Writing an Actor
================

A Simple Hello World
--------------------

Here's a simple Hello World actor.  It is a global actor (not associated with a given browser tab).
It has two parts: a spec and an implementation. The spec would go somewhere like
`devtools/shared/specs/hello-world.js` and would look like:

    const {Arg, RetVal, generateActorSpec} = require("devtools/shared/protocol");

    const helloWorldSpec = generateActorSpec({
      typeName: "helloWorld", // I'll explain types later, I promise.

      methods: {
        sayHello: {
          // The request packet template.  There are no arguments, so
          // it is empty.  The framework will add the "type" and "to"
          // request properties.
          request: {},

          // The response packet template.  The return value of the function
          // will be plugged in where the RetVal() appears in the template.
          response: {
            greeting: RetVal("string") // "string" is the return value type.
          }
        },
      },
    });

    // Expose the spec so it can be imported by the implementation.
    exports.helloWorldSpec = helloWorldSpec;

The actor implementation would go somewhere like
`devtools/server/actors/hello-world.js` and would look like:

    const { Actor } = require("devtools/shared/protocol");
    const {helloWorldSpec} = require("devtools/shared/specs/hello-world");

    class HelloActor extends Actor {
      constructor(conn) {
        super(conn, helloWorldSpec);
      }

      sayHello() {
       return "hello";
      }
    }

    // You also need to export the actor class in your module for discovery.
    exports.HelloActor = HelloActor;

To activate your actor, register it in the `addBrowserActors` method in `server/actors/utils/actor-registry.js`.
The registration code would look something like this:

    this.registerModule("devtools/server/actors/hello-world", {
      prefix: "hello",
      constructor: "HelloActor",
      type: { global: true }
    });

Your spec allows the actor to support a `sayHello` request.
A request/reply will look like this:

    -> { to: <actorID>, type: "sayHello" }
    <- { from: <actorID>, greeting: "hello" }

Now we can create a client side object.  We call these *front* objects and
they typically go in `devtools/client/fronts/`.

Here's the front for the HelloActor:

    const HelloFront = protocol.FrontClassWithSpec(helloWorldSpec, {
      initialize: function (client, form) {
        protocol.Front.prototype.initialize.call(this, client, form);
        // This call may not be required but it's a good idea. It will
        // guarantee that your instance is managed in the pool.
        this.manage(this);
      }
    });

Note that there is no `sayHello` method.  The FrontClass will generate a method on the Front object that matches the method declaration in the Actor class.

The generated methods will return a Promise.  That promise will resolve to the RetVal of the actor method.

So if we have a reference to a HelloFront object, we can issue a `sayHello` request:

    hello.sayHello().then(greeting => {
      console.log(greeting);
    });

How do you get an initial reference to the front?  That's a bit tricky, but basically there are two ways:

* Manually
* Magically

Manually - If you're using a DevToolsClient instance, you can discover the actorID manually and create a Front for it:

    let hello = new HelloFront(this.client, { actor: <hello actorID> });

Magically - Once you have an initial reference to a protocol.js object, it can return other protocol.js objects and fronts will automatically be created.

Arguments
---------

`sayHello` has no arguments, so let's add a method that does take arguments.
Here's an adjustment to the spec:

    methods: {
      echo: {
        request: { echo: Arg(0, "string") },
        response: { echoed: RetVal("string") }
      }
    }

Here's an adjustment to the implementation:

    echo: function (str) {
      return str + "... " + str + "...";
    }

This tells the library to place the 0th argument, which should be a string, in the `echo` property of the request packet.


This will generate a request handler whose request and response packets look like this:

    { to: <actorID>, type: "echo", echo: <str> }
    { from: <actorID>, echoed: <str> }

The client usage should be predictable:

    hello.echo("hello").then(str => { assert(str === "hello... hello...") })

The library tries hard to make using fronts feel like natural javascript (or as natural as you believe promises are, I guess).  When building the response it will put the return value of the function where RetVal() is specified in the response template, and on the client side it will use the value in that position when resolving the promise.

