Jetpack Compose has revolutionized Android UI development by introducing a declarative UI paradigm. At its core, Compose relies on two fundamental concepts: State and Recomposition. These concepts are crucial for building efficient, responsive, and reactive user interfaces.

What is State in Jetpack Compose?

State is any value that can change over time and affect the UI. In the context of UI development:

• State represents any piece of data that can change over time and impacts the UI.
• For example: A counter value in a button click scenario, a text field’s current input, or the visibility of a UI component( something like loading spinner).
• Example:

var count by remember { mutableStateOf(0) }

I have a question: Is State quite similar to a regular Kotlin variable?

To answer this, we first need to understand what recomposition is and how it works. Once we understand that, we’ll be able to see the difference between state and regular Kotlin variables.

So, What is recomposition?

In Jetpack Compose, we build apps using hierarchies of composable functions. Each composable function takes data as input and uses it to create parts of the user interface, which are then displayed on the screen by the Compose runtime system.

Typically, the data passed between composable functions comes from a state variable declared in a parent function. If the state value changes in the parent, the child composables relying on that state must also reflect the updated value. Compose handles this with a process called recomposition.

Recomposition happens whenever a state value changes. Compose detects this change, identifies the affected composable functions, and calls them again with the updated state value.

Why is recomposition efficient?

Instead of rebuilding the entire UI tree, Compose uses intelligent recomposition. This means only the functions that actually use the changed state value are recomposed. This approach ensures that updates are fast and efficient, avoiding unnecessary processing.

In short, Compose efficiently updates only the parts of the UI affected by state changes, keeping performance smooth and responsive.

Now, back to our question: What’s the difference between a state variable and a regular Kotlin variable?

At first glance, a state variable in Jetpack Compose might seem similar to a regular Kotlin variable, which can also hold changing values during an app’s execution. However, state differs from a regular variable in two important ways:

1. Remembering the state variable value: When you use a state variable inside a composable function (a UI function in Jetpack Compose), its value should be remembered between function calls. This means that the state doesn’t get reset every time the composable function is called again. If it were a normal variable, it would get reset each time the function is called, but for state variables, Jetpack Compose makes sure they “remember” their previous values so the UI stays consistent.
2. Implications of changing a state variable: When you change the value of a state variable, it triggers a rebuild of not just the composable function where the state is defined, but also all the composable functions that depend on that state. This means that the entire UI tree (the hierarchy of composables) can be affected, and parts of it may need to be redrawn to reflect the new state. This is different from a regular function where changes in local variables only affect that specific function.

Let’s compare how state behaves differently from a regular Kotlin variable in code.

@Composable
fun Counter() {
    // State variable that remembers its value
    var count by remember { mutableStateOf(0) }

    Button(onClick = { count++ }) {
        Text("Clicked $count times")
    }
}

Here,

• count is a state variable. Its value is “remembered” across recompositions.
• When count changes (on clicking the button), the UI updates automatically.

On the other hand, with a regular Kotlin variable…

@Composable
fun Counter() {
    // Regular Kotlin variable
    var count = 0

    Button(onClick = { count++ }) {
        Text("Clicked $count times")
    }
}

Here,

• count is a regular Kotlin variable.
• When the button is clicked, count is incremented, but it doesn’t “remember” its previous value.
• As a result, the composable doesn’t re-render with the updated value because the value of count is reset every time the composable is called.

In short: State in composables is “sticky” (it remembers its value), and changing the state affects the UI by causing relevant parts to be updated automatically. These two differences make state a powerful tool for managing dynamic and reactive UIs in Compose.

Mutable vs Immutable State

• Mutable State: Can change over time (e.g., mutableStateOf)
• Immutable State: Cannot change once initialized (e.g., val)

Why is State Important?

State drives how your UI looks and behaves. In Compose:

• State updates trigger recompositions (UI updates).
• Compose ensures the UI is always consistent with the current state.

Conclusion

State and Recomposition are foundational concepts in Jetpack Compose. Properly managing state and understanding how recomposition works can lead to efficient and scalable Android applications.

Understanding these concepts is not just about syntax but about building a mental model for how Jetpack Compose interacts with state changes. Start experimenting with small examples, and you’ll soon master the art of managing state in Compose effectively.