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Extending Kotlin and Java classes ​

Subclassing Kotlin and Java classes ​

The following examples demonstrate how to subclass Kotlin/Java classes in NativeScript:

kotlin
class MyButton(context: Context) : android.widget.Button(context) {
    override fun setEnabled(enabled: Boolean) {
        super.setEnabled(enabled)
    }
}

val btn = MyButton(context)
java
public class MyButton extends android.widget.Button {
    public MyButton(Context context) {
        super(context);
    }

    @Override
    public void setEnabled(boolean enabled) {
        super.setEnabled(enabled);
    }
}

MyButton btn = new MyButton(context);

Doing the same in NativeScript:

ts
let constructorCalled = false

@NativeClass
class MyButton extends android.widget.Button {
  constructor() {
    super()
    constructorCalled = true

    // necessary when extending TypeScript constructors
    return global.__native(this)
  }

  setEnabled(enabled: boolean): void {
    this.super.setEnabled(enabled)
  }
}

const button = new MyButton(context)
js
let constructorCalled = false
const MyButton = android.widget.Button.extend({
  // constructor
  init: function () {
    constructorCalled = true
  },

  setEnabled: function (enabled) {
    this.super.setEnabled(enabled)
  },
})

const button = new MyButton(context)

Note

In the above example, the setEnabled function's use of this keyword points to the JavaScript object that proxies the extended native instance. The this.super usage provides access to the base class method implementation.

Creating an anonymous Java class which extends from the base Java java.lang.Object class ​

ts
@NativeClass
class MyClass extends java.lang.Object {
  constructor() {
    super()
    // necessary when extending TypeScript constructors
    return global.__native(this)
  }

  toString(): string {
    // override Object's toString
  }
}

const myClassInstance = new MyClass()
js
const MyClass = java.lang.Object({
  init() {
    // constructor
  },

  toString() {
    // override Object's toString
  },
})

const myClassInstance = new MyClass()

Creating named Java classes ​

To create a named Java class which extends from the java.lang.Object class:

ts
@NativeClass
@JavaProxy('org.nativescript.example.MyClass')
class MyClass extends java.lang.Object {
  // constructor
  constructor() {
    super()
    // necessary when extending TypeScript constructors
    return global.__native(this)
  }

  toString(): string {
    // override Object's toString
  }
}

const myClassInstance = new MyClass()

// note: this may result in a TypeScript error because the namespace is not typed
// this can be safely ignored with // @ts-ignore
// or by type-casting to `any` for example.
const myClassInstance2 = new org.nativescript.example.MyClass()
const myClassInstance3 = new (org as any).nativescript.example.MyClass()
js
const MyClass = java.lang.Object('org.nativescript.example.MyClass', {
  init() {
    // constructor
  },

  toString() {
    // override Object's toString
  },
})

const myClassInstance = new MyClass()
const myClassInstance2 = new org.nativescript.example.MyClass()

TIP

One important thing to note when dealing with extending classes and implementing interfaces in NativeScript is that, unlike in Java—where you can extend an Abstract class with a new java.arbitrary.abstract.Class() { }, in NativeScript the class needs to be extended as per the previous examples - using the extend function on the java.arbitrary.abstract.Class, or using the extends class syntax in TypeScript.

Custom Android Application and Activity ​

NativeScript provides a way to create custom android.app.Application and android.app.Activity implementations.

Extending Android Application ​

Create a new TypeScript file in the root of your project folder - name it application.android.ts or application.android.js if you are using plain JS.

Note

Note the *.android suffix - we want this file packaged for Android only.

A minimal example of the custom Application class:

ts
// the `JavaProxy` decorator specifies the package and the name for the native *.JAVA file generated.
@NativeClass()
@JavaProxy('org.nativescript.example.Application')
class Application extends android.app.Application {
  public onCreate(): void {
    super.onCreate()

    // At this point modules have already been initialized
    // Enter custom initialization code here
  }

  public attachBaseContext(baseContext: android.content.Context) {
    super.attachBaseContext(baseContext)

    // This code enables MultiDex support for the application (if needed)
    // androidx.multidex.MultiDex.install(this);
  }
}
js
const superProto = android.app.Application.prototype

// the first parameter of the `extend` call defines the package and the name for the native *.JAVA file generated.
android.app.Application.extend('org.nativescript.example.Application', {
  onCreate: function () {
    superProto.onCreate.call(this)

