java - 为什么视口(viewport)更改监听器被多次调用

Question

我有一个视口(viewport)，并且我已将一个更改监听器附加到它。每当我滚动视口(viewport)时，我的更改监听器就会被调用大约四次。我不知道如何避免这种情况；我只想调用一次？

Answer 1

没有办法解决这个问题，JViewport 将触发多个 stateChanged 事件，因为它提供有关许多属性更改的通知...

来自JavaDocs ...

Adds a ChangeListener to the list that is notified each time the view's size, position, or the viewport's extent size has changed.

此时，很难知道该建议什么，因为我们不知道您想要实现什么目标，但是，如果您必须使用 ChangeListener，您可以建立联合机制。也就是说，您基本上要等到事件之间发生足够长的延迟才响应它，而不是响应每个事件......

例如...

public class DelayedChangeHandler implements ChangeListener {

    private Timer timer;
    private ChangeEvent last;

    public DelayedChangeHandler() {
        timer = new Timer(250, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                stableStateChanged();
            }
        });
        timer.setRepeats(false);
    }

    @Override
    public void stateChanged(ChangeEvent e) {
        last = e;
        timer.restart();
    }

    protected void stableStateChanged() {
        System.out.println("Finally...");
    }

}

基本上，这是一个 ChangeListener 实现，它使用具有短延迟的非重复 javax.swing.Timer 。每次调用stateChanged时，计时器都会重新启动。最后，当允许计时器“滴答”时，它会调用 stableStateChanged 指示自上次事件引发以来已经过去了足够的时间。

这假设您不太关心导致事件的原因，只关心事件发生了......

一个可运行的示例...

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

public class TestViewport {

    public static void main(String[] args) {
        new TestViewport();
    }

    public TestViewport() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                }

                JPanel pane = new JPanel() {
                    @Override
                    public Dimension getPreferredSize() {
                        return new Dimension(1000, 1000);
                    }
                };

                JScrollPane sp = new JScrollPane(pane);
                sp.getViewport().addChangeListener(new DelayedChangeHandler());
                sp.getViewport().addPropertyChangeListener(new PropertyChangeListener() {
                    @Override
                    public void propertyChange(PropertyChangeEvent evt) {
                        System.out.println(evt.getPropertyName());
                    }
                });

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(sp);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class DelayedChangeHandler implements ChangeListener {

        private Timer timer;
        private ChangeEvent last;

        public DelayedChangeHandler() {
            timer = new Timer(250, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    stableStateChanged();
                }
            });
            timer.setRepeats(false);
        }

        @Override
        public void stateChanged(ChangeEvent e) {
            last = e;
            timer.restart();
        }

        protected void stableStateChanged() {
            System.out.println("Finally...");
        }

    }

}