Jetpack Compose: Interoperability with Java

Jetpack Compose is revolutionizing Android UI development with its declarative approach and Kotlin-based syntax. However, many existing Android projects are built with Java. Ensuring seamless interoperability between Jetpack Compose and Java is crucial for gradual adoption and integration of Compose in existing codebases.

Understanding Interoperability

Interoperability refers to the ability of different programming languages to work together. In the context of Android development, it means that you can use Jetpack Compose code within Java code, and vice versa.

Why is Interoperability Important?

• Gradual Migration: Allows developers to adopt Compose incrementally in existing Java-based projects.
• Code Reuse: Enables leveraging existing Java components in Compose UIs.
• Flexibility: Provides the ability to mix and match the strengths of both languages.

Using Compose in Java

Integrating Jetpack Compose into a Java codebase requires setting up your project to support Compose and then hosting Compose UI elements within Java-based activities or fragments.

Step 1: Set up Your Project for Compose

First, ensure your build.gradle file includes the necessary dependencies and configurations for Compose. This usually involves:

• Enabling Compose in buildFeatures.
• Adding Compose dependencies.
• Setting the Kotlin compiler version.

Here’s an example of a build.gradle file configured for Compose interoperability:

android {
    buildFeatures {
        compose true
    }
    
    composeOptions {
        kotlinCompilerExtensionVersion compose_version
    }
}

dependencies {
    implementation("androidx.compose.ui:ui:$compose_version")
    implementation("androidx.compose.material:material:$compose_version")
    implementation("androidx.compose.ui:ui-tooling-preview:$compose_version")
    debugImplementation("androidx.compose.ui:ui-tooling:$compose_version")
    implementation("androidx.activity:activity-compose:1.8.2")
}

// Define the compose_version in your buildscript ext block or gradle.properties
ext {
    compose_version = "1.5.4"
}

Step 2: Hosting Compose UI in a Java Activity

To use Compose UI elements in a Java activity, you can use the ComposeView.

Here’s a Java activity that integrates a Compose UI:

import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.compose.ui.platform.ComposeView;
import androidx.compose.material.MaterialTheme;
import androidx.compose.material.Text;
import androidx.compose.runtime.Composable;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        ComposeView composeView = new ComposeView(this);
        composeView.setContent(() -> {
            return MaterialTheme.INSTANCE.getApplyDefaults() ? MaterialTheme.INSTANCE.getProvideTextStyle() != null ?
                    MaterialTheme.INSTANCE.ProvideTextStyle(
                        null,
                        (c,u) -> HelloWorldText()
                    ) : HelloWorldText()
                   : HelloWorldText();
        });
        setContentView(composeView);
    }

    @Composable
    private Text HelloWorldText() {
      return new Text("Hello from Compose!");
    }
}

Explanation:

• ComposeView: Acts as a bridge, allowing Compose UI elements to be rendered in a traditional Android view hierarchy.
• setContent: Takes a Composable function and sets it as the content of the ComposeView.

Note In order for `HelloWorldText` to be a proper `@Composable` you may need to make it a Kotlin method that has the `@Composable` annotation attached.

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

@Composable
fun HelloWorldText() {
    Text("Hello from Compose!")
}

Which now makes the java version look like:

import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;
import androidx.compose.ui.platform.ComposeView;
import androidx.compose.material.MaterialTheme;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        ComposeView composeView = new ComposeView(this);
        composeView.setContent(() -> {
            return MaterialTheme.INSTANCE.getApplyDefaults() ? MaterialTheme.INSTANCE.getProvideTextStyle() != null ?
                    MaterialTheme.INSTANCE.ProvideTextStyle(
                        null,
                        (c,u) -> MainActivityKt.HelloWorldText()
                    ) : MainActivityKt.HelloWorldText()
                   : MainActivityKt.HelloWorldText();
        });
        setContentView(composeView);
    }
}

Using Java Code in Compose

You can seamlessly call Java code from your Compose composables. Just ensure that your Java classes are accessible from your Kotlin/Compose code.

// Java class
public class MyJavaClass {
    public String getMessage() {
        return "Message from Java!";
    }
}

Now, in your Compose code:

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

@Composable
fun JavaInteropExample() {
    val javaClass = MyJavaClass()
    val message = javaClass.message
    Text(text = message)
}

Considerations for Interoperability

• Lifecycle Management: Ensure that lifecycle-aware components (like ViewModel) are handled correctly across Java and Compose boundaries.
• Threading: Manage threading properly to avoid blocking the UI thread. Both Java and Compose code should interact with UI elements on the main thread.
• Data Passing: When passing data between Java and Compose, be mindful of nullability and data types. Use appropriate conversions where necessary.
• State Management: Prefer Compose’s state management tools (like remember and mutableStateOf) for UI state within Compose elements.

Practical Examples

Example 1: Displaying Java Data in Compose

public class User {
    private String name;
    private int age;

    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }
}

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

@Composable
fun UserInfo(user: User) {
    Text("Name: ${user.name}, Age: ${user.age}")
}

Example 2: Using Java Callbacks in Compose

public interface OnClickListener {
    void onClick();
}

public class ButtonHandler {
    private OnClickListener listener;

    public void setOnClickListener(OnClickListener listener) {
        this.listener = listener;
    }

    public void handleClick() {
        if (listener != null) {
            listener.onClick();
        }
    }
}

import androidx.compose.material.Button
import androidx.compose.material.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.tooling.preview.Preview

@Composable
fun JavaCallbackExample() {
    val buttonHandler = ButtonHandler()

    Button(onClick = { buttonHandler.handleClick() }) {
        Text("Click Me")
    }

    buttonHandler.setOnClickListener(object : OnClickListener {
        override fun onClick() {
            println("Button clicked from Java!")
        }
    })
}

@Preview(showBackground = true)
@Composable
fun DefaultPreview() {
    JavaCallbackExample()
}

Conclusion

Jetpack Compose offers seamless interoperability with Java, enabling developers to gradually integrate Compose into existing projects. By using ComposeView to host Compose UI in Java activities and calling Java code directly from Compose, you can leverage the benefits of both languages. Understanding the key considerations for interoperability ensures a smooth and efficient transition, allowing you to modernize your Android applications while preserving valuable existing code.