Defines ExUnit callbacks.

This module defines both setup_all and setup callbacks, as well as the on_exit/2, start_supervised/2 and stop_supervised/1 functions.

The setup callbacks are defined via macros and each one can optionally receive a map with metadata, usually referred to as context. The callback may optionally put extra data into the context to be used in the tests.

The setup_all callbacks are invoked only once per module, before any test runs. All setup callbacks are run before each test. No callback runs if the test case has no tests or all tests have been filtered out.

start_supervised/2 is used to start processes under a supervisor. The supervisor is linked to the current test process. The supervisor as well as all child processes are guaranteed to terminate before any on_exit/2 callback runs.

on_exit/2 callbacks are registered on demand, usually to undo an action performed by a setup callback. on_exit/2 may also take a reference, allowing callback to be overridden in the future. A registered on_exit/2 callback always runs, while failures in setup and setup_all will stop all remaining setup callbacks from executing.

Finally, setup_all callbacks run in a separate process per module, while all setup callbacks run in the same process as the test itself. on_exit/2 callbacks always run in a separate process, as implied by their name. The test process always exits with reason :shutdown, which means any process linked to the test process will also exit, although asynchronously. Therefore it is preferred to use start_supervised/2 to guarantee synchronous termination.

Here is a run down of the life-cycle of the test process:

1. the test process is spawned
2. it runs setup/2 callbacks
3. it runs the test itself
4. it stops all supervised processes
5. the test process exits with reason :shutdown
6. on_exit/2 callbacks are executed in a separate process

Context

If you return a keyword list, a map, or {:ok, keywords | map} from setup_all, the keyword list/map will be merged into the current context and be available in all subsequent setup_all, setup, and the test itself.

Similarly, returning a keyword list, map, or {:ok, keywords | map} from setup means that the returned keyword list/map will be merged into the current context and be available in all subsequent setup and the test itself.

Returning :ok leaves the context unchanged (both in setup and setup_all callbacks).

Returning anything else from setup_all will force all tests to fail, while a bad response from setup causes the current test to fail.

Examples

defmodule AssertionTest do
  use ExUnit.Case, async: true

  # "setup_all" is called once per module before any test runs
  setup_all do
    IO.puts "Starting AssertionTest"

    # No context is returned here
    :ok
  end

  # "setup" is called before each test
  setup do
    IO.puts "This is a setup callback for #{inspect self()}"

    on_exit fn ->
      IO.puts "This is invoked once the test is done. Process: #{inspect self()}"
    end

    # Returns extra metadata to be merged into context
    [hello: "world"]
  end

  # Same as above, but receives the context as argument
  setup context do
    IO.puts "Setting up: #{context.test}"
    :ok
  end

  # Setups can also invoke a local or imported function that returns a context
  setup :invoke_local_or_imported_function

  test "always pass" do
    assert true
  end

  test "uses metadata from setup", context do
    assert context[:hello] == "world"
  end

  defp invoke_local_or_imported_function(context) do
    [from_named_setup: true]
  end
end