Jetpack Compose for Kotlin: Complete Guide with Examples

What is Jetpack Compose and why is it needed?

Jetpack Compose is a modern toolkit for building user interfaces (UI) for Android, developed by Google. Unlike the traditional approach based on XML markup and the View system, Compose uses a declarative approach. This means you describe what the UI should look like for each state of data, rather than describing a sequence of steps to change it.

Main advantages of Jetpack Compose:

• Declarative: You describe the UI as a function of state. When the state changes, Compose automatically redraws only those parts of the UI that have changed.
• Less code: Compared to XML + View, Compose code is usually shorter and more readable.
• Intuitiveness: The Kotlin language is used, making the code more natural for Android developers.
• Performance: Compose is optimized to work with new Android architectures and minimizes redraws.
• Compatibility: Compose can be gradually introduced into existing projects, using it alongside traditional View.

Jetpack Compose is not just a library, but a whole ecosystem that includes tools for working with animation, navigation, testing, and much more. It is the future of Android development, and Google actively recommends switching to it.

Installing Jetpack Compose

Since Jetpack Compose is a library for Android, it is not installed via pip (Python) or npm (JavaScript). Instead, it is added to an Android project through the Gradle build system. Here is a step-by-step guide:

1. Setting up a project in Android Studio

Create a new Android project by selecting the "Empty Compose Activity" template. Android Studio will automatically configure all necessary dependencies.

2. Manual setup (if the project already exists)

Open the build.gradle.kts file (at the project level) and make sure the plugins block contains:

plugins {    id("com.android.application") version "8.2.0" apply false    id("org.jetbrains.kotlin.android") version "1.9.20" apply false}

Then open the build.gradle.kts file (at the module level, usually app) and add:

plugins {    id("com.android.application")    id("org.jetbrains.kotlin.android")}

android {    namespace = "com.example.myapp"    compileSdk = 34

    defaultConfig {        applicationId = "com.example.myapp"        minSdk = 24        targetSdk = 34        versionCode = 1        versionName = "1.0"    }

    buildFeatures {        compose = true    }

    composeOptions {        kotlinCompilerExtensionVersion = "1.5.5"    }

    kotlinOptions {        jvmTarget = "17"    }}

dependencies {    implementation(platform("androidx.compose:compose-bom:2024.01.00"))    implementation("androidx.compose.ui:ui")    implementation("androidx.compose.ui:ui-graphics")    implementation("androidx.compose.ui:ui-tooling-preview")    implementation("androidx.compose.material3:material3")    implementation("androidx.activity:activity-compose:1.8.2")    debugImplementation("androidx.compose.ui:ui-tooling")    debugImplementation("androidx.compose.ui:ui-test-manifest")}

After syncing Gradle, you can start using Jetpack Compose.

Quick start with a code example

The easiest way to start is to create a screen with the text "Hello, World!". To do this, in your Activity (or anywhere else you use Compose), call the setContent function:

import android.os.Bundleimport androidx.activity.ComponentActivityimport androidx.activity.compose.