# `GenServerVirtualTime`
[🔗](https://github.com/d-led/gen_server_virtual_time/blob/v1.1.1/packages/gen_server_virtual_time/lib/gen_server_virtual_time.ex#L1)

Virtual time for `GenServer` and `:gen_statem`: test time-dependent processes
by advancing a clock a test owns, instead of sleeping through the timeline.

This package is the clock and the OTP wrappers, and nothing else. The rest of
the library is published separately, with the same module names, so what you
need is a dependency rather than a rewrite:

| Package | Add it when |
| --- | --- |
| `gen_server_virtual_time` (this one) | you test code with timers |
| `actor_simulation` | you model parts of a system talking to each other |
| `actor_simulation_codegen` | you want the same model in C++, Go, Pony, Rust, Java or OMNeT++ |

`mix virtual_time.doctor` reads a project and says which of them a missing
module needs.

## Quick Example

    # Define a time-dependent GenServer
    defmodule MyServer do
      use VirtualTimeGenServer

      def init(_) do
        VirtualTimeGenServer.send_after(self(), :tick, 1000)
        {:ok, %{count: 0}}
      end

      def handle_info(:tick, state) do
        {:noreply, %{state | count: state.count + 1}}
      end
    end

    # Test it with virtual time (instant!)
    test "ticks 10 times in 10 seconds" do
      {:ok, clock} = VirtualClock.start_link()
      VirtualTimeGenServer.set_virtual_clock(clock)

      {:ok, server} = MyServer.start_link(nil)
      VirtualClock.advance(clock, 10_000)  # Instant!

      assert get_count(server) == 10
    end

## Modules

- `VirtualClock` - the virtual timeline and its scheduled events
- `VirtualTimeGenServer` - a `GenServer` that schedules on that timeline
- `VirtualTimeGenStateMachine` - the same for `:gen_statem`
- `VirtualTimeGenServer.Instrumentation` - the port for measuring a server
- `VirtualTimeGenServer.Doctor` - reports modules a project is missing

See the README for examples and the design docs for the clock.

# `version`

Returns the version of the library.

---

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