With the ability to add, modify, and delete notes, our app is nearly finished! The last step is to persist data between app launches so that user's notes won't get deleted every time they close the app.

Realm is a fast and easy-to-use mobile database that's used by many companies, including Google and Amazon. Realm is used to save and retrieve objects locally on our user's device. For Make School Notes, we want to save all of a user's notes when they close the app, and retrieve all of their notes when they reopen the app.

Importing the Realm Framework

When using the Realm framework in Swift you must use the following import statement:

import RealmSwift

We will assume that RealmSwift has been properly imported for the remainder of this tutorial. That means if something Realm related gives you an error, remember to check to see if you imported it!

Realm's Object Type

When using Realm to store objects, the object we are storing must inherit from a Realm provided class called Object. For instance, if we wanted to store and retrieve objects of type Person, we would declare the Person class as follows:

class Person: Object {

If our hypothetical Person class had name and age properties, they would need to be declared dynamic like so:

class Person: Object {
  dynamic var name = ""
  dynamic var age = 0

Despite the fact that we added a special superclass and a fancy keyword to the properties of our Person class, it will still behave the same as a regular class. We would create an instance of the Person class like this:

var chris = Person()
chris.name = "Chris"
chris.age = 23

Realm's Realm Type

Realm is the name of the framework; the framework also has an object that has type Realm. We save and retrieve objects from the default Realm. Realm (the framework) can be configured in many different ways, but for our purposes, the default Realm will suffice.

Before adding, modifying, retrieving, or deleting objects in Realm, we must get access to the default Realm:

let realm = try! Realm()

The try keyword in the above code signals that the call to Realm() can throw an error. (Throwing an error is a fancy way of saying that a method can fail.) By using the try! keyword (note the exclamation point) we are indicating that we know the Realm() method can throw an error, but that we are sure that it will not, and therefore will not handle the error case.

In Swift, errors are handled using the do/try/catch paradigm. For more information on the do/try/catch paradigm of error handling, check out these lecture slides or read the official documentation.

Write Transactions in Realm

Write transactions are a special way to inform Realm that we want to change something in the default Realm. For instance, if we wanted to add, modify, or delete an object we would do so inside of a write transaction. (We do not need to retrieve objects inside write transactions because object retrieval does not change anything in the default Realm.)

To start a write transaction, we do the following:

//write transaction
try! realm.write() {
  // save, modify, or delete some object(s) here

Saving Objects

Once inside the write transaction, we can save an object using the add() method. We pass the object we want to save to the add() method. If we wanted to save the chris variable from above, we would do the following:

try! realm.write() {

The above code saves the chris variable to the default Realm.

Updating Objects

Updating objects in the default Realm is really easy. All we have to do is create a write transaction and modify the object as desired. If we wanted to change the age property of the chris variable, we would do the following:

try! realm.write() {
  chris.age = 100

The above code will both update the age of chris and save it in the default realm.

Retrieving Objects

We use the objects() method to retrieve objects. We pass the type of the desired object to the object() method to specify what kind of objects we want to retrieve. If we wanted to retrieve all of our Person objects we would do the following:

let realm = try! Realm()
let people = realm.objects(Person)

The call to realm.objects(Person) will return all of the objects of type Person that have been saved to the default Realm.

Realm's Results Type

In the above code, the call to realm.objects(Person) returns an object of type Results<Person>. The objects() method is special because it returns a template type, indicated by the angled brackets <>. Template types are useful because they can take on many different types. For instance, the return type in the code above is Results<Person> because we requested all Person objects from our default Realm. However, if we had a Dog type and requested all Dogs from our default Realm, the return type would be Results<Dog>.

The Results type is similar to arrays and can be indexed using square bracket [] notation or using for..in loops.

Filtering Objects

What if we wanted to retrieve a specific instance of the Person class?

In Realm, there is no direct way to retrieve a single object. We must retrieve all the objects of the required type and then filter the returned Results object. For instance, if we wanted to retrieve the Person object where the name property is set to "Chris" and the age property is set to 23, we would do the following:

let realm = try! Realm()
let chris = realm.objects(Person).filter("name = 'Chris' AND age = 23")

Deleting Objects

Once inside the write transaction, we can delete an object using the delete() method. We pass the object we want to delete to the delete() method. If we wanted to delete the chris variable, we would do the following:

try! realm.write() {

The above code deletes the chris variable from the default realm.

Wrapping Up

We have discussed how to add, modify, retrieve, and delete objects in Realm and are ready to add data persistence to Make School Notes!

If you would like more practice with Realm, check out this demo project.

If you are interested in some of the more advanced features that Realm offers, check out the full documentation.

On this page, you should have:

  1. Learned the basic ways you can interact with Realm to persist data.


If you have feedback on this tutorial or find any mistakes, please open issues on the GitHub Repository or comment below.

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