    // At this point modules have already been initialized
    // Enter custom initialization code here
  },
  attachBaseContext: function (base) {
    superProto.attachBaseContext.call(this, base)
    // This code enables MultiDex support for the application (if needed compile androidx.multidex:multidex)
    // androidx.multidex.MultiDex.install(this);
  },
})

To use the custom Application class, modify the application entry in the AndroidManifest.xml in App_Resources/Android/:

xml
<application
  android:name="org.nativescript.example.Application"
  android:allowBackup="true"
  android:icon="@mipmap/ic_launcher"
  android:label="@string/app_name"
  android:theme="@style/AppTheme">

Lastly, in order to correctly include the custom Application class, the extended Android application should be added to the webpack.config.js file.

js
const webpack = require('@nativescript/webpack')

module.exports = (env) => {
  webpack.init(env)

  webpack.chainWebpack((config) => {
    if (webpack.Utils.platform.getPlatformName() === 'android') {
      // make sure the path to the applicatioon.android.(js|ts)
      // is relative to the webpack.config.js
      config.entry('application').add('./application.android')
    }
  })

  return webpack.resolveConfig()
}

The source code of application.android.ts is bundled separately as application.js file which is loaded from the native Application.java class on launch.

The bundle.js and vendor.js files are not loaded early enough in the application launch. That's why the logic in application.android.ts is needed to be bundled separately in order to be loaded as early as needed in the application lifecycle.

Note

This approach will not work if application.android.ts requires external modules.

Extending Android Activity ​

@nativescript/core ships with a default androidx.appcompat.app.AppCompatActivity implementation, that bootstraps the NativeScript application, without forcing users to declare their custom Activity in every project. In some cases you may need to implement a custom Android Activity.

Create a new activity.android.ts or activity.android.js when using plain JS.

Note

Note the .android suffix - we only want this file on Android.

A basic Activity can be implemented as follows:

ts
import {
  Frame,
  Application,
  setActivityCallbacks,
  AndroidActivityCallbacks,
} from '@nativescript/core'

@NativeClass()
@JavaProxy('org.nativescript.example.CustomActivity')
class CustomActivity extends androidx.appcompat.app.AppCompatActivity {
  public isNativeScriptActivity

  private _callbacks: AndroidActivityCallbacks

  public onCreate(savedInstanceState: android.os.Bundle): void {
    Application.android.init(this.getApplication())
    // Set the isNativeScriptActivity in onCreate (as done in the original NativeScript activity code)
    // The JS constructor might not be called because the activity is created from Android.
    this.isNativeScriptActivity = true
    if (!this._callbacks) {
      setActivityCallbacks(this)
    }

    this._callbacks.onCreate(
      this,
      savedInstanceState,
      this.getIntent(),
      super.onCreate
    )
  }

  public onNewIntent(intent: android.content.Intent): void {
    this._callbacks.onNewIntent(
      this,
      intent,
      super.setIntent,
      super.onNewIntent
    )
  }

  public onSaveInstanceState(outState: android.os.Bundle): void {
    this._callbacks.onSaveInstanceState(
      this,
      outState,
      super.onSaveInstanceState
    )
  }

  public onStart(): void {
    this._callbacks.onStart(this, super.onStart)
  }

  public onStop(): void {
    this._callbacks.onStop(this, super.onStop)
  }

  public onDestroy(): void {
    this._callbacks.onDestroy(this, super.onDestroy)
  }

  public onPostResume(): void {
    this._callbacks.onPostResume(this, super.onPostResume)
  }

  public onBackPressed(): void {
    this._callbacks.onBackPressed(this, super.onBackPressed)
  }

  public onRequestPermissionsResult(
    requestCode: number,
    permissions: Array<string>,
    grantResults: Array<number>
  ): void {
    this._callbacks.onRequestPermissionsResult(
      this,
      requestCode,
      permissions,
      grantResults,
      undefined /*TODO: Enable if needed*/
    )
  }

  public onActivityResult(
    requestCode: number,
    resultCode: number,
    data: android.content.Intent
  ): void {
    this._callbacks.onActivityResult(
      this,
      requestCode,
      resultCode,
      data,
      super.onActivityResult
    )
  }
}
js
import { Frame, Application, setActivityCallbacks } from '@nativescript/core'

const superProto = androidx.appcompat.app.AppCompatActivity.prototype

androidx.appcompat.app.AppCompatActivity.extend(
  'org.nativescript.example.CustomActivity',
  {
    onCreate(savedInstanceState) {
      // Used to make sure the App is inited in case onCreate is called before the rest of the framework
      Application.android.init(this.getApplication())

