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

Behaviour for stateful fake handler modules.

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

    # Instead of:
    Double.fake(Repo, &Repo.OpenInMemory.dispatch/4, Repo.OpenInMemory.new())

    # Write:
    Double.fake(Repo, Repo.OpenInMemory)

## Callbacks

  * `new/2` — build initial state from seed data and options
  * `dispatch/4` — stateful handler `(contract, operation, args, state) -> {result, new_state}`
  * `dispatch/5` — stateful handler with cross-contract state access

Implement either `dispatch/4` or `dispatch/5` (or both). When both
are implemented, `dispatch/5` takes priority.

## Example

    defmodule MyApp.InMemoryStore do
      @behaviour DoubleDown.Contract.Dispatch.FakeHandler

      @impl true
      def new(seed, _opts), do: seed

      @impl true
      def dispatch(_contract, :get, [id], state), do: {Map.get(state, id), state}
      def dispatch(_contract, :put, [id, val], state), do: {:ok, Map.put(state, id, val)}
    end

# `dispatch`
*optional* 

```elixir
@callback dispatch(
  contract :: module(),
  operation :: atom(),
  args :: [term()],
  state :: term()
) :: {term(), term()}
```

Stateful dispatch handler.

Receives the contract module, operation name, argument list, and
current state. Returns `{result, new_state}`.

# `dispatch`
*optional* 

```elixir
@callback dispatch(
  contract :: module(),
  operation :: atom(),
  args :: [term()],
  state :: term(),
  all_states :: map()
) :: {term(), term()}
```

Stateful dispatch handler with cross-contract state access.

Same as `dispatch/4` but receives a read-only snapshot of all
contract states as the 5th argument.

# `new`

```elixir
@callback new(seed :: term(), opts :: keyword()) :: term()
```

Build initial state from seed data and options.

Called by `Double.fake/2..4` to construct the initial state for the
stateful handler.

  * `seed` — seed data (e.g. `%{User => %{1 => %User{}}}` for Repo.OpenInMemory)
  * `opts` — additional options (e.g. `fallback_fn: fn ... end`)

---

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