Custom Native Android Code

Many apps will want to add custom Java or Kotlin code to implement native features, without the overhead of building and publishing a proper Capacitor plugin.

There are two ways to do this depending on whether or not you need to access that code from the WebView:

WebView Accessible Native Code

The easiest way to build custom native code that needs to be accessible in the WebView is to build a local Capacitor plugin for it. In this case, building the plugin is as simple as building a class that inherits from com.getcapacitor.Plugin and uses the @NativePlugin() and @PluginMethod() annotations.

Here are examples of custom code in Java and Kotlin:

Java

com/example/myapp/CustomNativePlugin.java in android/app/src/main/java:

package com.example.myapp;

import com.getcapacitor.NativePlugin;
import com.getcapacitor.Plugin;
import com.getcapacitor.PluginCall;
import com.getcapacitor.PluginMethod;

@NativePlugin()
public class CustomNativePlugin extends Plugin {

  @PluginMethod()
  public void customCall(PluginCall call) {
    String message = call.getString("message");
    // More code here...
    call.success();
  }

  @PluginMethod()
  public void customFunction(PluginCall call) {
    // More code here...
    call.resolve();
  }
}

Kotlin

It is also possible to develop custom code with Kotlin. When adding new Kotlin files in Android Studio, you will be prompted to configure Kotlin in your project if necessary. When doing this, make sure to only configure Kotlin in your app module, not the Capacitor or third-party modules.

com/example/myapp/CustomNativePlugin.kt in android/app/src/main/java:

package com.example.myapp;

import com.getcapacitor.NativePlugin;
import com.getcapacitor.Plugin;
import com.getcapacitor.PluginCall;
import com.getcapacitor.PluginMethod;

@NativePlugin
class CustomNativePlugin : Plugin() {

  @PluginMethod
  fun customCall(call: PluginCall) {
    val message = call.getString("message")
    // More code here...
    call.success()
  }

  @PluginMethod
  fun customFunction(call: PluginCall) {
    // More code here...
    call.resolve()
  }
}

Registering Plugin Code

The final step is to register the plugin in your Activity. Registering a Kotlin plugin class in the Activity is the same as registering a Java class:

// Other imports...
import com.example.myapp.CustomNativePlugin;

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

    // Initializes the Bridge
    this.init(savedInstanceState, new ArrayList<Class<? extends Plugin>>() {{
      // Additional plugins you've installed go here
      // Ex: add(TotallyAwesomePlugin.class);
      add(CustomNativePlugin.class);
    }});
  }
}

Then you can use your functions in your webView code:

// Other codes...
import { Plugins } from '@capacitor/core';
const { CustomNativePlugin } = Plugins;
// Other codes...
CustomNativePlugin.customCall({ message: 'CUSTOM MESSAGE' });
CustomNativePlugin.customFunction();
// Other codes...

For more usages of plugin APIs, have a look at Capacitor Android Plugin Guide.

Private Native Code

If your code doesn't need to be accessible from the WebView, then simply add your code anywhere it needs to go. With Capacitor, you have full control over your native project. Need to add a new event handler in your Activity? Just update MainActivity and add it. The world is truly your oyster.