# ExWaiter

Helper for waiting on asynchronous conditions to be met.

Hexdocs found at

## Installation

Add the latest release to your `mix.exs` file:

defp deps do
    {:ex_waiter, "~> 0.2.1"}

Then run `mix deps.get` in your shell to fetch the dependencies.

## Why This Exists?

In some testing scenarios there is no obvious way to ensure that asynchronous
side effects have taken place without continuously checking for successful
completion. For example, perhaps an assertion is needed on click data being
asynchronously persisted to the database. It is not difficult to write a
recursive function to handle this one-off, but there is a bit of ceremony

Additionally, perhaps it is desirable to configure the amount of delay prior
to each check, the total number of attempts, a convention for handling
exhausted retries, an easy way to inject callbacks, and a record of the
history of each attempt.

This simple package provides all that and more! Well, actually just that.

## A Walkthrough

The package provides `await/2` and `await!/2` functions. Each requires an
anonymous function that may return `{:ok, value}`, `:ok`, or `true` for a
successful attempt or `{:error, value}`, `:error`, or `false` for a failed
attempt. The tagged tuples must be used if you need a return value or want
to track the history of value changes. Additional options are available for
setting the desired number of attempts and custom delay between attempts.

Let's use `await!/2` to check the database for the most recently persisted

click = ExWaiter.await!(fn ->
  case Clicks.most_recent() do
    %Click{} = click ->
      {:ok, click}

    value ->
      # This is a good place for a callback you might want to run each
      # time the condition is unmet (e.g. flushing jobs).
      {:error, value}


By default, this will check the database up to 5 times spaced out over 150ms.
If, at some point, the condition is met, the `%Click{}` will be returned. If
retries are exhausted, an exception will be raised that looks something like:

 ** (ExWaiter.Exceptions.RetriesExhausted) Tried 5 times over 150ms, but condition was never met.

   attempt_num: 5,
   attempts: [
     %ExWaiter.Attempt{attempt_num: 1, delay_before: 10, fulfilled?: false, value: nil},
     %ExWaiter.Attempt{attempt_num: 2, delay_before: 20, fulfilled?: false, value: nil},
     %ExWaiter.Attempt{attempt_num: 3, delay_before: 30, fulfilled?: false, value: nil},
     %ExWaiter.Attempt{attempt_num: 4, delay_before: 40, fulfilled?: false, value: nil},
     %ExWaiter.Attempt{attempt_num: 5, delay_before: 50, fulfilled?: false, value: nil},
   attempts_left: 0,
   delay_before_fn: #Function<...>,
   fulfilled?: false,
   checker_fn: #Function<...>,
   num_attempts: 5,
   total_delay: 150,
   value: nil

This displays a `Waiter` struct, which includes a recording of everything
that happened during attempts.

The `await/2` function would return either `{:ok, %Click{}, %Waiter{}}` or
`{:error, nil, %Waiter}`. It can be helpful to inspect this `Waiter`
struct for debugging and optics into timing.

### Additional Options

* `:delay_before_fn` - takes a function that receives the `%Waiter{}` struct at
  that moment and returns a number of milliseconds to delay prior to performing
  the next attempt. The default is `fn waiter -> waiter.attempt_num * 10 end`.
* `:num_attempts` - The number of attempts before retries are exhausted.
  (default: 5)