Returning JSON
--------------

Maybe your response is an object. Here's an example of a spec:

    methods: {
      addOneTwice: {
        request: { a: Arg(0, "number"), b: Arg(1, "number") },
        response: { ret: RetVal("json") }
      }
    }

Here's an example implementation:

    addOneTwice: function (a, b) {
      return { a: a + 1, b: b + 1 };
    }

This will generate a response packet that looks like:

    { from: <actorID>, ret: { a: <number>, b: <number> } }

That's probably unnecessary nesting (if you're sure you won't be returning an object with 'from' as a key!), so you can just replace `response` with:

    response: RetVal("json")

and now your packet will look like:

    { from: <actorID>, a: <number>, b: <number> }

Types and Marshalling
---------------------

Things have been pretty simple up to this point - all the arguments we've passed in have been javascript primitives.  But for some types (most importantly Actor types, which I'll get to eventually), we can't just copy them into a JSON packet and expect it to work, we need to marshal things ourselves.

Again, the protocol lib tries hard to provide a natural API to actors and clients, and sometime that natural API might involve object APIs. I'm going to use a wickedly contrived example, bear with me.  Let's say I have a small class that contains a number and has a few methods associated with it:

    class Incrementor {
      constructor (i) {
        this.value = i;
      }
      increment() { this.value++ }
      decrement() { this.value-- }
    }


and I want to return it from a backend function:

    // spec:
    methods: {
      getIncrementor: {
        request: { number: Arg(0, "number") },
        response: { value: RetVal("incrementor") } // We'll define "incrementor" below.
      }
    }

    // implementation:
    getIncrementor: function (i) {
      return new Incrementor(i)
    }

I want that response to look like `{ from: <actorID>, value: <number> }`, but the client side needs to know to return an Incrementor, not a primitive number.  So let's tell the protocol lib about Incrementors:

    protocol.types.addType("incrementor", {
        // When writing to a protocol packet, just send the value
        write: (v) => v.value,

        // When reading from a protocol packet, wrap with an Incrementor
        // object.
        read: (v) => new Incrementor(v)
    });

And now our client can use the API as expected:

    front.getIncrementor(5).then(incrementor => {
        incrementor.increment();
        assert(incrementor.value === 6);
    });

You can do the same thing with arguments:

    // spec:
    methods: {
      passIncrementor: {
        request: { Arg(0, "incrementor") },
      }
    }

    // implementation:
    passIncrementor: function (inc) {
      w.increment();
      assert(incrementor.value === 6);
    }

    front.passIncrementor(new Incrementor(5));

The library provides primitiive `boolean`, `number`, `string`, and `json` types.

Moving right along, let's say you want to pass/return an array of Incrementors.  You can just prepend `array:` to the type name:

    // spec:
    methods: {
      incrementAll: {
        request: { incrementors: Arg(0, "array:incrementor") },
        response: { incrementors: RetVal("array:incrementor") }
      }
    }

    // implementation:
    incrementAll: function (incrementors) {
      incrementors.forEach(incrementor => {
        incrementor.increment();
      }
      return incrementors;
    }

You can use an iterator in place of an array as an argument or return value, and the library will handle the conversion automatically.

Or maybe you want to return a dictionary where one item is a incrementor.  To do this you need to tell the type system which members of the dictionary need custom marshallers:

    protocol.types.addDictType("contrivedObject", {
      incrementor: "incrementor",
      incrementorArray: "array:incrementor"
    });

    // spec:
    methods: {
      reallyContrivedExample: {
        response: RetVal("contrivedObject")
      }
    }

    // implementations:
    reallyContrivedExample: function () {
      return {
        /* a and b are primitives and so don't need to be called out specifically in addDictType */
        a: "hello", b: "world",
        incrementor: new Incrementor(1),
        incrementorArray: [new Incrementor(2), new Incrementor(3)]
      }
    }

    front.reallyContrivedExample().then(obj => {
      assert(obj.a == "hello");
      assert(obj.b == "world");
      assert(incrementor.i == 1);
      assert(incrementorArray[0].i == 2);
      assert(incrementorArray[1].i == 3);
    });

Nullables
---------

If an argument, return value, or dict property can be null/undefined, you can prepend `nullable:` to the type name:

    "nullable:incrementor",  // Can be null/undefined or an incrementor
    "array:nullable:incrementor", // An array of incrementors that can have holes.
    "nullable:array:incrementor" // Either null/undefined or an array of incrementors without holes.


