reactive-tree

A simple library for reactive programming.

Requires RxJS 6.

Link to Changelog.

Installation

npm install reactive-tree

Definition

Leaves

A leaf defines a reactive state. It reacts whenever its value changes.

Example: Create a leaf

import { createLeaf } from "reactive-tree";

const leaf = createLeaf("world");

console.log(`Hello, ${leaf.value}.`); // Hello, world.

A second way to create a leaf is using defineLeaf().

This example doesn't show how a leaf reacts whenever its value changes. Let's keep reading.

---

Twigs

A twig defines a computed state. It has a value that is computed from its handler function. It computes only when someone try to get its value and it's dirty.

A twig gets dirty when:

• it's just created, or
• it reads leaves (or other twigs) inside its handler function and any of those leaves (or other twigs) reacts, or
• its dirty property is set to true.

A twig also behaves like a reactive state if, inside its handler function, it reads values from leaves (or other twigs).

Example: Create a twig

import { createLeaf, createTwig } from "reactive-tree";

const leaf = createLeaf("world");
const twig = createTwig(() => `Hello, ${leaf.read()}.`);

console.log(twig.value); // Hello, world.
leaf.write("kitty");
console.log(twig.value); // Hello, kitty.

Now you can see that this line leaf.write("kitty"); causes the leaf to react, which causes the twig to become dirty, because the twig reads the leaf inside its handler function. As a result, the second twig.value now has a different value.

A second way to create a twig is using defineTwig().

---

Branches

A branch starts a reactive procedure: it collects reactive states by calling its handler function; then it waits until any of those reactive states reacts, it schedules to restart this procedure (by default, using setTimeout function).

Example: Create a branch

import { createBranch, createLeaf, createTwig } from "reactive-tree";

const leaf = createLeaf("world");
const twig = createTwig(() => `Hello, ${leaf.read()}.`);

createBranch(() => {
  console.log(twig.read());
});

leaf.write("kitty");

// Output:
//   Hello, world.
//   Hello, kitty.

Basically, this example illustrates how a branch reacts whenever any of reactive states inside its handler function changes. Things you should know that:

• createBranch() immediately calls its sole argument, the handler function, which produces the first line of output;
• the second line of output does not immediately show up, but the gap is too small to be noticed.

Branches can be nested inside each other.

Example: Nesting

import { createBranch, defineLeaf } from "reactive-tree";

class Book {
  constructor(public name: string) {
    defineLeaf(this, "name");
  }
}

class MyApp {
  showThisBook = null as Book | null;
  constructor() {
    defineLeaf(this, "showThisBook");
  }
}

const myApp = new MyApp();

createBranch(() => {
  const book = myApp.showThisBook;
  if (book === null) {
    console.log("No book is showing.");
    return;
  }
  console.log("We are showing a book.");
  createBranch(() => {
    console.log(`Name of the book is "${book.name}".`);
  });
});

setTimeout(() => {
  const book = new Book("How To Make Cookies");
  myApp.showThisBook = book;
  setTimeout(() => {
    book.name = "How To Plant A Tree";
    setTimeout(() => {
      myApp.showThisBook = null;
      book.name = "This One Will Not Show Up";
    }, 2000);
  }, 2000);
}, 2000);

// Output:
//   No book is showing.
//   (2 seconds later)
//   We are showing a book.
//   Name of the book is "How To Make Cookies".
//   (2 seconds later)
//   Name of the book is "How To Plant A Tree".
//   (2 seconds later)
//   No book is showing.

API

class Leaf

function createLeaf<T>(value: T): Leaf<T>;
function defineLeaf<T, K extends keyof T>(
  target: T,
  propertyKey: K
): Leaf<T[K]>;
function defineLeaf<T>(
  target: object,
  propertyKey: string | symbol,
  value: T
): Leaf<T>;

class Leaf<T> implements Signal {
  static defaultSelector = distinctUntilChanged();
  static create = createLeaf;
  static define = defineLeaf;
  value: T;
  selector: OperatorFunction<T, T>;
  readonly subject: Subject<T>;
  read(): T;
  write(value: T): void;
  observe(observable: ObservableInput<T>): Subscription;
  unobserve(): void;
}

function createLeaf()

Create a leaf with a value.

function defineLeaf()

Create a leaf with a value, like createLeaf(), but also defines a property for an object, which corresponds with this leaf:

• when you get this property, it returns leaf.read();
• when you set this property to something, it calls leaf.write(something).

class Leaf: value

Get or set the value.

Generally, you should consider using read() or write() instead of getting or setting this property.

class Leaf: selector

Determine whether the leaf should react when a new value writes to it.

By default, leaves react only when a different value writes to them. You can change this behavior by setting this property.

