java - 现在用Applet画线全乱了

Question

我们最近开始使用 Applet 并用它做一些事情，所以最后一个类是画线。因此，在编写成功的代码后，我出于某种我不知道的原因更改了一行代码，现在它不画一条线，而是画一条奇怪的射线。 这是代码，因此我们将不胜感激。

import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;

public class SimplePaint extends Applet {
    private int lastx;
    private int lasty;
    @Override
    public void init() {
        this.setForeground(Color.BLUE);
        this.addMouseListener(new PositionRecorder());
        this.addMouseMotionListener(new LineDrawer());
    }
    public int getLastx() {
        return lastx;
    }
    public void setLastx(int lastx) {
        this.lastx = lastx;
    }
    public int getLasty() {
        return lasty;
    }
    public void setLasty(int lasty) {
        this.lasty = lasty;
    }
    private class PositionRecorder extends MouseAdapter{
        public void mousePressed(MouseEvent e) {
            setLastx(e.getX());
            setLasty(e.getY());

        }
    }
    private class LineDrawer extends MouseMotionAdapter{
        public void mouseDragged(MouseEvent e) {
            int x=e.getX();
            int y=e.getY();
            Graphics g;
            g=getGraphics();
            g.drawLine(lastx, lasty, x, y);

        }

    }
}

我清楚地记得弄乱了这部分代码(如果有帮助)

private class PositionRecorder extends MouseAdapter{
   public void mousePressed(MouseEvent e) {
     setLastx(e.getX());
     setLasty(e.getY());

Answer 1

在 AWT 中绘画是一个有据可查的过程。您应该首先阅读:

• Painting in AWT and Swing ，特别是，我建议您阅读有关 The Paint Method 的部分
• A Closer Look at the Paint Mechanism

引自《仔细观察绘画机制》

the paint method (defined by java.awt.Component.) This method will be executed by the painting subsystem whenever you component needs to be rendered.

在 AWT 中，绘制是破坏性的，任何时候 paint 被调用时，您都将完全重新绘制组件状态。这提出了许多您需要解决的问题。

对于需要绘制的任何内容，您需要一种可靠的方式来存储这些状态并在调用 paint 时重新应用它们。在您的情况下，像 ArrayList 这样简单的东西就适合，但您也可以使用 BufferedImage 作为后备缓冲区并将提交的绘图元素绘制到其中。 p>

第一种方法的好处是提供撤消更改的能力，但就绘画而言，并不像简单地绘画图像那么快。

更聪明的方法是同时使用两者。维护 ArrayList 中的绘制状态，在提交到 BufferedImage 时渲染每个状态，并且当您想要执行撤消时，从新的 BufferedImage 开始code> 并将所有状态绘制到您不需要的程度...但这超出了问题的范围

import java.applet.Applet;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;

public class SimplePaint extends Applet {

    private Point clickPoint;
    private Point dragPoint;

    @Override
    public void init() {
        this.setForeground(Color.BLUE);
        this.addMouseListener(new PositionRecorder());
        this.addMouseMotionListener(new LineDrawer());
    }

    @Override
    public void paint(Graphics g) {
        super.paint(g);
        if (clickPoint != null && dragPoint != null) {
            g.drawLine(clickPoint.x, clickPoint.y, dragPoint.x, dragPoint.y);
        }
    }

    private class PositionRecorder extends MouseAdapter {

        public void mousePressed(MouseEvent e) {
            clickPoint = e.getPoint();
        }

        @Override
        public void mouseReleased(MouseEvent e) {
            // This is where I'd reset the clickPoint and dragPoint
            // to null and apply the line to some structure which can
            // redraw it in the `paint` method, maybe something like
            // a ArrayList
            // Alternativly, you could render the result to a BufferedImage
            // and paint that inside the paint method
        }
    }

    private class LineDrawer extends MouseMotionAdapter {

        public void mouseDragged(MouseEvent e) {
            dragPoint = e.getPoint();
            repaint();
        }

    }
}