Defining a matcher supporting block expectations

When you wish to support block expectations (e.g. expect { ... }.to matcher) with your custom matchers you must specify this. You can do this manually (or determinately based on some logic) by defining a supports_block_expectation? method or by using the DSL's supports_block_expectations shortcut method.

Define a block matcher manually

Given a file named "block_matcher_spec.rb" with:

RSpec::Matchers.define :support_blocks do
  match do |actual|
    actual.is_a? Proc
  end

  def supports_block_expectations?
    true # or some logic
  end
end

RSpec.describe "a custom block matcher" do
  specify { expect { }.to support_blocks }
end

When I run rspec ./block_matcher_spec.rb

Then the example should pass.

Define a block matcher using shortcut

Given a file named "block_matcher_spec.rb" with:

RSpec::Matchers.define :support_blocks do
  match do |actual|
    actual.is_a? Proc
  end

  supports_block_expectations
end

RSpec.describe "a custom block matcher" do
  specify { expect { }.to support_blocks }
end

When I run rspec ./block_matcher_spec.rb

Then the example should pass.

Define a block matcher using shortcut

Given a file named "block_matcher_spec.rb" with:

RSpec::Matchers.define :support_blocks_with_errors do
  match(:notify_expectation_failures => true) do |actual|
    actual.call
    true
  end

  supports_block_expectations
end

RSpec.describe "a custom block matcher" do
  specify do
    expect {
      expect(true).to eq false
    }.to support_blocks_with_errors
  end
end

When I run rspec ./block_matcher_spec.rb

Then it should fail with:

Failures:

  1) a custom block matcher is expected to support blocks with errors
     Failure/Error: expect(true).to eq false

       expected: false
            got: true

       (compared using ==)