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Builds on the Mox pattern — generates behaviours and dispatch facades from `defcallback` declarations — and adds stateful test doubles powerful enough to test Ecto.Repo operations without a database.
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lib/double_down/dispatch/stateless_handler.ex
defmodule DoubleDown.Dispatch.StatelessHandler do
@moduledoc """
Behaviour for stateless stub handler modules.
Implement this behaviour to make a stateless stub usable by module
name in `DoubleDown.Double.fallback/2..4`:
# Instead of:
Double.fallback(Repo, Repo.Stateless.new())
# Write:
Double.fallback(Repo, Repo.Stateless)
# With a fallback function:
Double.fallback(Repo, Repo.Stateless, fn _contract, :all, [User] -> [] end)
## Callbacks
* `new/2` — build a 3-arity dispatch function from a fallback
function and options. The returned function has the signature
`fn contract, operation, args -> result end`.
## Example
defmodule MyApp.TestStore do
@behaviour DoubleDown.Dispatch.StatelessHandler
@impl true
def new(fallback_fn, _opts) do
fn contract, operation, args ->
case {operation, args} do
{:get, [id]} -> %{id: id}
_ when is_function(fallback_fn) -> fallback_fn.(contract, operation, args)
_ -> raise "unhandled"
end
end
end
end
"""
@doc """
Build a 3-arity dispatch function from a fallback function and options.
* `fallback_fn` — an optional 3-arity function `(contract, operation, args) -> result`
for operations the stub doesn't handle directly. `nil` if not provided.
* `opts` — additional options for configuring the stub.
Returns a 3-arity function `fn contract, operation, args -> result end` suitable
for use as a `Double.fallback` function fallback.
"""
@callback new(
fallback_fn :: DoubleDown.Dispatch.Types.stateless_fun() | nil,
opts :: keyword()
) :: DoubleDown.Dispatch.Types.stateless_fun()
end