Note that setting this property has no effect if the leaf has been read by twigs or branches. Setting this property just after the leaf is created is the recommended way.

class Leaf: subject

Get a Subject for this leaf. Subsequent calls return the same one.

The subject responds to write() and observe().

class Leaf: read()

read() returns value. Additionally, calling read() inside a handler function causes the leaf to be collected by the owner of that handler function, which must be a twig or a branch.

class Leaf: write()

Set value property to a new value. It also causes the leaf to react if this new value differs from the old one (To change this behavior, see selector).

class Leaf: observe()

Subscribe an Observable and returns a Subscription. A write() cancels this subscription. Each value emitted by this observable is written to the leaf, like write() but without canceling this subscription.

class Leaf: unobserve()

Cancel all subscriptions created by observe(). A write() also cancels all those subscriptions.

---

class Twig

function createTwig<T>(handler: () => T): Twig<T>;
function defineTwig<T>(
  target: object,
  propertyKey: string | symbol,
  handler: () => T
): Twig<T>;

class Twig<T> implements Signal {
  static create = createTwig;
  static define = defineTwig;
  dirty: boolean;
  handler: () => T;
  readonly value: T;
  read(): T;
  write(value: T): void;
  clean(): void;
  notify(): void;
  connect(signal: Signal): void;
}

function createTwig()

Create a twig with a handler.

function defineTwig()

Create a twig with a handler, like createTwig(), but also defines a property for an object, which corresponds with this twig:

• when you get this property, it returns twig.read().
• when you set this property to something, it calls twig.write(something).

class Twig: dirty

Indicate whether the twig should update the cached value.

class Twig: handler

Get or set the handler.

If set, you may also want to set dirty to true and make a call to notify().

class Twig: value

Get the cached value computed from handler function.

If dirty is true, a new value returned by handler function will be cached and used instead. And then dirty is set to false.

Generally, you should consider using read() instead of getting this property.

class Twig: read()

read() returns value. Additionally, calling read() inside a handler function causes the twig to be collected by the owner of that handler function, which must be a twig or a branch.

class Twig: write()

By default, write() throws an error. Overwrite this property if you need it.

class Twig: clean()

Clean the twig if it's dirty.

class Twig: notify()

Force the twig to react.

class Twig: connect()

Connect a signal with the twig.

connect() should only be called inside the handler function.

---

class Branch

function createBranch(handler?: (branch: Branch) => void): Branch;
function createBranch(
  scheduler?: Scheduler,
  handler?: (branch: Branch) => void
): Branch;

class Branch {
  static create = createBranch;
  handler?: (branch: Branch) => void;
  scheduler?: Scheduler;
  readonly stopped: boolean;
  readonly disposed: boolean;
  run(): void;
  stop(): void;
  dispose(): void;
  freeze(): void;
  unfreeze(): void;
  schedule(): void;
  unschedule(): void;
  connect(signal: Signal): void;
  teardown(x: TeardownLogic): void;
  finalize(x: TeardownLogic): void;
}

function createBranch()

Create a branch with a handler and/or a scheduler.

If scheduler is not specified, but the parent branch has one, that one will be used. That is to say, inner branches share the same scheduler from their parent branch, if their scheduler are not set.

Schedulers are used to change the way how branches schedule when they react.

class Branch: handler

Get or set the handler.

If set, you need to call run() or schedule() to take effect.

class Branch: scheduler

Get or set the scheduler.

If not set, Scheduler.default is used.

class Branch: stopped

Check if the branch stops.

class Branch: disposed

Check if the branch disposes.

class Branch: run()

Force the branch to restart its procedure immediately.

An error throws if run() is called inside the handler function.

class Branch: stop()

Stop the branch.

class Branch: dispose()

Dispose the branch.

You can run() or schedule() a stopped branch again, but not a disposed one.

class Branch: freeze()

Freeze the branch. Subsequent reactive states will NOT be collected by the branch.

freeze() should only be called inside the handler function.

class Branch: unfreeze()

Unfreeze the branch, ready to collect subsequent reactive states.

unfreeze() should only be called inside the handler function.

unfreeze() need not be called if there is no subsequent reactive states after.

class Branch: schedule()

Make a schedule to run().

class Branch: unschedule()

Undo schedule().

class Branch: connect()

Connect a signal with the branch.

connect() should only be called inside the handler function.

class Branch: teardown()

Add something to do when the branch restarts or stops or disposes. This is useful if you need to undo something that is done inside the handler function.

teardown() should only be called inside the handler function.

class Branch: finalize()

Add something to do when the branch disposes.

finalize() should only be called outside the handler function.

