#Listen to atom value changes

This is what happens with useAtom hook, it listens to the atom's value change using onChange method.

// anywhere in your app
import { currencyAtom } from "~/src/atoms";

currencyAtom.onChange((newValue, oldValue, atom) => {
  //
});

Please note the onChange is returning an EventSubscription we can remove the listener anytime, for example when unmounting the component. >

// anywhere in your app
import { currencyAtom } from "~/src/atoms";

// in your component...
const [currency, setCurrency] = useState(currencyAtom.value);
useEffect(() => {
  const onCurrencyChange = currencyAtom.onChange(setCurrency);
  return () => onCurrencyChange.unsubscribe();
}, []);

#Watch For Partial Change

Sometimes you may need to watch for only a key in the atom's value object, the atom.watch function is the perfect way to achieve this.

Please note this only works if the atom's default is an object or an array. >

// anywhere in your app
import { atom } from "@mongez/react-atom";

const userAtom = atom({
  key: "user",
  default: {
    key: "Hasan",
    address: {
      city: "New York",
    },
  },
});

userAtom.watch("key", (newName, oldName) => {
  console.log(newName, oldName); // 'Hasan', 'Ali'
});

// later in the app
userAtom.update({
  ...userAtom.value,
  key: "Ali",
});

Dot notation is allowed too.

// anywhere in your app
import { atom } from "@mongez/react-atom";

const userAtom = atom({
  key: "user",
  default: {
    key: "Hasan",
    address: {
      city: "New York",
    },
  },
});

userAtom.watch("address.cty", (newCity, oldCity) => {
  console.log(newName, oldName); // 'New York', 'Cairo'
});

// later in the app
userAtom.update({
  ...userAtom.value,
  address: {
    ...userAtom.value.address,
    city: "Cairo",
  },
});

#Atom Watch Hook

In some scenarios, we may need to watch for a key in the atom's value object for change and perform an action inside a component, the atom.useWatch hook is the perfect way to achieve this.

export function SomeComponent() {
  const [city, setCity] = useState(userAtom.get("address.city"));

  userAtom.useWatch("address.city", setCity);

  // first time will render New York then it will render Cairo

  return <>Current City: {city};
}

#Value Mutation Before Update

Sometimes it's useful to mutate the value before updating it in the atom, this can be achieved via defining beforeUpdate method in the atom declaration.

This is very useful especially when dealing with objects/arrays and you want to make some operations before using the final value.

beforeUpdate(newValue: any, oldValue: any, atom: Atom)

import { atom, Atom } from "@mongez/react-atom";

export const multipleAtom: Atom = atom({
  key: "multiple",
  default: 0,
  beforeUpdate(newNumber: number): number {
    return newNumber * 2;
  },
});

multipleAtom.update(4);

console.log(multipleAtom.value); // 8

#Listen to atom destruction

To detect atom destruction when destroy() method, use onDestroy.

// anywhere in your app
import { currencyAtom } from "~/src/atoms";

const subscription = currencyAtom.onDestroy((atom) => {
  //
});

#Atom Type

We can get the type of the atom's value using atom.type property.

const currencyAtom = atom({
  key: "currency",
  default: "USD",
});

console.log(currencyAtom.type); // string

If the default value is an array it will be returned as array not object.

const todoListAtom = atom({
  key: "todo",
  default: [],
});

console.log(todoListAtom.type); // array

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