Actors
------

Probably the most common objects that need custom martialing are actors themselves.  These are more interesting than the Incrementor object, but by default they're somewhat easy to work with.  Let's add a ChildActor implementation that will be returned by the HelloActor (which is rapidly becoming the OverwhelminglyComplexActor):

    // spec:
    const childActorSpec = generateActorSpec({
      actorType: "childActor",
      methods: {
        getGreeting: {
          response: { greeting: RetVal("string") },
        }
      }
    });

    // implementation:
    class ChildActor extends Actor {
      constructor(conn, id) {
        super(conn, childActorSpec);

        this.greeting = "hello from " + id;
      }

      getGreeting() {
        return this.greeting;
      }
    }

    exports.ChildActor = ChildActor;

    const ChildFront = protocol.FrontClassWithSpec(childActorSpec, {
      initialize: function (client, form) {
        protocol.Front.prototype.initialize.call(this, client, form);
      },
    });

The library will register a marshaller for the actor type itself, using typeName as its tag.

So we can now add the following code to HelloActor:

    // spec:
    methods: {
      getChild: {
        request: { id: Arg(0, "string") },
        response: { child: RetVal("childActor") }
      }
    }

    // implementation:
    getChild: function (id) {
      return ChildActor(this.conn, id);
    }

    front.getChild("child1").then(childFront => {
      return childFront.getGreeting();
    }).then(greeting => {
      assert(id === "hello from child1");
    });

The conversation will look like this:

    { to: <actorID>, type: "getChild", id: "child1" }
    { from: <actorID>, child: { actor: <childActorID> }}
    { to: <childActorID>, type: "getGreeting" }
    { from: <childActorID>, greeting: "hello from child1" }

But the ID is the only interesting part of this made-up example.  You're never going to want a reference to a ChildActor without checking its ID.  Making an extra request just to get that id is wasteful.  You really want the first response to look like `{ from: <actorID>, child: { actor: <childActorID>, greeting: "hello from child1" } }`

You can customize the marshalling of an actor by providing a `form` method in the `ChildActor` class:

    form: function () {
        return {
            actor: this.actorID,
            greeting: this.greeting
        }
    },

And you can demarshal in the `ChildFront` class by implementing a matching `form` method:

    form: function (form) {
        this.actorID = form.actor;
        this.greeting = form.greeting;
    }

Now you can use the id immediately:

    front.getChild("child1").then(child => { assert(child.greeting === "child1) });

You may come across a situation where you want to customize the output of a `form` method depending on the operation being performed.  For example, imagine that ChildActor is a bit more complex, with a, b, c, and d members:

    ChildActor:
        form: function () {
            return {
                actor: this.actorID,
                greeting: this.greeting,
                a: this.a,
                b: this.b,
                c: this.c,
                d: this.d
            }
        }
    ChildFront:
        form: function (form) {
            this.actorID = form.actorID;
            this.id = form.id;
            this.a = form.a;
            this.b = form.b;
            this.c = form.c;
            this.d = form.d;
        }

And imagine you want to change 'c' and return the object:

    // Oops!  If a type is going to return references to itself or any other
    // type that isn't fully registered yet, you need to predeclare the type.
    types.addActorType("childActor");

    ...

    // spec:
    methods: {
      changeC: {
        request: { newC: Arg(0) },
        response: { self: RetVal("childActor") }
      }
    }

    // implementation:
    changeC: function (newC) {
      c = newC;
      return this;
    }

    ...

    childFront.changeC('hello').then(ret => { assert(ret === childFront); assert(childFront.c === "hello") });

Now our response will look like:

    { from: <childActorID>, self: { actor: <childActorID>, greeting: <id>, a: <a>, b: <b>, c: "hello", d: <d> }


Lifetimes
---------

No, I don't want to talk about lifetimes quite yet.

