Android Native Module Example

EmpManToast.js

/**
 * This exposes the native EmpManToast module as a JS module. This has a
 * function 'show' which takes the following parameters:
 *
 * 1. String message: A string with the text to toast
 * 2. int duration: The duration of the toast. May be ToastExample.SHORT or
 *    ToastExample.LONG
 */

import {NativeModules} from 'react-native';

module.exports = NativeModules.EmpManToast;

EmpToastPackage.java

package com.emp_man;

import com.facebook.react.ReactPackage;
import com.facebook.react.bridge.NativeModule;
import com.facebook.react.bridge.ReactApplicationContext;
import com.facebook.react.uimanager.ViewManager;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class EmpToastPackage implements ReactPackage {

  @Override
  public List<ViewManager> createViewManagers(ReactApplicationContext reactContext) {
    return Collections.emptyList();
  }

  @Override
  public List<NativeModule> createNativeModules(
                              ReactApplicationContext reactContext) {
    List<NativeModule> modules = new ArrayList<>();

    modules.add(new ToastModule(reactContext));

    return modules;
  }

}

login.js

import EmpManToast from './EmpManToast';

if (Platform.OS == "android") {
  EmpManToast.show('Awesome', EmpManToast.SHORT);
}

MainApplication.java

...
  ...
  ...
  
protected List<ReactPackage> getPackages() {
    return Arrays.<ReactPackage>asList(
            new MainReactPackage(),
            new EmpToastPackage()   // <-- Add this line with your package name
    ); 
}

...
  ...
  ...

ToastModule.java

package com.emp_man;

import android.widget.Toast;

import com.facebook.react.bridge.NativeModule;
import com.facebook.react.bridge.ReactApplicationContext;
import com.facebook.react.bridge.ReactContext;
import com.facebook.react.bridge.ReactContextBaseJavaModule;
import com.facebook.react.bridge.ReactMethod;

import java.util.Map;
import java.util.HashMap;

public class ToastModule extends ReactContextBaseJavaModule {

  private static final String DURATION_SHORT_KEY = "SHORT";
  private static final String DURATION_LONG_KEY = "LONG";

  public ToastModule(ReactApplicationContext reactContext) {
    super(reactContext);
  }
  
  @Override
  public String getName() {
    return "EmpManToast";
  }
  
  @Override
  public Map<String, Object> getConstants() {
    final Map<String, Object> constants = new HashMap<>();
    constants.put(DURATION_SHORT_KEY, Toast.LENGTH_SHORT);
    constants.put(DURATION_LONG_KEY, Toast.LENGTH_LONG);
    return constants;
  }
  
  @ReactMethod
  public void show(String message, int duration) {
    Toast.makeText(getReactApplicationContext(), message, duration).show();
  }
}