TimeTravel

TimeTravel is a record & replay debugger for Phoenix LiveView applications.

TimeTravel allows you to record the state of your LiveView as you interact with the page and replay the interactions while viewing the state of the socket's assigns at any given point in time.

By attaching to the Telemetry events Phoenix LiveView emits we are able to construct a timeline of events & state to rewind and replay as many times as you like in order to track down bugs in the socket's state.

TimeTravel was inspired by Elm Reactor and the rr-project

Demo

See the Time Travel Demo Repo for a working example

See a short example video here:

https://user-images.githubusercontent.com/18315178/209589137-0bf0bec3-ebdc-4983-aeb6-7f2e25825ef5.mov

Installation

The package can be installed by adding time_travel to your list of dependencies in mix.exs:

def deps do
  [
    {:time_travel, "~> 0.3"}
  ]
end

Steps

1. Download and install the Chrome Extension

2. Add :time_travel as an Elixir dependency in mix.exs (see above)

3. (If you have already set up Phoenix Channels) Add the lvdbg channel specification in your_app_web/channels/user_socket.ex:

## Channels
channel "lvdbg:*", TimeTravel.LiveViewDebugChannel

• If you have not set up a Phoenix socket, run mix phx.gen.socket User
• Make sure you have a socket configuration set up in lib/your_app_web/endpoint.ex:

socket "/socket", YourAppWeb.UserSocket,
  websocket: true,
  longpoll: false

4. Import time travel and declare the socket in app.js (before you declare the liveSocket):

import {Socket} from "phoenix"
import {TimeTravel} from "time_travel"
let timeTravel = new TimeTravel(Socket);

let csrfToken = document.querySelector("meta[name='csrf-token']").getAttribute("content")
let liveSocket = new LiveSocket("/live", Socket, {params: {_csrf_token: csrfToken}})

5. Attach to the telemetry handlers in the init callback in lib/your_app_web/telemetry.ex:

# telemetry.ex
# init callback
:ok =
  :telemetry.attach_many(
    "live-view-handler",
    [
      [:phoenix, :live_view, :mount, :stop],
      [:phoenix, :live_view, :handle_event, :stop],
      [:phoenix, :live_view, :handle_params, :stop],
      [:phoenix, :live_component, :update, :stop],
      [:phoenix, :live_component, :handle_event, :stop]
    ],
    &TimeTravel.TelemetryHandler.live_view_event_handler/4,
    %{}
  )

6. Finally, use TimeTravel in the live_view and live_component definitions in my_app_web.ex:

def live_view do
  quote do
    use Phoenix.LiveView,
      layout: {TimeTravelDemoWeb.LayoutView, "live.html"}

    # Import TimeTravel handle_cast callbacks for each LiveView
    use TimeTravel, :live_view

    # ...
  end
end

def live_component do
  quote do
    use Phoenix.LiveComponent

    use TimeTravel, :live_component
    # ...
  end
end

For a full example see the Time Travel Demo Repo

Usage

1. With the LiveView open, Right click > Inspect
2. Press the arrows >> for more options and select LiveView DevTools
3. Interact with your LiveView (LiveComponents not supported at this time)
4. Drag the slider back and forth to replace the socket assigns

Notes

• If you run out of chrome storage, press the "Clear Storage" button (The Extension & Library is memory-heavy at this time)