Dynamic dispatch facades for existing modules.

Enables Mimic-style bytecode interception — replace any module with a dispatch shim at test time, then use the full DoubleDown.Double API (expects, stubs, fakes, stateful responders, passthrough) without defining a contract or facade.

Setup

Call setup/1 in test/test_helper.exs before ExUnit.start():

DoubleDown.DynamicFacade.setup(MyApp.EctoRepo)
DoubleDown.DynamicFacade.setup(SomeThirdPartyModule)

ExUnit.start()

Setup is lazy: modules are registered for potential shimming but no bytecode manipulation happens until a test installs a handler via DoubleDown.Double.fallback/2, expect/3, or stub/2. This means a test run that never exercises a module pays zero shimming cost.

Usage in tests

setup do
  DoubleDown.Double.fallback(MyApp.EctoRepo, DoubleDown.Repo.OpenInMemory)
  :ok
end

test "insert then get" do
  {:ok, user} = MyApp.EctoRepo.insert(User.changeset(%{name: "Alice"}))
  assert ^user = MyApp.EctoRepo.get(User, user.id)
end

Tests that don't install a handler get the original module's behaviour — zero impact on unrelated tests.

Struct modules

If the original module defines a struct (defstruct), the shim preserves full struct support:

• %Module{} literal syntax works at compile time in tests
• __info__(:struct) returns correct field metadata
• @enforce_keys and default values are preserved
• __struct__/0 and __struct__/1 calls route through dispatch/3, so Double.fallback / Double.expect handlers can intercept struct construction at runtime

Behaviour and macro modules

• @behaviour declarations are copied from the original module to the shim, so behaviour-based dispatch and compliance checks work correctly.
• Macros (defmacro) are proxied via defmacro wrappers that delegate to the original implementation. Macros expand at compile time so they always use the original — they cannot be intercepted by Double handlers.

Constraints

• Call setup/1 before tests start (in test_helper.exs). Bytecode replacement is VM-global; calling it during async tests may cause flaky behaviour.
• Cannot set up dynamic facades for DoubleDown contracts (use DoubleDown.ContractFacade instead), DoubleDown internals, NimbleOwnership, or Erlang/OTP modules.

See also

• DoubleDown.ContractFacade — dispatch facades for defcallback contracts (typed, LSP-friendly, recommended for new code).
• DoubleDown.BehaviourFacade — dispatch facades for vanilla @behaviour modules (typed, but no pre_dispatch or combined contract + facade).