Setting Up the Android Studio Project

1. Create a new project:

   • In Android Studio, navigate to File > New > New Project.
   • Enter a suitable Application Name and Company Domain, ensuring the Package Name matches the Unity project's Bundle Identifier.
   • Click Next, selecting "Phone and Tablet" and specifying the Minimum SDK.
   • Choose "Empty Activity" and use default values for the Activity and Layout Names.
   • Click Finish to generate the project.

2. Add Unity's classes.jar:

   • Copy the Editor\Data\PlaybackEngines\AndroidPlayer\Variations\mono\Release\Classes\classes.jar file from the Unity installation directory.
   • Place it in the libs directory of your Android Studio project.
   • Right-click on Project, select Open Module Settings, and add classes.jar as a dependency.

Developnig the Android Code

Edit the main activity to include the necessary Unity integration:

import android.os.Bundle;
import android.widget.Toast;
import com.unity3d.player.UnityPlayer;
import com.unity3d.player.UnityPlayerActivity;

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

    public void ShowToast(final String message){
        runOnUiThread(() -> Toast.makeText(getApplicationContext(), message, Toast.LENGTH_LONG).show());
    }

    public static String GetInformation() {
        return "This is a Plugin's content!";
    }
}

Modifying the Build Configuration

Adjust build.gradle to configure the project for AAR export:

1. Replace apply plugin: 'com.android.application' with apply plugin: 'com.android.library'.
2. Remove the applicationId entry.
3. The modified build.gradle:

apply plugin: 'com.android.library'

android {
    compileSdkVersion 24
    buildToolsVersion "24.0.1"
    defaultConfig {
        minSdkVersion 18
        targetSdkVersion 24
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(include: ['*.jar'], dir: 'libs')
    implementation files('libs/classes.jar')
}

Finalizing the Android Manifest File

Ensure the manifest file contains the appropriate <meta-data> information for the main activity. Remove unnecessary resources such as the styles.xml file.

Exporting and Integrating the Plugin with Unity

1. Build the AAR file by selecting Build > Build APK.
2. Remove the classes.jar file from the generated AAR, as Unity includes its copy during APK generation.

Importing the Plugin into Unity

1. Create a Unity project.
2. Import the generated AAR file:
   • Create the Plugins/Android directory in your Unity project.
   • Place the AAR file and Android Manifest in the Android folder.

Implementing the Unity Code

Create a script to interface with the Android functionality:

using UnityEngine;

public class Call : MonoBehaviour {
    string information_ = null;

    void OnGUI() {
        if(GUI.Button(new Rect(0, 0, 200, 20), "Show Toast - Hello World!")) {
            using (AndroidJavaClass jc = new AndroidJavaClass("com.unity3d.player.UnityPlayer")) {
                using (AndroidJavaObject jo = jc.GetStatic<AndroidJavaObject>("currentActivity")) {
                    jo.Call("ShowToast", "Hello World!");
                }
            }
        }

        if (GUI.Button(new Rect(0, 40, 200, 20), "Get Plugin's Information")) {
            using (AndroidJavaClass jc = new AndroidJavaClass("com.unity3d.player.UnityPlayer")) {
                using (AndroidJavaObject jo = jc.GetStatic<AndroidJavaObject>("currentActivity")) {
                    information_ = jo.CallStatic<string>("GetInformation");
                }
            }
        }

        GUI.Label(new Rect(220, 40, Screen.width - 220, 20), information_);
    }
}

Set the Unity project's Bundle Identifier and Minimum API Level to match the plugin's configuration. Build the Unity APK and test the functionality.