# FRP tools

Observer and Computed value stores designed to work together for storing real state and derived
state.

# [observable](./src/observable.js)

`observable` is a simple value store that can report when its value changes. It is good for wrapping
external props passed into a component so compute types can dependent on them. It can also be used
to receive events such as _window.onresize_ to always provide the current viewport size.

NOTE: Javascript has a proposal for a thing called an
[Observable](https://github.com/tc39/proposal-observable). This is not an implementation of that.
They do serve similar functions (provide a way to communicate when a value changes) but the tc39
proposal is more about event input sources and can communicate errors and be extended. This
implementation is designed to be as small and simple as possible. Extending it is done via
[Computed] instances depending on them. I may rename `observable` to `property` in a future major
release to avoid this confusion.

## Usage

### Creation

Creates and sets initial value to `true`

```js
const inViewport = observable(true);
```

### Read

Call it to receive the stored value.

```js
if (inViewport()) {
	/* inViewport is truthy */
}
```

### Change

Call it passing the new value. If any computed stores depend on this value they will be marked dirty
and re-evaluated the next time they are read from.

```js
inViewport(false);
```

### Subscribe to changes

Call the `subscribe` method with a callback that will be called when the observable is changed to a
different value. The returned function can be called to unsubscribe from the observable. When called
it will provide the count of remaining subscriptions.

```js
const unsubscribe = inViewport.subscribe(console.log.bind(console));
const remainingSubscriptionCount = unsubscribe();

inViewport.unsubscribeAll(); // Call unsubscribeAll to remove child observables/computeds.
```

### Provide a comparator for complex types

When storing a type that is not determined to be equal with simple equality (===), provide a
function to determine in the new provided value should be propagated to dependents.

```js
function setEquals(a, b) {
	return (
		a instanceof Set &&
		b instanceof Set &&
		[...a].reduce((acc, d) => acc && b.has(d), true) &&
		[...b].reduce((acc, d) => acc && a.has(d), true)
	);
}

const a = observable(new Set([1, 2]), setEquals);
```

# [computed](./src/computed.js)

`computed` is a functional store that depends on the values of observables or other computeds. They
derive value from observables rather than store value and hence cannot be set directly.

## Behavior

A `computed` will subscribe to its dependencies in such a way that it will be marked as _dirty_ when
any dependency changes. Whenever it is read from, if will recompute its result if the _dirty_ flag
is set, otherwise it just return the stored result from the last time it computed.

## Usage

### Creation

```js
const showDialog = computed(
	(inVP, shouldShow) => inVP && shouldShow, // computation function
	[inViewport, shouldShow] // array of dependencies, can be either observable or computed
);
```

### Read

```js
if (showDialog()) {
	/* showDialog() is truthy */
}
```

Call it to receive the stored value, recomputing if necessary.

### Subscribe to changes

Call the subscribe method with a callback that will be called when the computed result changes to a
different value. The returned function can be called to unsubscribe from the observable. When called
it will provide the count of remaining subscriptions.

```js
const unsubscribe = inViewport.subscribe(console.log.bind(console));
const remainingSubscriptionCount = unsubscribe();
```

**NOTE**: Subscribing to a computed forces it to recompute every time an upstream dependency
changes. This could negatively performance if it depends on multiple values that change sequentially
and the computation function is non-trivial. For example:

```js
const inViewport = observable(false);
const shouldShow = observable(false);

const showDialog = computed((inVP, shouldShow) => inVP && shouldShow, [inViewport, shouldShow]);

inViewport(true); // showDialog marked as dirty but does not recompute its stored result.
shouldShow(true); // showDialog is already marked as dirty. Nothing else happens.
showDialog(); // showDialog recomputes its stored result and unsets the dirty flag.

// adding a subscription will change showDialog's internal behavior
showDialog.subscribe(console.log.bind(console));

inViewport(false); // showDialog result recomputed and `false` is written to the console.
shouldShow(false); // showDialog result recomputed, console.log is not called.
showDialog(); // showDialog does not recompute, console.log is not called. `false` is returned.

showDialog.detach(); // Call detach to remove this computed from the logic tree.
showDialog.unsubscribeAll(); // Call unsubscribeAll to remove child observables/computeds.
```

### Provide a comparator for complex types

When the computed result is a type that is not determined to be equal with simple equality (===),
provide a function to determine in the new provided value should be propagated to dependents.

```js
function setEquals(a, b) {
	return (
		a instanceof Set &&
		b instanceof Set &&
		[...a].reduce((acc, d) => acc && b.has(d), true) &&
		[...b].reduce((acc, d) => acc && a.has(d), true)
	);
}

function _intersection(a, b) {
	return new Set([...a].filter(x => b.has(x)));
}

const a = observable(new Set([1, 2]), setEquals);
const b = observable(new Set([2, 3]), setEquals);
const intersection = computed(_intersection, [a, b], setEquals);
```

# [bundle](./src/bundle.js)

`bundle` is a wrapper around a group of `observables` for the purpose of applying changes to all of
them at once without having to trigger a subscription that may depend on more than observable in the
group.

Another way to think of a `bundle` is an `observable` that takes an object and exposes the
properties as individual observables.

## Behavior

A `bundle` wraps observables to intercept dependency hooks in such a way that updating all
observables can happen at once before any downstream `computeds` are evaluated. A bundle returns a
function that can be called with an object to set values for the mapped member observables.

## Usage

### Creation

```js
const layoutEventBundle = bundle({
	width: observable(1),
	height: observable(2)
});
const ratio = computed((a, b) => a / b, [layoutEventBundle.width, layoutEventBundle.height]);
ratio.subscribe(render);
```

### Change Member Observables atomically

```js
layoutEventBundle({ width: 640, height: 480 });
```

`ratio` would normally be evaluated twice and `render` would be called after each intermediate
change. But bundle allows both values to change and `ratio` will only be evaluated once and `render`
called once.

### Change Member Observables individually

```js
layoutEventBundle.width(640);
layoutEventBundle.height(480);
```

The observables exposed by the bundle can also be updated apart from their grouping.