Hi
Warning: this goes on quite a bit. It contains early-morning caffeinated
ramblings and many "hmmm I wonder what this does..." snippets.
I'm looking for the best way to parameterise shared examples. Imagine (as an
academic example...) you were doing it for subclasses of Struct instances (a
more realistic example might be ActiveRecord subclasses, or DataMapper
resources), such as:
class MyStruct < Struct.new(:a, :b)
end
class MyOtherStruct < Struct.new(:foo, :bar)
end
I've seen it done with #let, eg:
shared_examples_for "a Struct" do
it "has methods" do
properties.each do |property|
struct.should respond_to(property)
end
end
end
describe MyStruct do
let(:struct) { MyStruct.new }
let(:properties) { [:a, :b] }
it_should_behave_like "a Struct"
end
describe MyOtherStruct do
let(:struct) { MyOtherStruct.new }
let(:properties) { [:foo, :bar] }
it_should_behave_like "a Struct"
end
Which is not a bad solution, but does feel a bit too much like using (scoped)
global variables for my liking. There's no explicit association between the
shared examples and their parameters (and the arguments actually passed in each
example group.
So I started to wonder if this could be done with metadata. My first naive
stab was this:
describe MyStruct do
it_should_behave_like "a Struct", properties: [:a, :b]
end
But this fails:
Could not find shared example group named {:properties=>[:a, :b]}
Anyway, I dug in a bit and found that the metadata is only available to the
example group anyway, not the examples themselves. So you can't do:
describe MyStruct, properties: [:a, :b] do
let(:struct) { MyStruct.new }
it "has methods" do
metadata[:properties].each do |property|
struct.should respond_to(property)
end
end
end
But (more digging), you can do this:
describe MyStruct, properties: [:a, :b] do
let(:struct) { MyStruct.new }
it "has methods" do
example.metadata[:properties].each do |property|
struct.should respond_to(property)
end
end
end
Which means I can get this close to my original dreamed-up syntax:
shared_examples_for "a Struct with metadata" do
it "has methods" do
example.metadata[:properties].each do |property|
struct.should respond_to(property)
end
end
end
describe MyStruct, properties: [:a, :b] do
let(:struct) { MyStruct.new }
it_should_behave_like "a Struct with metadata"
end
I don't object so much to having "struct" floating around, as it's fairly safe
to say all the shared examples will depend on #struct being available.
Although, arguably, #subject would be better:
shared_examples_for "a subject Struct with metadata" do
it "has methods" do
example.metadata[:properties].each do |property|
subject.should respond_to(property)
end
end
end
describe MyStruct, properties: [:a, :b] do
subject { MyStruct.new }
it_should_behave_like "a subject Struct with metadata"
end
or even:
shared_examples_for "a subject Struct with metadata" do
metadata[:properties].each do |property|
it { should respond_to(property) }
end
end
describe MyStruct, properties: [:a, :b] do
subject { MyStruct.new }
it_should_behave_like "a subject Struct with metadata"
end
I tried to be a bit clever to see if I could clean up the example definitions
in the shared spec, but I got this far before hitting weirdness that was beyond
my understanding of RSpec (and the reach of my spade...). But, this was a bit
of a tangent anyway:
shared_examples_for "a subject Struct with metadata" do
metadata[:params].each { |key, value| define_method(key) { value } }
p self.inspect # outputs nil (!!!)
properties.each do |property|
it { should respond_to(property) }
end
end
describe MyStruct, params: {properties: [:a, :b]} do
subject { MyStruct.new }
it_should_behave_like "a subject Struct with metadata"
end
Sooooo... after all this, I just wondered if anyone had any ideas what the best
way to achieve this is, and how it could be extended.
For example, would there be any merit in being able to write:
it_should_behave_like "a Struct", properties: [:a, :b]
?
Also I figure that as the metadata system is new, it's potentially unfinished
and/or in flux. What are the plans/intentions/opportunities for expansion for
it?
Cheers
Ash
--
http://www.patchspace.co.uk/
http://www.linkedin.com/in/ashleymoran
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