Events
------

Your actor has great news!

Actors are subclasses of jetpack `EventTarget`, so you can just emit events.
Here's how you'd set it up in a spec:

    events: {
      "good-news": {
        type: "goodNews", // event target naming and packet naming are at odds, and we want both to be natural!
        news: Arg(0)
      }
    }

    methods: {
      giveGoodNews: {
        request: { news: Arg(0) }
      }
    }

Here's how the implementation would look:

    // In your Actor class:
    giveGoodNews(news) {
      this.emit("good-news", news);
    }

Now you can listen to events on a front:

    front.on("good-news", news => {
      console.log(`Got some good news: ${news}\n`);
    });
    front.giveGoodNews().then(() => { console.log("request returned.") });

If you want to modify the argument that will be passed to event listeners callbacks, you
can use `before(eventName, fn)` in the front definition. This can only be used once for a
given `eventName`. The `fn` function will be called before emitting the event via
the EventEmitter API on the Front, and its return value will be passed to the event
listener callbacks. If `fn` is async, the event will only be emitted after `fn` call resolves.

    // In front file, most probably in the constructor:
    this.before("good-news", function(news) {
      return news.join(" - ");
    });

    // In any consumer
    front.on("good-news", function(news) {
      console.log(news);
    });

So if the server sent the following array: `[1, 2, 3]`, the console.log in the consumer
would print `1 - 2 - 3`.

On a somewhat related note, not every method needs to be request/response.  Just like an actor can emit a one-way event, a method can be marked as a one-way request.  Maybe we don't care about giveGoodNews returning anything:

    // spec:
    methods: {
      giveGoodNews: {
        request: { news: Arg(0, "string") },
        oneway: true
      }
    }

    // implementation:
    giveGoodNews: function (news) {
      emit(this, "good-news", news);
    }

Lifetimes
---------

No, let's talk about custom front methods instead.

Custom Front Methods
--------------------

You might have some bookkeeping to do before issuing a request.  Let's say you're calling `echo`, but you want to count the number of times you issue that request.  Just use the `custom` tag in your front implementation:

    echo: custom(function (str) {
        this.numEchos++;
        return this._echo(str);
    }, {
        impl: "_echo"
    })

This puts the generated implementation in `_echo` instead of `echo`, letting you implement `echo` as needed.  If you leave out the `impl`, it just won't generate the implementation at all.  You're on your own.

Lifetimes
---------

OK, I can't think of any more ways to put this off.  The remote debugging protocol has the concept of a *parent* for each actor.  This is to make distributed memory management a bit easier.  Basically, any descendents of an actor will be destroyed if the actor is destroyed.

Other than that, the basic protocol makes no guarantees about lifetime.  Each interface defined in the protocol will need to discuss and document its approach to lifetime management (although there are a few common patterns).

The protocol library will maintain the child/parent relationships for you, but it needs some help deciding what the child/parent relationships are.

The default parent of an object is the first object that returns it after it is created.  So to revisit our earlier HelloActor `getChild` implementation:

    // spec:
    methods: {
      getChild: {
        request: { id: Arg(0) },
        response: { child: RetVal("childActor") }
      }
    }

    // implementation:
    getChild: function (id) {
      return new ChildActor(this.conn, id);
    }

The ChildActor's parent is the HelloActor, because it's the one that created it.

You can customize this behavior in two ways.  The first is by defining a `marshallPool` property in your actor.  Imagine a new ChildActor method:

    // spec:
    methods: {
      getSibling: {
        request: { id: Arg(0) },
        response: { child: RetVal("childActor") }
      }
    }

    // implementation:
    getSibling: function (id) {
      return new ChildActor(this.conn, id);
    }

This creates a new child actor owned by the current child actor.  But in this example we want all actors created by the child to be owned by the HelloActor.  So we can define a `defaultParent` property that makes use of the `parent` property provided by the Actor class:

    get marshallPool() { return this.parent }

The front needs to provide a matching `defaultParent` property that returns an owning front, to make sure the client and server lifetimes stay synced.
