Method: RSpec::Core::MemoizedHelpers::ClassMethods#let

Defined in:
lib/rspec/core/memoized_helpers.rb

#let(name, &block) ⇒ void

Note:

let can enhance readability when used sparingly (1,2, or maybe 3 declarations) in any given example group, but that can quickly degrade with overuse. YMMV.

Note:

let can be configured to be threadsafe or not. If it is threadsafe, it will take longer to access the value. If it is not threadsafe, it may behave in surprising ways in examples that spawn separate threads. Specify this on RSpec.configure

Note:

Because let is designed to create state that is reset between each example, and before(:context) is designed to setup state that is shared across all examples in an example group, let is not intended to be used in a before(:context) hook.

Generates a method whose return value is memoized after the first call. Useful for reducing duplication between examples that assign values to the same local variable.

Examples:


RSpec.describe Thing do
  let(:thing) { Thing.new }

  it "does something" do
    # First invocation, executes block, memoizes and returns result.
    thing.do_something

    # Second invocation, returns the memoized value.
    thing.should be_something
  end
end


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# File 'lib/rspec/core/memoized_helpers.rb', line 306

def let(name, &block)
  # We have to pass the block directly to `define_method` to
  # allow it to use method constructs like `super` and `return`.
  raise "#let or #subject called without a block" if block.nil?

  # A list of reserved words that can't be used as a name for a memoized helper
  # Matches for both symbols and passed strings
  if [:initialize, :to_s].include?(name.to_sym)
    raise ArgumentError, "#let or #subject called with reserved name `#{name}`"
  end

  our_module = MemoizedHelpers.module_for(self)

  # If we have a module clash in our helper module
  # then we need to remove it to prevent a warning.
  #
  # Note we do not check ancestor modules (see: `instance_methods(false)`)
  # as we can override them.
  if our_module.instance_methods(false).include?(name)
    our_module.__send__(:remove_method, name)
  end
  our_module.__send__(:define_method, name, &block)

  # If we have a module clash in the example module
  # then we need to remove it to prevent a warning.
  #
  # Note we do not check ancestor modules (see: `instance_methods(false)`)
  # as we can override them.
  if instance_methods(false).include?(name)
    remove_method(name)
  end

  # Apply the memoization. The method has been defined in an ancestor
  # module so we can use `super` here to get the value.
  if block.arity == 1
    define_method(name) { __memoized.fetch_or_store(name) { super(RSpec.current_example, &nil) } }
  else
    define_method(name) { __memoized.fetch_or_store(name) { super(&nil) } }
  end
end