1. Basic principles of listener mode
The listener mode consists of the following parts:
- Event Source: The object that generates the event.
- Event Object: Encapsulated event details.
- Listener Interface: Defines a method to respond to a specific event.
- Listener Implementation: Implement the listener interface, including specific event processing logic.
Steps to Listener Mode
- Define event classes: Create an event class to encapsulate event information.
- Define the listener interface: Create an interface to define a method to respond to events.
- Implement the listener interface: Create a class, implement the listener interface, and write event processing logic in the method.
- Register the listener: Register the listener object into the event source. When an event occurs, the event source will notify the listener and call the corresponding method.
2. Practical application scenarios
2.1 Graphical User Interface (GUI) Event Processing
The listener mode is widely used in graphical user interface (GUI) programming and is used to handle user interaction events.
- Button click event: When the user clicks the button, the corresponding event handler is triggered.
- Mouse Events: Handle mouse click, move, enter, exit and other events.
- Keyboard events: Handle keyboard pressing, release and other events.
- Window Events: Handle events such as window opening, closing, minimizing, and maximizing.
JButton button = new JButton("Click Me");
(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
("Button clicked!");
}
});
2.2 Data model update notification
Listener mode is used to inform changes in the data model and is usually used to implement observer mode. When the data changes, all registered listeners will be notified to be updated.
- MVC mode: In the Model-View-Controller (MVC) architecture, when the model data changes, the view needs to be updated to reflect the changes.
- Data binding: In some frameworks, listeners are used to implement data binding and automatically update the interface when the data source changes.
public class DataModel {
private List<DataChangeListener> listeners = new ArrayList<>();
public void addDataChangeListener(DataChangeListener listener) {
(listener);
}
public void removeDataChangeListener(DataChangeListener listener) {
(listener);
}
public void notifyDataChange() {
for (DataChangeListener listener : listeners) {
();
}
}
}
public interface DataChangeListener {
void dataChanged();
}
2.3 Log system
The log system can use the listener mode to send log information to multiple different destinations (such as consoles, files, remote servers, etc.).
- Multiple log processing: A log event can be listened to and processed by multiple log handlers to ensure that log information is saved to multiple places.
public class Logger {
private List<LogListener> listeners = new ArrayList<>();
public void addLogListener(LogListener listener) {
(listener);
}
public void removeLogListener(LogListener listener) {
(listener);
}
public void log(String message) {
for (LogListener listener : listeners) {
(message);
}
}
}
public interface LogListener {
void onLog(String message);
}
2.4 File system monitoring
Listener mode is used to monitor changes in the file system, such as file creation, modification, deletion and other events.
- File monitoring tool: Some tools can listen to changes in specific directories and trigger corresponding processing when there are changes in files.
Path path = ("/path/to/directory");
WatchService watchService = ().newWatchService();
(watchService, StandardWatchEventKinds.ENTRY_CREATE, StandardWatchEventKinds.ENTRY_DELETE, StandardWatchEventKinds.ENTRY_MODIFY);
while (true) {
WatchKey key = ();
for (WatchEvent<?> event : ()) {
<?> kind = ();
Path fileName = (Path) ();
(kind + ": " + fileName);
}
();
}
2.5 Network and distributed systems
Listener mode is used to handle asynchronous events in networks and distributed systems, such as arrival of messages, establishment or disconnection of connections, etc.
- Message Queue: Listener mode is used to process messages in the message queue, and triggers the corresponding handler when the message arrives.
- Network connection: In network programming, listeners are used to handle events such as connection establishment, disconnection and data reception.
public interface MessageListener {
void onMessage(String message);
}
public class MessageQueue {
private List<MessageListener> listeners = new ArrayList<>();
public void addMessageListener(MessageListener listener) {
(listener);
}
public void removeMessageListener(MessageListener listener) {
(listener);
}
public void receiveMessage(String message) {
for (MessageListener listener : listeners) {
(message);
}
}
}
2.6 Task scheduling and asynchronous processing
Listener mode is used for task scheduling and asynchronous processing, allowing tasks to notify the listener for subsequent processing after completion.
- Task completion notification: When the task is completed, the listener is triggered to perform the corresponding subsequent operations.
- Asynchronous event processing: Handle asynchronous events and notify the listener when the event occurs.
public interface TaskCompleteListener {
void onTaskComplete();
}
public class Task {
private List<TaskCompleteListener> listeners = new ArrayList<>();
public void addTaskCompleteListener(TaskCompleteListener listener) {
(listener);
}
public void removeTaskCompleteListener(TaskCompleteListener listener) {
(listener);
}
public void complete() {
for (TaskCompleteListener listener : listeners) {
();
}
}
}
3 Custom listener
3.1 Common listener interfaces in Java
- ActionListener: Used to handle action events, such as button clicks.
- MouseListener: Used to handle mouse events, such as mouse click, entry, exit, etc.
- KeyListener: Used to handle keyboard events, such as key pressing, release, etc.
- WindowListener: Used to handle window events, such as window opening and closing, etc.
- FocusListener: Used to handle component focus events.
3,2 Use a custom listener
Sometimes you may need to define your own events and listeners. Here is an example showing how to create and use a custom listener.
3,2,1 Define event classes
import ;
public class CustomEvent extends EventObject {
public CustomEvent(Object source) {
super(source);
}
}
3.2.2 Defining the listener interface
import ;
public interface CustomEventListener extends EventListener {
void handleEvent(CustomEvent event);
}
3.2.3 Implement event source class
import ;
import ;
public class EventSource {
private final List<CustomEventListener> listeners = new ArrayList<>();
public void addCustomEventListener(CustomEventListener listener) {
(listener);
}
public void removeCustomEventListener(CustomEventListener listener) {
(listener);
}
public void triggerEvent() {
CustomEvent event = new CustomEvent(this);
for (CustomEventListener listener : listeners) {
(event);
}
}
}
3.2.4 Using a custom listener
public class CustomEventListenerExample {
public static void main(String[] args) {
EventSource eventSource = new EventSource();
// Add a custom event listener (new CustomEventListener() {
@Override
public void handleEvent(CustomEvent event) {
("Custom event triggered!");
}
});
// Trigger event ();
}
}
4 Summary
Listener mode is a powerful design pattern for scenarios where asynchronous events and notifications are required. By using the listener mode, loosely coupled event processing logic can be implemented to improve the maintainability and scalability of the code. Whether in GUI programming, data model update, log system, file system monitoring, network programming, or task scheduling, the listener mode provides an efficient event processing mechanism.
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