Saving Data to File and Loading it Later

Often we need to save application data so that it can be reloaded when the application launches again and we can carry on where we left off.

When we have complex data structures (e.g. MutableLists of OOP objects), the simplest way to save the data within then to a file for later access, is to:

1. Serialize the data into a text format such as JSON
2. Write this JSON string to a text file (e.g. data.json)

In your IntelliJ IDEA project, you need to make sure that you're using the most recent version of Kotlin and the Compose library. You can find the latest versions listed here (most recent at the bottom of the table).

Edit the gradle.properties configuration file:

(As of writing thes are the latest versions)

                    kotlin.version = 1.9.10
                    compose.version = 1.5.1
                

You also need to add the serialization library to your build.

Edit the build.gradle.kts configuration file, adding the following lines to the existing plugins and dependencies sections:

                    plugins {
                        ...
                        kotlin("plugin.serialization") version "1.9.10"
                    }

                    dependencies {
                        ...
                        implementation("org.jetbrains.kotlinx:kotlinx-serialization-json:1.5.1")
                    }
                

When creating classes for data inside your application, you should use data class. This is a simple type of class that is specifically for data-based objects.

To allow serialization of the data, the data class needs to be labelled as @Serializable.

Data Classes come with useful, built-in functionality, such as a pre-configured .toString() function, a .copy() function, etc.

Note: Date / Time Properties

If you are using dates (via the LocalDate class), things are a little more complex. See the Serializing Dates section below.

You would usually want to load your data when your application first loads, and before the UI is first rendered...

Generally, you want to save your data each time the data is modified (i.e. new item added, item deleted, or item edited).

So you might have a button that creates a new object and adds it to the MutableList, then the list is saved...

                Button(
                    onClick = {
                        val newUser = User(...)
                        users.add(newUser)

                        // Data has changed, so save it
                        users.saveToFile()
                    }
                ) {
                    Text("Add New User")
                }
            

Note that you could chose to wait until the application is closing before you save any of your data, but this risks data loss if the application crashes.