      // Set the isNativeScriptActivity in onCreate (as done in the original NativeScript activity code)
      // The JS constructor might not be called because the activity is created from Android.
      this.isNativeScriptActivity = true
      if (!this._callbacks) {
        setActivityCallbacks(this)
      }
      // Modules will take care of calling super.onCreate, do not call it here
      this._callbacks.onCreate(
        this,
        savedInstanceState,
        this.getIntent(),
        superProto.onCreate
      )

      // Add custom initialization logic here
    },
    onNewIntent(intent) {
      this._callbacks.onNewIntent(
        this,
        intent,
        superProto.setIntent,
        superProto.onNewIntent
      )
    },
    onSaveInstanceState(outState) {
      this._callbacks.onSaveInstanceState(
        this,
        outState,
        superProto.onSaveInstanceState
      )
    },
    onStart() {
      this._callbacks.onStart(this, superProto.onStart)
    },
    onStop() {
      this._callbacks.onStop(this, superProto.onStop)
    },
    onDestroy() {
      this._callbacks.onDestroy(this, superProto.onDestroy)
    },
    onPostResume() {
      this._callbacks.onPostResume(this, superProto.onPostResume)
    },
    onBackPressed() {
      this._callbacks.onBackPressed(this, superProto.onBackPressed)
    },
    onRequestPermissionsResult(requestCode, permissions, grantResults) {
      this._callbacks.onRequestPermissionsResult(
        this,
        requestCode,
        permissions,
        grantResults,
        undefined
      )
    },
    onActivityResult(requestCode, resultCode, data) {
      this._callbacks.onActivityResult(
        this,
        requestCode,
        resultCode,
        data,
        superProto.onActivityResult
      )
    },
    /* Add any other events you need to capture */
  }
)

Note

The this._callbacks property is automatically assigned to your extended class by the frame.setActivityCallbacks method. It implements the AndroidActivityCallbacks interface and allows core to get notified for important Activity events. It is important to use these callbacks, as many parts of NativeScript rely on them!

Next, modify the activity in App_Resources/Android/src/main/AndroidManifest.xml

xml
<activity
  android:name="org.nativescript.example.CustomActivity"
  android:label="@string/title_activity_kimera"
  android:configChanges="keyboardHidden|orientation|screenSize">

To include the new Activity in the build, make sure it's added to the webpack.config.js with the following:

js
const webpack = require('@nativescript/webpack')

module.exports = (env) => {
  env.appComponents = (env.appComponents || []).concat(['./activity.android'])

  webpack.init(env)

  return webpack.resolveConfig()
}

Implementing Kotlin and Java interfaces ​

The next example shows how to implement an interface in Kotlin/Java with NativeScript. The main difference between inheriting classes and implementing interfaces in NativeScript is the use of the extend keyword. Implement an interface is done by passing the implementation object to the interface constructor function. The syntax is identical to Java Anonymous Classes.

kotlin
button.setOnClickListener {
    // Perform action on click
}
java
button.setOnClickListener(new View.OnClickListener() {
    public void onClick(View v) {
        // Perform action on click
    }
});

The same can be implemented in NativeScript:

ts
button.setOnClickListener(
  new android.view.View.OnClickListener({
    onClick() {
      // Perform action on click
    },
  })
)

Alternatively you can use the following pattern for a named interface implementation using TypeScript:

ts
@NativeClass()
@Interfaces([android.view.View.OnClickListener])
class ClickListener
  extends java.lang.Object
  implements android.view.View.OnClickListener
{
  constructor() {
    super()

    // necessary when extending TypeScript constructors
    return global.__native(this)
  }

  onClick(view: android.view.View): void {
    // Perform action on click
  }
}

nativeView.setOnClickListener(new ClickListener())

Or using JavaScript:

js
const ClickListener = java.lang.Object.extend({
  // the interfaces to use
  interfaces: [android.view.View.OnClickListener],
  onClick() {
    // Perform action on click
  },
})

nativeView.setOnClickListener(new ClickListener())
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