Poorman's Ruby Spy

poorman_ruby_spy.rb

class Foo
	def bar(val, key:)
		puts("BAR #{val} #{key}")
	end
end

def spy_on(object, method)
	did_call = false
	aliased = "#{method}_spied".to_sym

	object.class.instance_eval do
		alias_method aliased, method
	end

	object.instance_eval do
		self.class.define_method(method) do |*args, **kwargs|
			did_call = true
			send(aliased, *args, **kwargs)
		end
	end

	yield

	raise unless did_call
end

foo = Foo.new
spy_on(foo, :bar) do
	foo.bar(42, key: "real")
end