JTextField：获取鼠标按下前的光标位置

给JTextField添加了MouseListener，需要获取鼠标按下前的光标位置，但监听器触发时光标位置已改变，且无法设置监听器优先级让其在光标改变前执行。添加CaretListener也无效，因为它仅在鼠标松开时触发。运行以下代码可复现问题：

package test;

import java.awt.EventQueue;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;

@SuppressWarnings("serial")
public class TestCaret extends JFrame
{
  public TestCaret()
  {
    JTextField   text;
    TestListener listener;
    text = new JTextField("This is my text");
    listener = new TestListener();
    text.addMouseListener(listener);
    text.addCaretListener(listener);
    add(text);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
  }

  private class TestListener extends MouseAdapter implements CaretListener
  {
    @Override
    public void caretUpdate(CaretEvent e)
    {
      System.out.println("Got CaretEvent " + e);
    }

    @Override
    public void mousePressed(MouseEvent e)
    {
      JTextField textField;
      textField = (JTextField)e.getComponent();
      System.out.println("Got MouseEvent with caret position " + textField.getCaretPosition());
    }

  }

  public static void main(String[] args)
  {
    Runnable runner;
    runner = new Runnable()
    {
      @Override
      public void run()
      {
        TestCaret frame;
        frame = new TestCaret();
        frame.pack();
        frame.setVisible(true);
      }
    };
    EventQueue.invokeLater(runner);

  }

} // class TestCaret

方案1：自定义JTextField拦截鼠标事件 #

核心思路是在JTextField默认处理鼠标按下事件（即改变光标位置）之前，先记录当前光标位置，通过重写processMouseEvent方法实现：

public class CustomTextField extends JTextField {
    private int prePressCaretPosition;

    public CustomTextField(String text) {
        super(text);
    }

    @Override
    protected void processMouseEvent(MouseEvent e) {
        // 仅在鼠标按下事件触发时记录光标位置
        if (e.getID() == MouseEvent.MOUSE_PRESSED) {
            prePressCaretPosition = getCaretPosition();
        }
        // 调用父类方法，执行默认的光标位置更新逻辑
        super.processMouseEvent(e);
    }

    // 提供获取鼠标按下前光标位置的方法
    public int getPrePressCaretPosition() {
        return prePressCaretPosition;
    }
}

使用时替换原JTextField为CustomTextField，在MouseListener中调用getPrePressCaretPosition()即可拿到目标位置：

@Override
public void mousePressed(MouseEvent e) {
    CustomTextField textField = (CustomTextField)e.getComponent();
    System.out.println("鼠标按下前的光标位置：" + textField.getPrePressCaretPosition());
    System.out.println("当前光标位置：" + textField.getCaretPosition());
}

方案2：维护全局光标位置变量 #

通过CaretListener实时记录当前光标位置（CaretEvent会在光标每次改变时触发），在鼠标按下事件中直接调用记录的历史位置：

private class TestListener extends MouseAdapter implements CaretListener {
    private int lastCaretPosition;

    @Override
    public void caretUpdate(CaretEvent e) {
        lastCaretPosition = ((JTextField)e.getSource()).getCaretPosition();
        System.out.println("Got CaretEvent " + e);
    }

    @Override
    public void mousePressed(MouseEvent e) {
        JTextField textField = (JTextField)e.getComponent();
        // lastCaretPosition 即为鼠标按下前的光标位置
        System.out.println("鼠标按下前的光标位置：" + lastCaretPosition);
        System.out.println("当前光标位置：" + textField.getCaretPosition());
    }
}

注意：需确保CaretListener在程序启动时就已注册，否则初始光标位置无法被记录。

内容的提问来源于stack exchange，提问作者Robert Kock