New Way Handle State, Event With Sealed Classes in Android

Sometimes, we want to represent an Event, a State in Android. Such as handle action click item ViewHolder in RecyclerView, or return state when calling API from the server (SUCCESS, FAIL, LOADING)… We have a lot of solutions for this case, for example use interface, abstract class, or enum class…

With interface or abstract class, it is cumbersome as well as not suitable for expansion. Enum class is often used to represent a state, however, it can show multi values but only allow a single instance of each value and it cannot contain additional information for each type.

For example we want a return state when calling API from the server with retrofit

enum class StateApi {
    LOADING,
    SUCCESS,
    ERROR
}

The downside here is that we cannot insert additional data into each StateApi type.

Such as ERROR(val exeption: Exption), SUCCESS(val data: Data) …

It’s great to be in Kotlin, Sealed class is the best solution 👌

Intro Sealed classes

“Sealed classes = abstract classes + enum++ classes”

Sealed classes are used for representing restricted class hierarchies, when a value can have one of the types from a limited set, but cannot have any other type

Can be defined Sealed classes is an extension of enum classes, and whereas a subclass of a sealed class can have multiple instances that can contain a state. Each instance of Sealed class can contain its own data

To declare a sealed class, we put the sealed modifier before the name of the class. A sealed class can have subclasses (object, data class…), but all of them must be declared in the same file as the sealed class itself

sealed class StateApi {
    object Loading : StateApi()
    data class Success(val data: String) : StateApi()
    data class Error(val error: Exception) : StateApi()
}

The example above: Loading, Success, Error is a instance of StateApi. The special thing here is Success is data class contain params is data: String and Error contain params is an error: Exception. So we can easily return states when request data from API.

A sealed class is abstract by itself, it cannot be instantiated directly and can have abstract members. Sealed classes are not allowed to have non-private constructors

Some use cases for Sealed classes

Handle state request API

Handle event action item ViewHolder replace for interface class

Besides, we can use sealed classes for ViewHolder type, the action of view in fragment or activity…

Hopefully, this article will help you to understand more about how to use sealed classes in real situations.

Let me know your thoughts on this article.

Thanks!!! Happy Coding! 😇

Reference

Sealed Classes