feat: Async store and compute() function

[mnasyrov]

Async store

This is a new implementation of the store. The previous one was based on BehaviourSubject which emitted synchronous notifications for state updates.

The new store is based on its own subscription collection and schedules a notification of subscribers on the next micro task. Also subscribers receive only on notification with a current state. All subsequent events are dropped because they don't matter.

Behaviour of a subscription is the same: a new subscriber receives a current state on subscribing synchronously.

Subscribers can receive a current state synchronously at any time by invoking store.notify() manually.

compute()

This function creates a computable query which calculates its values by provided "computation" function and dependencies.

Rules of "Computation" function:

• it must have no side effects
• it recalculates only when specified dependencies are updated
• its "formula" may have other sources of values, however they don't trigger updates

"Computation" function provides a resolver for using a dependency withing a calculation expression.

Dependency can be declared explicitly as an array by the second argument. It has the following advantages:

• Faster dependency subscription
• Ability to specify extra queries or observables as dependencies

A custom value comparator can be specified by "options" object as the second argument.
It helps to decide if a new value differs from a previous one in complex cases.

Example

const greeting = createStore('Hello');
const username = createStore('World');

// Dependency are implicitly resolved
const message = compute((get) => get(greeting) + ' ' + get(username) + '!');

// Dependency declared explicitly
const messageUppercase = compute(() => message.get().toUpperCase(), [message]);

expect(message.get()).toBe('Hello World!');
expect(messageUppercase.get()).toBe('HELLO WORLD!');

Perfomance

[coveralls]

Pull Request Test Coverage Report for Build 4180689321

• 174 of 174 (100.0%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 4068088631:	0.0%
Covered Lines:	512
Relevant Lines:	512

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💛 - Coveralls