn’t think that the overhead introduced by DI was worth the benefits it provided, so I looked for another solution.
As with all things in Ruby, there’s always another way. I wouldn’t reach for this solution often, but it is certainly a nice addition to your Ruby toolbelt for these situations.