Building Composable Functions

Introduction

Jetpack Compose is Android’s modern toolkit for building native UI. It simplifies and accelerates UI development on Android with less code, powerful tools, and intuitive Kotlin APIs. In this tutorial, we will cover how to build composable functions from start to finish.

What are Composable Functions?

Composable functions are the fundamental building blocks of the Jetpack Compose UI. These functions, annotated with @Composable, define the layout and behavior of a part of the UI. They are designed to be easily reusable and composable, allowing you to build complex UIs from smaller, simpler components.

Creating Your First Composable Function

Let’s start by creating a simple composable function that displays a greeting message.

import androidx.compose.material.Text
import androidx.compose.runtime.Composable

@Composable
fun Greeting(name: String) {
    Text(text = "Hello, $name!")
}

This function takes a single parameter, name, and uses the Text composable to display a greeting message.

Using Composable Functions

To use your composable function, call it from another composable function. For example, you could call it from the setContent block of your main activity.

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material.MaterialTheme
import androidx.compose.material.Surface

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MaterialTheme {
                Surface {
                    Greeting(name = "World")
                }
            }
        }
    }
}

Combining Composable Functions

Composable functions can call other composable functions, allowing you to build complex UIs. For example, let’s create another composable function that displays a list of greetings.

import androidx.compose.foundation.layout.Column

@Composable
fun Greetings(names: List) {
    Column {
        for (name in names) {
            Greeting(name = name)
        }
    }
}

In this example, the Greetings function takes a list of names and calls the Greeting function for each name, displaying them in a column.

State in Composable Functions

State is an important concept in Jetpack Compose. It allows your UI to react to data changes. You can use the remember function to hold state in a composable function.

import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember

@Composable
fun ClickCounter() {
    val count = remember { mutableStateOf(0) }
    Button(onClick = { count.value++ }) {
        Text("Clicked ${count.value} times")
    }
}

In this example, the ClickCounter function displays a button that increments a counter each time it is clicked. The state of the counter is remembered across recompositions.

Conclusion

In this tutorial, we covered the basics of building composable functions with Jetpack Compose. We created simple composable functions, combined them to build more complex UIs, and used state to make our UIs reactive. With these fundamentals, you can start building your own Jetpack Compose UIs.