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React to Changes - Node.js SDK

On this page

  • Register a Realm Change Listener
  • Register a Collection Change Listener
  • Register an Object Change Listener
  • Remove All Change Listeners
  • Change Notification Limits

Data in Realm is live, which means that an object always reflects its most recent saved state and read operations never block. Objects automatically update in response to changes, so you can see up-to-date data in your application without running a new query. Objects and queries emit notifications that can update your app whenever data changes.

You can register three types of notification listeners:

  • A realm listener fires whenever any object in a realm changes.

  • A collection listener fires whenever a specific query matches a new set of objects or when any matched object changes.

  • An object listener fires whenever a specific object is deleted or has one or more properties modified.

Register a Realm Change Listener

To register a change listener for an entire realm, pass a callback function to the realm's addListener() method. Realm calls the listener asynchronously whenever an operation adds, changes, or removes objects in the realm.

To remove a realm listener, pass the callback to the realm's removeListener() method.

Tip

Use Object & Collection Listeners for Change Details

Realm does not pass any information about what changed to realm listener callback functions. If you need to know more information about what changed in an object or collection, use object listeners and collection listeners.

Tip

Handling Exceptions Inside a Listener

To handle exceptions thrown from a change listener, wrap your addListener() call within a try...catch statement.

// Define a listener callback function
function onRealmChange() {
  console.log("Something changed!");
}
// Add the listener callback to the realm
try {
  realm.addListener("change", onRealmChange);
} catch (error) {
  console.error(
    `An exception was thrown within the change listener: ${error}`
  );
}
// Remember to remove the listener when you're done!
realm.removeListener("change", onRealmChange);

Register a Collection Change Listener

To register a change listener for a collection of Realm objects, pass a callback function to the collection's addListener() method. Realm calls the listener asynchronously when it's registered as well as whenever an operation adds, changes, or removes objects in the collection.

To remove a collection listener, pass the callback to the collection's removeListener() method.

Important

Order Matters

In collection notification handlers, always apply changes in the following order: deletions, insertions, then modifications. Handling insertions before deletions may result in unexpected behavior.

// You can define a listener for any collection of Realm objects
const dogs = realm.objects("Dog");
// Define a listener callback function for changes to any Dog
function onDogsChange(dogs, changes) {
  // Handle deleted Dog objects
  changes.deletions.forEach((index) => {
    // You cannot directly access deleted objects,
    // but you can update a UI list, etc. based on the index.
    console.log(`Looks like Dog #${index} has left the realm.`);
  });
  // Handle newly added Dog objects
  changes.insertions.forEach((index) => {
    const insertedDog = dogs[index];
    console.log(`Welcome our new friend, ${insertedDog.name}!`);
  });
  // Handle Dog objects that were modified
  changes.modifications.forEach((index) => {
    const modifiedDog = dogs[index];
    console.log(`Hey ${modifiedDog.name}, you look different!`);
  });
}
// Add the listener callback to the collection of dogs
try {
  dogs.addListener(onDogsChange);
} catch (error) {
  console.error(
    `An exception was thrown within the change listener: ${error}`
  );
}
// Remember to remove the listener when you're done!
dogs.removeListener(onDogsChange);

Register an Object Change Listener

To register a change listener on a specific Realm object, pass a callback function to the object's addListener() method. Realm calls the listener if any of the object's properties change or if someone deletes the object.

To remove an object listener, pass the callback to the object's removeListener() method.

// Define a listener callback function for changes to a specific Dog
function onDogChange(dog, changes) {
  if (changes.deleted) {
    console.log(`dog is deleted: ${changes.deleted}`);
  } else {
    changes.changedProperties.forEach((prop) => {
      console.log(`* the value of "${prop}" changed to ${dog[prop]}`);
    });
  }
}
// You can define a listener for any Realm object
try {
  dog.addListener(onDogChange);
} catch (error) {
  console.error(
    `An exception was thrown within the change listener: ${error}`
  );
}
// Remember to remove the listeners when you're done!
dog.removeListener(onDogChange);

Remove All Change Listeners

To remove all listeners on a given realm, object, or collection instance, call the instance's removeAllListeners() function:

// Remove all listeners from a realm
realm.removeAllListeners();
// Remove all listeners from a collection
dogs.removeAllListeners();
// Remove all listeners from an object
dog.removeAllListeners();

Change Notification Limits

Changes in nested documents deeper than four levels down do not trigger change notifications.

If you have a data structure where you need to listen for changes five levels down or deeper, workarounds include:

  • Refactor the schema to reduce nesting.

  • Add something like "push-to-refresh" to enable users to manually refresh data.

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