# `DoubleDown.Contract.Dispatch.StubHandler`
[🔗](https://github.com/mccraigmccraig/double_down/blob/main/lib/double_down/contract/dispatch/stub_handler.ex#L1)

Behaviour for stateless stub handler modules.

Implement this behaviour to make a stateless stub usable by module
name in `DoubleDown.Double.stub/2..4`:

    # Instead of:
    Double.stub(Repo, Repo.Stub.new())

    # Write:
    Double.stub(Repo, Repo.Stub)

    # With a fallback function:
    Double.stub(Repo, Repo.Stub, fn :all, [User] -> [] end)

## Callbacks

  * `new/2` — build a 2-arity dispatch function from a fallback
    function and options. The returned function has the signature
    `fn operation, args -> result end`.

## Example

    defmodule MyApp.TestStore do
      @behaviour DoubleDown.Contract.Dispatch.StubHandler

      @impl true
      def new(fallback_fn, _opts) do
        fn operation, args ->
          case {operation, args} do
            {:get, [id]} -> %{id: id}
            _ when is_function(fallback_fn) -> fallback_fn.(operation, args)
            _ -> raise "unhandled"
          end
        end
      end
    end

# `new`

```elixir
@callback new(
  fallback_fn :: (atom(), [term()] -> term()) | nil,
  opts :: keyword()
) :: (atom(), [term()] -> term())
```

Build a 2-arity dispatch function from a fallback function and options.

  * `fallback_fn` — an optional 2-arity function `(operation, args) -> result`
    for operations the stub doesn't handle directly. `nil` if not provided.
  * `opts` — additional options for configuring the stub.

Returns a 2-arity function `fn operation, args -> result end` suitable
for use as a `Double.stub` function fallback.

---

*Consult [api-reference.md](api-reference.md) for complete listing*