如何编写组件监听器

The Component listener is a listener interface for receiving component events. A component is an object having a graphical representation that can be displayed on the screen and that can interact with the user. Some of the examples of components are the buttons, checkboxes, and scrollbars of a typical graphical user interface.

The class that is interested in processing a component event either implements this interface and all the methods it contains, or extends the abstract ComponentAdapter class overriding only the methods of interest. The listener object created from that class is then registered with a component using the component's addComponentListener method. When the component's size, location, or visibility changes, the relevant method in the listener object is invoked, and the ComponentEvent is passed to it.

One or more component events are fired by a Component object just after the component is hidden, made visible, moved, or resized.

The component-hidden and component-shown events occur only as the result of calls to a Component 's setVisible method. For example, a window might be miniaturized into an icon (iconified) without a component-hidden event being fired.

To write a simple Component listener program, follow the steps mentioned below:

• Declare a class which implements Component listener. 例如：

  public class ComponentEventDemo ... implements ComponentListener
  

• Identify the components that you would like to catch the events for. For example: pane, label, checkbox, etc.
• Add the Component Listener to the identified components. 例如：

  ....
  label.addComponentListener(this);
  .....
  checkbox.addComponentListener(this);
  ....
  panel.addComponentListener(this);
  ...
  frame.addComponentListener(this);
  

• Finally, catch the different events of these components by using four methods of Component Listener as shown below:

  public void componentHidden(ComponentEvent e) {
          displayMessage(e.getComponent().getClass().getName() + " --- Hidden");
      }
  
      public void componentMoved(ComponentEvent e) {
          displayMessage(e.getComponent().getClass().getName() + " --- Moved");
      }
  
      public void componentResized(ComponentEvent e) {
          displayMessage(e.getComponent().getClass().getName() + " --- Resized ");            
      }
  
      public void componentShown(ComponentEvent e) {
          displayMessage(e.getComponent().getClass().getName() + " --- Shown");
  
      }
  

The following example demonstrates component events. The window contains a panel that has a label and a check box. The check box controls whether the label is visible. A text area displays a message every time the window, panel, label, or check box fires a component event.

You can find the demo's code in ComponentEventDemo.java. Here is just the code related to handling component events:

public class ComponentEventDemo ... implements ComponentListener {
    static JFrame frame;
    JLabel label;
    ...
    public ComponentEventDemo() {
        ...
        JPanel panel = new JPanel(new BorderLayout());
        label = new JLabel("This is a label", JLabel.CENTER);
        label.addComponentListener(this);
        panel.add(label, BorderLayout.CENTER);

        JCheckBox checkbox = new JCheckBox("Label visible", true);
        checkbox.addComponentListener(this);
        panel.add(checkbox, BorderLayout.PAGE_END);
        panel.addComponentListener(this);
        ...
        frame.addComponentListener(this);
    }
    ...
     public void componentHidden(ComponentEvent e) {
        displayMessage(e.getComponent().getClass().getName() + " --- Hidden");
    }

    public void componentMoved(ComponentEvent e) {
        displayMessage(e.getComponent().getClass().getName() + " --- Moved");
    }

    public void componentResized(ComponentEvent e) {
        displayMessage(e.getComponent().getClass().getName() + " --- Resized ");            
    }

    public void componentShown(ComponentEvent e) {
        displayMessage(e.getComponent().getClass().getName() + " --- Shown");

    }

    public static void main(String[] args) {
        ...
        //Create and set up the window.
        frame = new JFrame("ComponentEventDemo");
        ...
        JComponent newContentPane = new ComponentEventDemo();
        frame.setContentPane(newContentPane);
        ...
    }
}

The Component Listener API

The ComponentListener Interface

All of these methods are also in the adapter class, ComponentAdapter.

The ComponentEvent Class

Examples that Use Component Listeners