`addValueListener`

The addValueListener method registers a listener function with the Store that will be called whenever data in a Value changes.

addValueListener(
  valueId: IdOrNull,
  listener: ValueListener,
  mutator?: boolean,
): Id

	Type	Description
`valueId`	`IdOrNull`	The `Id` of the `Value` to listen to, or `null` as a wildcard.
`listener`	`ValueListener`	The function that will be called whenever data in the matching `Value` changes.
`mutator?`	`boolean`	An optional boolean that indicates that the listener mutates `Store` data.
returns	`Id`	A unique `Id` for the listener that can later be used to call it explicitly, or to remove it.

The provided listener is a ValueListener function, and will be called with a reference to the Store, the Id of the Value that changed, the new Value value, the old Value, and a GetValueChange function in case you need to inspect any changes that occurred.

You can either listen to a single Value (by specifying the Value Id) or changes to any Value (by providing a null wildcard).

Use the optional mutator parameter to indicate that there is code in the listener that will mutate Store data. If set to false (or omitted), such mutations will be silently ignored. All relevant mutator listeners (with this flag set to true) are called before any non-mutator listeners (since the latter may become relevant due to changes made in the former). The changes made by mutator listeners do not fire other mutating listeners, though they will fire non-mutator listeners.

Examples

This example registers a listener that responds to any changes to a specific Value.

const store = createStore().setValues({open: true, employees: 3});
const listenerId = store.addValueListener(
  'employees',
  (store, valueId, newValue, oldValue, getValueChange) => {
    console.log('employee value changed');
    console.log([oldValue, newValue]);
    console.log(getValueChange('employees'));
  },
);

store.setValue('employees', 4);
// -> 'employee value changed'
// -> [3, 4]
// -> [true, 3, 4]

store.delListener(listenerId);

This example registers a listener that responds to any changes to any Value.

const store = createStore().setValues({open: true, employees: 3});
const listenerId = store.addValueListener(null, (store, valueId) => {
  console.log(`${valueId} value changed`);
});

store.setValue('employees', 4);
// -> 'employees value changed'
store.setValue('open', false);
// -> 'open value changed'

store.delListener(listenerId);

This example registers a listener that responds to any changes to a specific Value, and which also mutates the Store itself.

const store = createStore().setValues({open: true, employees: 3});
const listenerId = store.addValueListener(
  'employees',
  (store, valueId) => store.setValue('updated', true),
  true,
);

store.delValue('employees');
console.log(store.getValues());
// -> {open: true, updated: true}

store.delListener(listenerId);

Since

v3.0.0