code: // Licensed to the Software Freedom Conservancy (SFC) under one
  // or more contributor license agreements.  See the NOTICE file
  // distributed with this work for additional information
  // regarding copyright ownership.  The SFC licenses this file
  // to you under the Apache License, Version 2.0 (the
  // "License"); you may not use this file except in compliance
  // with the License.  You may obtain a copy of the License at
  //
  //   http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing,
  // software distributed under the License is distributed on an
  // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  // KIND, either express or implied.  See the License for the
  // specific language governing permissions and limitations
  // under the License.
  
  package org.openqa.selenium.devtools.idealized;
  
  import java.util.LinkedList;
  import java.util.List;
  import java.util.function.Consumer;
  import org.openqa.selenium.JavascriptException;
  import org.openqa.selenium.devtools.Command;
  import org.openqa.selenium.devtools.DevTools;
  import org.openqa.selenium.devtools.Event;
  import org.openqa.selenium.devtools.events.ConsoleEvent;
  import org.openqa.selenium.internal.Require;
  
  public abstract class Events<CONSOLEEVENT, EXCEPTIONTHROWN> {
  
    private final DevTools devtools;
    private final List<Consumer<ConsoleEvent>> consoleListeners = new LinkedList<>();
    private final List<Consumer<JavascriptException>> exceptionListeners = new LinkedList<>();
    private boolean consoleListenersEnabled = false;
  
    public Events(DevTools devtools) {
      this.devtools = Require.nonNull("DevTools", devtools);
    }
  
    public void addConsoleListener(Consumer<ConsoleEvent> listener) {
      Require.nonNull("Event handler", listener);
  
      consoleListeners.add(listener);
  
      initializeConsoleListeners();
    }
  
    public void addJavascriptExceptionListener(Consumer<JavascriptException> listener) {
      Require.nonNull("Listener", listener);
  
      exceptionListeners.add(listener);
  
      initializeConsoleListeners();
    }
  
    private void initializeConsoleListeners() {
      if (consoleListenersEnabled) {
        return;
      }
  
      devtools.send(enableRuntime());
  
      devtools.addListener(
          consoleEvent(),
          event -> {
            ConsoleEvent consoleEvent = toConsoleEvent(event);
  
            consoleListeners.forEach(l -> l.accept(consoleEvent));
          });
  
      devtools.addListener(
          exceptionThrownEvent(),
          event -> {
            JavascriptException exception = toJsException(event);
  
            exceptionListeners.forEach(l -> l.accept(exception));
          });
  
      consoleListenersEnabled = true;
    }
  
    public void disable() {
      devtools.send(disableRuntime());
      consoleListeners.clear();
      consoleListenersEnabled = false;
    }
  
    protected abstract Command<Void> enableRuntime();
  
    protected abstract Command<Void> disableRuntime();
  
    protected abstract Event<CONSOLEEVENT> consoleEvent();
  
    protected abstract Event<EXCEPTIONTHROWN> exceptionThrownEvent();
  
    protected abstract ConsoleEvent toConsoleEvent(CONSOLEEVENT event);
  
    protected abstract JavascriptException toJsException(EXCEPTIONTHROWN event);
  }
  