addHasValueListener
The addHasValueListener
method registers a listener function with the Store
that will be called when a Value
is added to or removed from the Store
.
addHasValueListener(
valueId: IdOrNull,
listener: HasValueListener<Store>,
mutator?: boolean,
): string
Type | Description | |
---|---|---|
valueId | IdOrNull | |
listener | HasValueListener<Store> | The function that will be called whenever the matching |
mutator? | boolean | An optional boolean that indicates that the listener mutates |
returns | string | A unique |
The provided listener is a HasValueListener
function, and will be called with a reference to the Store
and the Id
of Value
that changed. It is also given a flag to indicate whether the Value
now exists (having not done previously), or does not (having done so previously).
You can either listen to a single Value
being added or removed (by specifying the Value
Id
) or any Value
being added or removed (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 a specific Value
being added or removed.
import {createStore} from 'tinybase';
const store = createStore().setValues({open: true, employees: 3});
const listenerId = store.addHasValueListener(
'employees',
(store, valueId, hasValue) => {
console.log('employee value ' + (hasValue ? 'added' : 'removed'));
},
);
store.delValue('employees');
// -> 'employee value removed'
store.setValue('employees', 4);
// -> 'employee value added'
store.delListener(listenerId);
This example registers a listener that responds to any Value
being added or removed.
import {createStore} from 'tinybase';
const store = createStore().setValues({open: true, employees: 3});
const listenerId = store.addHasValueListener(
null,
(store, valueId, hasValue) => {
console.log(valueId + ' value ' + (hasValue ? 'added' : 'removed'));
},
);
store.delValue('employees');
// -> 'employees value removed'
store.setValue('website', 'https://pets.com');
// -> 'website value added'
store.delListener(listenerId);
This example registers a listener that responds to a specific Value
being added or removed, and which also mutates the Store
itself.
import {createStore} from 'tinybase';
const store = createStore().setValues({open: true, employees: 3});
const listenerId = store.addHasValueListener(
'employees',
(store) => store.setValue('updated', true),
true,
);
store.delValue('employees');
console.log(store.getValues());
// -> {open: true, updated: true}
store.delListener(listenerId);
Since
v4.4.0