如何自定义ActionEvent的事件源？

Question

I'm working on a custom component and came up with a problem. Here is the component:

public class MyComponent extends JPanel {

    private final JButton jButton;
    private final JLabel jLabel;

    public MyComponent(){
        jButton = new JButton();
        //etc..
    }

    public void addActionListener(ActionListener l){
         //The problem with this is that ActionEvent has source attribute 
         //set to jButton which is not desirable. How can I set it to this?
         jButton.addActionListener(l);
    }

    //other component-specific methods
}

The thing is I'm trying to hide MyComponent 's implementation details. But setting listener this way is not good since a caller may observe that source attrubte is jButton . How can I set to the enclosing MyComponent instance?

Answer 1

Instead of allowing the client to pass in an ActionListener , have the client pass in a different callback, create your own listener, then have your listener invoke the callback:

public class MyComponent extends JPanel {
    private final JButton jButton;

    public MyComponent(){
        jButton = new JButton();
    }

    public void addActionListener(SomeCallback callback){
        jButton.addActionListener(event -> { //create listener
            callback.execute(); //invoke callback
        });
    }
}

interface SomeCallback {
    void execute();
}

If you want to pass the client the ActionEvent without the ability to access ActionEvent#getSource() , create a wrapper:

class ActionEventWrapper {
    private ActionEvent event;

    public MyActionEvent(ActionEvent event) {
        this.event = event;
    }

    //expose methods that aren't getSource()
    public String getActionCommand() {
        return event.getActionCommand();
    }
}

Simply add this type to the callback's method parameter:

interface SomeCallback {
    void execute(ActionEventWrapper event);
}

You could then create a new ActionEventWrapper anytime an event is triggered:

    public void addActionListener(SomeCallback callback){
        jButton.addActionListener(event -> {
            callback.execute(new ActionEventWrapper(event));
        });
    }

If you really want to adjust the source of the component's listener's events, simply create a new ActionEvent , specifying whichever source you want via the constructor:

public void addActionListener(ActionListener listener) {
    jButton.addActionListener(event -> {
        listener.actionPerformed(new ActionEvent(..., event.getID(), event.getActionCommand()));
    });
}

The ... is where you specify which component you want to act as the source.

Answer 2

Following should work.

@SuppressWarnings("all")
public class MyComponent extends JPanel {
    private final JButton jButton = new JButton();
    private final JLabel jLabel = new JLabel();

    public void addActionListener(final ActionListener listener) {
        final MyComponent self = this;

        ActionListener newListener = new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                ActionEvent newEvent = new ActionEvent(e.getSource(), e.getID(), e.getActionCommand()) {
                    @Override
                    public Object getSource() {
                        return self;
                    }
                };
                listener.actionPerformed(newEvent);
            }
        };
        jButton.addActionListener(newListener);
    }
}

With Lambda expression (singular)

@SuppressWarnings("all")
public class MyComponent2 extends JPanel {
    private final JButton jButton = new JButton();
    private final JLabel jLabel = new JLabel();

    public void addActionListener(final ActionListener listener) {
        MyComponent2 self = this;

        jButton.addActionListener(e-> {
            ActionEvent newEvent = new ActionEvent(e.getSource(), e.getID(), e.getActionCommand()) {
                @Override
                public Object getSource() {
                    return self;
                }
            };
            listener.actionPerformed(newEvent);
        });
    }
}