Module: RSpec::Core::MemoizedHelpers::ClassMethods

### Home Classes Methods

### In Files

- lib/rspec/core/memoized_helpers.rb

### Methods

- #let
- #let!
- #subject
- #subject!

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# RSpec::Core::MemoizedHelpers::ClassMethods

This module is extended onto {ExampleGroup}, making the methods available to be called from within example group blocks. You can think of them as being analagous to class macros.

### Public Instance Methods

let(name, &block) click to toggle source

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.

@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` uses an `||=` conditional that has the potential to

    behave in surprising ways in examples that spawn separate threads,
    though we have yet to see this in practice. You've been warned.

@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.

@example

    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

    # File lib/rspec/core/memoized_helpers.rb, line 231
    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?
      MemoizedHelpers.module_for(self).__send__(:define_method, name, &block)

      # 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(name) { |k| __memoized[k] = super(RSpec.current_example, &nil) } }
      else
        define_method(name) { __memoized.fetch(name) { |k| __memoized[k] = super(&nil) } }
      end
    end

let!(name, &block) click to toggle source

Just like `let`, except the block is invoked by an implicit `before` hook. This serves a dual purpose of setting up state and providing a memoized reference to that state.

@example

    class Thing
      def self.count
        @count ||= 0
      end

      def self.count=(val)
        @count += val
      end

      def self.reset_count
        @count = 0
      end

      def initialize
        self.class.count += 1
      end
    end

    describe Thing do
      after(:example) { Thing.reset_count }

      context "using let" do
        let(:thing) { Thing.new }

        it "is not invoked implicitly" do
          Thing.count.should eq(0)
        end

        it "can be invoked explicitly" do
          thing
          Thing.count.should eq(1)
        end
      end

      context "using let!" do
        let!(:thing) { Thing.new }

        it "is invoked implicitly" do
          Thing.count.should eq(1)
        end

        it "returns memoized version on first invocation" do
          thing
          Thing.count.should eq(1)
        end
      end
    end

    # File lib/rspec/core/memoized_helpers.rb, line 299
    def let!(name, &block)
      let(name, &block)
      before { __send__(name) }
    end

subject(name=nil, &block) click to toggle source

Declares a `subject` for an example group which can then be wrapped with `expect` using `is_expected` to make it the target of an expectation in a concise, one-line example.

Given a `name`, defines a method with that name which returns the `subject`. This lets you declare the subject once and access it implicitly in one-liners and explicitly using an intention revealing name.

When given a `name`, calling `super` in the block is not supported.

@param name [String,Symbol] used to define an accessor with an

    intention revealing name

@param block defines the value to be returned by `subject` in examples

@example

    describe CheckingAccount, "with $50" do
      subject { CheckingAccount.new(Money.new(50, :USD)) }
      it { is_expected.to have_a_balance_of(Money.new(50, :USD)) }
      it { is_expected.not_to be_overdrawn }
    end

    describe CheckingAccount, "with a non-zero starting balance" do
      subject(:account) { CheckingAccount.new(Money.new(50, :USD)) }
      it { is_expected.not_to be_overdrawn }
      it "has a balance equal to the starting balance" do
        account.balance.should eq(Money.new(50, :USD))
      end
    end

@see MemoizedHelpers#should @see MemoizedHelpers#should_not @see MemoizedHelpers#is_expected

    # File lib/rspec/core/memoized_helpers.rb, line 338
    def subject(name=nil, &block)
      if name
        let(name, &block)
        alias_method :subject, name

        self::NamedSubjectPreventSuper.__send__(:define_method, name) do
          raise NotImplementedError, "`super` in named subjects is not supported"
        end
      else
        let(:subject, &block)
      end
    end

subject!(name=nil, &block) click to toggle source

Just like `subject`, except the block is invoked by an implicit `before` hook. This serves a dual purpose of setting up state and providing a memoized reference to that state.

@example

    class Thing
      def self.count
        @count ||= 0
      end

      def self.count=(val)
        @count += val
      end

      def self.reset_count
        @count = 0
      end

      def initialize
        self.class.count += 1
      end
    end

    describe Thing do
      after(:example) { Thing.reset_count }

      context "using subject" do
        subject { Thing.new }

        it "is not invoked implicitly" do
          Thing.count.should eq(0)
        end

        it "can be invoked explicitly" do
          subject
          Thing.count.should eq(1)
        end
      end

      context "using subject!" do
        subject!(:thing) { Thing.new }

        it "is invoked implicitly" do
          Thing.count.should eq(1)
        end

        it "returns memoized version on first invocation" do
          subject
          Thing.count.should eq(1)
        end
      end
    end

    # File lib/rspec/core/memoized_helpers.rb, line 404
    def subject!(name=nil, &block)
      subject(name, &block)
      before { subject }
    end

[Validate]

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