---

class Scheduler

function createAsyncScheduler(schedule?: (cb: () => void) => void): Scheduler;

interface Scheduler {
  schedule(branch: Branch): void;
  unschedule(branch: Branch): void;
}

class Scheduler {
  static createAsync = createAsyncScheduler;
  static async: Scheduler;
  static sync: Scheduler;
  static default = Scheduler.async;
}

function createAsyncScheduler()

Create an async scheduler with a schedule function.

An async scheduler opens a window when scheduling a branch. During the lifetime of the window, multiple branches will then be put together. When the window closes, all these branches will then run() one by one in the order that oldest runs first and then the scheduler is ready to open another window.

The schedule function defines how a window opens and when it closes. If not provided, a default one will be used.

interface Scheduler: schedule()

Make a schedule to run() a branch.

interface Scheduler: unschedule()

Undo what schedule() does to a branch.

class Scheduler: async

An async scheduler. See createAsyncScheduler().

class Scheduler: sync

A scheduler. When scheduling a branch, sync immediately run()s it.

class Scheduler: default

The default scheduler, which is async.

---

class Signal

function createSignal(source: ObservableInput<any>): Signal;
function connectSignal(signal: Signal): void;
function collectSignals(cb: () => void): Signal[];

interface Signal {
  readonly name?: string | symbol;
  readonly identity: number;
  readonly observable: Observable<Signal>;
}

class Signal {
  static create = createSignal;
  static connect = connectSignal;
  static collect = collectSignals;
}

function createSignal()

Create a signal from an observable.

function connectSignal()

Connect a signal. connectSignal() should only be called inside a handler function (twigs' or branches'). Any emission from this signal causes those twigs or branches to react (twigs become dirty, branches schedule to run again).

function collectSignals()

Collect signals inside the callback function, return an array of them.

interface Signal: name

The name of the signal.

For leaves that are created by defineLeaf() or reactive(), their name would be the value passed to the function for the propertyKey parameter.

For twigs that are created by defineTwig() or computed(), their name would be the value passed to the function for the propertyKey parameter.

interface Signal: identity

The identity for the signal.

This property is for uniqueness, for ordering, the value itself has no meaning.

interface Signal: observable

The observable provided by the signal. Each emission of this observable represents a signal.

---

Decorators

function reactive(target: object, propertyKey: string | symbol): void;
function computed(
  target: object,
  propertyKey: string | symbol,
  descriptor: PropertyDescriptor
): PropertyDescriptor;

function reactive()

Add a get-setter property to a class. The first time you set a value to this property of an instance, a leaf is created and bound to it.

function computed()

Wrap an existing get-setter property of a class. The first time you get this property of an instance, a twig is created and bound to it.

---

Others

function when(predicate: () => boolean, effect: () => void): Branch;
function whenever<T>(
  expression: () => T,
  effect: (data: T, branch: Branch) => void,
  selector?: OperatorFunction<T, T>,
  fireImmediately?: boolean
): Branch;

function when()

When predicate returns true, efffect is called (and only called once).

To cancel when(), dispose() the returned branch. After effect is called, the returned branch will be disposed too.

function whenever()

Whenever expression returns a value that differs from the old one, effect is called with the returned value and a branch that can be used to cancel whenever() as parameters.

whenever() also returns the branch that passes to effect. To cancel whenever(), dispose() the branch.

selector can be used to determine which values, returned by expression, should pass to effect. By default, Leaf.defaultSelector is used.

If fireImmediately is true, effect will be immediately called with the first value returned by expression and the branch mentioned above as parameters.

Changelog

• v4.0.0:
  • Added finalize() method for branches;
  • Added when() and whenever();
  • Added Leaf.defaultSelector;
  • Renamed subscribe() to observe() for leaves;
  • Renamed unsubscribe() to unobserve() for leaves;
  • Renamed createScheduler() to createAsyncScheduler();
  • Renamed scheduleBranch() to schedule() for schedulers;
  • Renamed unscheduleBranch() to unschedule() for schedulers;
  • Changed Scheduler to be an interface and a class;
  • Changed the type of selector for leaves;
  • Changed subject() to subject for leaves;
  • Changed the implementation of createSignal().
• v3.0.1 - v3.0.3:
  • Minor fixes.
• v3.0.0:
  • Added clean() for twigs;
  • Added connect() for twigs;
  • Added connect() for branches;
  • Added stopped for branches;
  • Added disposed for branches;
  • Added collectSignals();
  • Added Scheduler.async and Scheduler.sync;
  • Added reactive() and computed();
  • Renamed addTeardown() to teardown();
  • Removed setInterval() and setTimeout() for branches.
• v2.2.0:
  • Added write() for twigs.
• v2.0.1 - v2.0.3:
  • Minor changes.
• v2.0.0:
  • Added class Scheduler and Signal;
  • Added selector for leaves;
  • Renamed remove() to dispose();
  • Changed signature of createBranch().

License

MIT