Documenting client/assert.js and client/fsm.js

api/payments/stripe.js

@@ -2,7 +2,7 @@ import { Product, Payment } from '../../lib/models.js';
 import { API, developer_admin } from "../../lib/api.js";
 import logging from '../../lib/logging.js';
 import dayjs from 'dayjs';
-import { product as product_config, register_enabled } from "../../client/config.js";
+import { product_id, register_enabled } from "../../client/config.js";
 import assert from 'assert';
 import * as queues from "../../lib/queues.js";
 import stripe_create from "stripe";
@@ -12,7 +12,7 @@ const log = logging.create("api/payments/stripe.js");
 
 const stripe = stripe_create(stripe_private.secret);
 
-const product = await Product.first({id: product_config.id});
+const product = await Product.first({id: product_id});
 
 const rules = {
   "payment_intent": "required",

client/assert.js

@@ -1,13 +1,19 @@
+/*
+  A stupid thing I wrote so I could have the same assert function in the browser that I
+  have in Node. This should just be standard in JavaScript.
+ */
 import { log } from "./logging.js";
 
-/* I can't believe I have to write this just so I can use the assert that should be standard in every javascript. */
-
+/*
+  This is the error that is thrown when an `assert()` fails.
+ */
 export class AssertionError extends Error {
-  /* This is from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error
-   * which claims you have to do this weird stuff to make your error actually work.
-   */
 
-  constructor(foo = 'bar', ...params) {
+  /*
+    This is from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error
+    which claims you have to do this weird stuff to make your error actually work.
+   */
+  constructor(...params) {
     // Pass remaining arguments (including vendor specific ones) to parent constructor
     super(...params);
 
@@ -19,15 +25,22 @@ export class AssertionError extends Error {
 
     this.name = 'AssertionError';
     // Custom debugging information
-    this.foo = foo;
     this.date = new Date();
   }
 }
 
-const assert = (test, message) => {
-  log.assert(test, message);
+/*
+  If `test` is false then it logs the message and raises the `AssertionError`.
+
+  ___TODO___: Implement a way to "compile out" these assertions.
 
+  + `test boolean` -- The code only does `if(!test)` so should work with most any falsy thing.
+  + `message string` -- The message to log and then attach to the thrown `AssertionError`.
+  + ___throws___ `AssertionError(message)`
+ */
+export const assert = (test, message) => {
   if(!test) {
+    log.assert(test, message);
     throw new AssertionError(message);
   }
 }

client/fsm.js

@@ -1,8 +1,145 @@
+/*
+  A very simple Finite State Machine system.  If you're struggling with managing
+  a mixture of JavaScript API styles (Promise, callback, events) then this will
+  help get things organized.  The best example is in the `client/components/HLSVideo.svelte` file.
+
+  The main advantage an FSM has is it's ability to constrain and organize random events, and it's
+  ability to fully log what's happening in a stream of random events.  The `HLSVideo.svelte` example
+  is a great demonstration of this because it has to juggle events from two different styles of APIs,
+  network events, user UI events, and __video__ loading events.  Since all of these can come in at
+  random the FSM will help order them and do the right thing at the right time.
+
+  ### Theory
+
+  One way to look at a Finite State Machine is an ___explicit if-while-loop___.
+  If you have a `while-loop` that is doing a lot of processing based on
+  variables, and you're using a lot of `if-statements` or `switch-case`
+  statements, then the `FSM` simply formalizes exactly __what__ states and when
+  they can run.
+
+  For example, if you have a mess of nested `if-statements`, and each one might
+  not cover every possible variable, then you'll have bugs as different
+  branches of the `if-statements` run or don't run.  The `FSM` simply says, "The only
+  allowed states at this point in time are X, Y, and Z.  The only events I expect are
+  event1, event2, and event3.  If I get anything else that's an error."
+
+  It's a different way to think about data processing, but it is very reliable _and_
+  easy to debug because you can log every event, state, and all the data while processing
+  random events.
+
+  ### FSMs in User Interfaces
+
+  Most UIs are simple enough that you can get away with the basic Svelte variables updating
+  the state.  When UIs get more complex--and especially when they trigger network events--you
+  run into problems with random events requiring different updates to the UI.  For example, if
+  a user clicks on a button that loads a video, but they're already loading a different video,
+  then you have to deal with the current click, the old video loading state, and the new video
+  playing state.  An FSM would be able to "serialize" the click, old video load, and new video
+  load and play better than the usual Svelte variables.
+
+  In short, if you're struggling with properly updating a user interface as random events come
+  in from the user and the network, then switch to an FSM.
+
+  ### Usage
+
+  The `class FSM` takes two things:  data and a event transition object.  The `data` is only
+  for logging and debugging the FSM, as it gets printed out with each log message.  The core
+  of the FSM is your event handler, which has functions named after each event:
+
+  ```javascript
+  class VideoEvents {
+    async mount(state) {
+      case "START":
+        return "COUNTING_DOWN";
+      default:
+        return "ERROR";
+    }
+  }
+  ```
+
+  In this snippet the `class VideoEvents` has a `async mount` event that works like this:
+
+  + This `mount()` event takes a single `state` which is just a string for the state the FSM is currently in.
+  + You then figure out what to do based on this `state`, typically using a `switch/case`.
+  + Based on what you've done, you return the next state as another string.
+  + The `FSM` then waits for a new event, and calls that function with this new state.
+
+  To trigger this `mount` event you would then write:
+
+  ```javascript
+  await fsm.do("mount");
+  ```
+
+  You can pass additional arguments to the `do` function and they will be passed to the
+  event as additional arguments:
+
+  ```javascript
+  // if fsm is in "START" state
+  await fsm.do("mount", 1, 2, 3);
+
+  // then becomes
+  ev.mount("START", 1, 2, 3);
+  ```
+
+  ### Logging Events
+
+  The `FSM` class will already call `log.debug` on many things, but if you want to do your own logging
+  you can use the `onTransition` callback:
+
+  ```javascript
+  fsm = new FSM(video_config, new VideoEvents());
+  fsm.onTransition(fsm => {
+    video_config.state = fsm.state
+    log.info("I'm logging something else too", the_other_thing);
+  });
+  ```
+
+  This is a slight modification of how the FSM is used in `HLSVideo.svelte`.  Remember that the `video_config` is only passed to `FSM()` so that it gets logged while the `FSM` is running. This is _incredibly_ useful when debugging why your `FSM` is doing something.
+
+  In the `onTransition` callback I'm simply updating the `state` in `video_config` and then I added a little `log.info` to show you could log anything else you wanted there.
+
+  ### Internal Transitions
+
+  If I'm remembering my graduate courses right, this is called an "epsilon transition", but it's
+  basically triggering another event inside the current event, effectively changing state without
+  waiting for a new event.  I could be wrong on the terminology, but you do this by returning an
+  `Array[state, cb()]` pair. The `FSM` will then set state to `state` and call your callback so
+  you can do further processing.
+
+  ___BUG___: This could be improved but for now it works.  Look at `HLSVideo.svelte:detect_video` for an example of doing this.
+
+  ### State Names
+
+  The first state is "START" by default, but after that you can use any string. I prefer to make
+  the states UPPERCASE so they aren't confused with events, which are `lowercase()` function
+  names in the event handler class (`VideoEvents` above).
+
+ */
 import assert from "./assert.js";
 import { log } from "./logging.js";
 
