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);

A state (with all its children) can also be removed from the store:

const store = Store.create ();
store.select ('a.b.c'); // generation 1
store.select ('a.b.d'); // generation 2
store.remove ('a.b'); // generation 3
expect (store.find ('a').generation).to.equal (3);
expect (store.find ('a.b')).to.not.exist ();
expect (store.find ('a.b.c')).to.not.exist ();
expect (store.find ('a.b.d')).to.not.exist ();

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);

Check the generation

State has an intrinsic generation which can be retrieved with the generation property. As soon as a state is attached to a store, its generation will be a strictly positive integer (1...n).

The helper method shouldUpdate() returns true when the provided generation is compatible with the state's own generation (i.e. equal or more recent to the state generation).

const store = Store.create ();
store.select ('a');
store.select ('a.b');
const state = store.select ('a.b');
expect (state.generation).to.equal (2);
expect (state.shouldUpdate (1)).to.be.false ();  // provided gen=1 older than state.generation 
expect (state.shouldUpdate (2)).to.be.true ();
expect (state.shouldUpdate (3)).to.be.true ();

State

State holds following information:

• id → the absolute path of the node (e.g. 'a.b.c').
• key → the local path of the node (e.g. 'c' if the id is '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).

Read back values from the state

• get (name) or get () → the value for name (or the default value if no name is specified), if it exists; otherwise undefined.
• getInherited (name) → the value for name if it can be found on the state or any of its parent nodes, otherwise undefined.
• contains (name) → true if a value exists for name, otherwise false.

States as arrays

When a state (a node of the tree) contains multiple child nodes, and when these nodes have an integer key (zero or positive), this can be considered as a poor men's array. The keys are then equivalent to the indexes into the array. They can have gaps and need not be consecutive: [23, 24, 37] would be valid index keys.

Explore the tree from the state

• any (id) or any () → true if the state specified by id exists and if it is non-empty.
• exists (id) → true if the state specified by id exists.
• keys → an array of all the keys found for the state's child nodes.
  Example: ['a', '0', '12', 'b']
• indexKeys → an array of all the indexes of the state's child nodes; nodes will only be reported if their key is zero or a positive integer. The values will always be sorted.
  Example: [0, 1, 34]

The state can also be used as a starting point for find() and select(). Without any argument, they return the state itself.

• select() creates missing nodes.
• find() returns undefined if it does not find the specified child.
• remove() removes the node (if called without arguments) or the specified child, if an id is provided.

select(), find(), remove() and any() accept a child id or an index, which will be converted to a key and used to look up the child.

Working with state ids

State ids are similar to paths where the elements are separated by .. Class State provides some static methods to manipulate these ids:

• State.join (a, b, c, ...) → returns the joined path.
• State.getLeafId (id) → returns the last element of the path.
• State.getParentId (id) → returns the path of the immediate parent.
• State.getAncestorId (id, part) → returns the path of the first ancestor which contains the specified part (path element); if the last element of the id matches part, the full id will be returned.

expect (State.join ('a', 'b', 'c')).to.equal ('a.b.c');
expect (State.getLeafId ('a.b.c')).to.equal ('c');
expect (State.getParentId ('a.b.c')).to.equal ('a.b');
expect (State.getAncestorId ('a.b.c', 'b')).to.equal ('a.b');

Adding items to an array

Arrays can be built using state.add():

• add () → a new child state, where the key is equal to the current highest index, plus one.

This is an easy way to add new states to an array of states.

const store = Store.create ();
store.select ('a.1');
store.select ('a.2');
store.select ('a.5');

expect (store.select ('a').add ().key).to.equal ('6');
expect (store.select ('a').indexKeys).to.deep.equal ([1, 2, 5, 6]);

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
const state4 = state1.set ('x', 1, 'y', 2);
expect (state2.get ('x')).to.equal (1);
expect (state3.get ()).to.equal ('a');
expect (state4.get ('x')).to.equal (1);
expect (state4.get ('y')).to.equal (2);

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);