electrum-store

Electrum Store

Electrum Store (electrum-store) provides a store implementation tailored for use with electrum-arc, the Electrum Agnostic Reactive Components.

The store maintains state organized as a tree. State is immutable. When the store is updated, new state is produced and nodes get replaced in the tree.

Neither the store nor its states will emit notifications when things change, since electrum does not need the feature.

Thanks to immutability, whole trees can be compared for equality with ===. Whenever a (sub-)tree changes, the store guarantees that the state objects change too, from the node in the tree where the change happened up to the root of the tree (change percolation).

Store

Create a store

To create a store, call Store.create(). The constructor is not available for public consumption and should not be called.

const store = Store.create ();

Access state in the store

The store maintains its state as a tree. Selecting state located at a.b.c will automatically create a, a.b and a.b.c if they did not yet exist in the tree. You can call select() on a state object, which can be used to navigate down the tree.

select() creates missing nodes whereas find() returns undefined if it does not find the specified nodes.

const store = Store.create ();
const state1 = store.select ('a.b.c');
const state2 = store.select ('a').select ('b.c');
const state3 = store.find ('a.b.c');
const state4 = store.find ('x.y');
expect (state1).to.equal (state2);
expect (state1).to.equal (state3);
expect (state4).to.equal (undefined);

Mutate the store

Whenever new state needs to be recorded in the store, the tree will be updated and new generation tags will be applied to the parts of the tree which changed as a result of this.

Setting a.b.c first will produce nodes a, b and c in generation 1. Adding a.b.d will mutate a.b (it contains a new child d) and also mutate a (it contains an updated a.b); all this will happen inside generation 2. Nodes a, b and d will have generation:2 whereas node c will remain at generation:1.

const store = Store.create ();
store.select ('a.b.c'); // generation 1
store.select ('a.b.d'); // generation 2
expect (store.find ('a').generation).to.equal (2);
expect (store.find ('a.b').generation).to.equal (2);
expect (store.find ('a.b.c').generation).to.equal (1);
expect (store.find ('a.b.d').generation).to.equal (2);

Explicitly set state

State is usually updated using with(), withValue() and withValues() or created implicitly by select(). It is also possible to set state explicitly:

const store = Store.create ();
const state1 = State.create ('x.y');
const state2 = store.setState (state1);
expect (state1.generation).to.equal (0);
expect (state1).to.not.equal (state2);
expect (state2.generation).to.equal (1);

State

State holds following information:

• id → the absolute path of the node (e.g. 'a.b.c')
• store → a reference to the containing store.
• generation → the generation number of last update.
• values → a collection of values - this is never accessed directly.

The default value is accessed with state.value. Named values can be accessed using state.get(name).

Create state

To create state with an initial value, use State.create().

const state1 = State.create ('empty');
expect (state1.value).to.equal (undefined);

const state2 = State.create ('message', {'': 'Hello'});
expect (state2.value).to.equal ('Hello');

const state3 = State.create ('person', {name: 'Joe', age: 78});
expect (state3.get ('name')).to.equal ('Joe');
expect (state3.get ('age')).to.equal (78);

Mutate state

State objects are immutable. Updating state will produce a copy of the state object with the new values.

const state1 = State.create ('a', {x: 1, y: 2});
const state2 = State.withValue (state1, 'x', 10);
const state3 = State.withValues (state1, 'x', 10, 'y', 20);
const state4 = State.with (state1, {values: {x: 10, y: 20}});

// Setting same values does not mutate state:
const state5 = State.with (state1, {values: {x: 1, y: 2}});
expect (state1).to.equal (state5); // same values, same state

It is also possible to use method set() to create a new state; this is just syntactic sugar over the State.withValue() static methods.

const state1 = State.create ('a');
const state2 = state1.set ('x', 1);
const state3 = state2.set ('a'); // set default value
expect (state2.get ('x')).to.equal (1);
expect (state3.get ()).to.equal ('a');

Mutate state in a store

When a state attached to a store is being mutated, the new state will be stored in the tree, and all nodes up to the root will get updated while doing so.

const store = Store.create ();
expect (store.select ('a.b.c').generation).to.equal (1);  // gen. 1
expect (store.select ('a.b.d').generation).to.equal (2);  // gen. 2
State.withValue (store.select ('a.b.c'), 'x', 10); // gen. 3
expect (store.select ('a.b.c').generation).to.equal (3);
expect (store.select ('a.b.d').generation).to.equal (2); // unchanged
expect (store.select ('a.b').generation).to.equal (3);
expect (store.select ('a').generation).to.equal (3);
State.withValue (store.select ('a.b'), 'y', 20); // gen. 4
expect (store.select ('a.b.c').generation).to.equal (3); // unchanged
expect (store.select ('a.b.d').generation).to.equal (2); // unchanged
expect (store.select ('a.b').generation).to.equal (4);
expect (store.select ('a').generation).to.equal (4);