+/*
+  A very simple Finite State Machine class.  Nothing fancy, just a state
+  and some transition callbacks based on the function names found in `this.events`.
+ */
 export class FSM {
 
+  /*
+    Sets up this FSM to use the `events` callback as the transitions, and then
+    "START" as the first state. It does ___not___ call any initial transitions
+    so you have to do the first call to `FSM.do()`.  The `data` isn't used
+    internally and only passed to logging functions for debugging.
+
+    ### Event Class Signature
+
+    The `events` can be anything, and you construct it so it will have your own data.
+    The only requirement is that the functions have a `name(state)` signature and that
+    `state` is going to be a string.  It should then return the next state string for
+    the next event to receive.
+
+    + `data Anything` -- Whatever you want to log while debugging.
+    + `events Event Object` -- An object/class that can handle event calls sent to it.
+   */
   constructor(data, events) {
     // data is really only for logging/debugging
     this.data = data;
@@ -11,10 +148,25 @@ export class FSM {
     this.on_state = undefined;
   }
 
+  /*
+    A callback that runs whenever an event handler returns a state. The `cb` callback
+    receives this `FSM` so it can debug it or do more to it.
+
+    ___WARNING___: This is called called even if the state didn't change, which
+    might confuse people if they think it's only when a "transition" happens.
+
+    + `cb(FSM)` -- Callback after the transition. Receives this `FSM`.
+   */
   onTransition(cb) {
     this.transition_cb = cb;
   }
 
+  /*
+    Mostly used internally to perform the transition then call the `onTransition` callback,
+    but sometimes you might need to force a new state externally.
+
+    + `next_state string` -- The new state name.
+   */
   transition(next_state) {
     this.state = next_state;
 
@@ -23,10 +175,22 @@ export class FSM {
     }
   }
 
+  /*
+    All event names available in the `this.events` class.
+   */
   event_names() {
     return Object.getOwnPropertyNames(Object.getPrototypeOf(this.events)).filter(k => k !== "constructor");
   }
 
+  /*
+    The core of this `FSM` is the `do()` function. It handles finding the next
+    event by it's name, loading it, and calling it with the given additional
+    arguments.
+
+    + `event string` -- The __name__ of the target event, not the actual function. It's a string.
+    + `...args` -- Additional arguments to pass to the event.
+    + ___return___ `this.state string` -- The current state of this `FSM`.
+   */
   async do(event, ...args) {
     const evhandler = this.events[event];
     assert(evhandler !== undefined, `Invalid event ${event}. Available ones are '${this.event_names()}'.`);