如何使用鼠标拖动事件在Java Applet上绘制矩形

Question

我正在使用Java。我想根据mousedrag事件绘制矩形。如果用户拖动鼠标，则小程序上的矩形应根据当前鼠标坐标增加或减少。我有以下代码。

在下面的代码中，我使用[b] SelectionArea [/ b]类扩展了在其上执行绘制操作的画布。我在此类中使用[b] image [/
b]变量进行双缓冲，以减少闪烁并保存小程序的先前状态（即，绘制小程序的内容）

但如果我画第一个矩形，代码工作正常。如果我开始绘制第二个矩形，则先前绘制的矩形将消失。我希望先前绘制的矩形出现在屏幕上

谁能告诉我如何解决这个问题。

import java.awt.*;
import java.applet.Applet;
import java.awt.event.*;

/* 
 * This displays a framed area.  When the user drags within
 * the area, this program displays a rectangle extending from
 * where the user first pressed the mouse button to the current
 * cursor location.
 */

public class RectangleDemo extends Applet {
SelectionArea drawingPanel;
Label label;

public void init() {
    GridBagLayout gridBag = new GridBagLayout();
    GridBagConstraints c = new GridBagConstraints();

    setLayout(gridBag);

    drawingPanel = new SelectionArea(this);
    c.fill = GridBagConstraints.BOTH;
    c.weighty = 1.0;
    c.gridwidth = GridBagConstraints.REMAINDER; //end row
    gridBag.setConstraints(drawingPanel, c);
    add(drawingPanel);

    label = new Label("Drag within the framed area.");
    c.fill = GridBagConstraints.HORIZONTAL;
    c.weightx = 1.0;
    c.weighty = 0.0;
    gridBag.setConstraints(label, c);
    add(label);
    drawingPanel.setVisible(true);

    validate();
}

public void paint(Graphics g){
    drawingPanel.repaint();
}

public void update(Graphics g){
    paint(g);
}

}

class SelectionArea extends Canvas implements ActionListener, MouseListener,    MouseMotionListener{
Rectangle currentRect;
RectangleDemo controller;
//for double buffering
Image image;
Graphics offscreen;
public SelectionArea(RectangleDemo controller) {
    super();
    this.controller = controller;
    addMouseListener(this);
    addMouseMotionListener(this);        
}

public void actionPerformed(ActionEvent ae){
    repaintoffscreen();
}

public void repaintoffscreen(){
    image = createImage(this.getWidth(), this.getHeight());
    offscreen = image.getGraphics();
    Dimension d = getSize();
    if(currentRect != null){
        Rectangle box = getDrawableRect(currentRect, d);

        //Draw the box outline.
        offscreen.drawRect(box.x, box.y, box.width - 1, box.height - 1);  
        //repaint();
    }
}

public void mouseEntered(MouseEvent me) {}
public void mouseExited(MouseEvent me){ }
public void mouseClicked(MouseEvent me){}
public void mouseMoved(MouseEvent me){}

public void mousePressed(MouseEvent me) {        
    currentRect = new Rectangle(me.getX(), me.getY(), 0, 0);
    repaintoffscreen();        
}

public void mouseDragged(MouseEvent me) {
    System.out.println("here in dragged()");
    currentRect.setSize(me.getX() - currentRect.x, me.getY() - currentRect.y);
    repaintoffscreen();    
    repaint();
}

public void mouseReleased(MouseEvent me) {
    currentRect.setSize(me.getX() - currentRect.x, me.getY() - currentRect.y);
    repaintoffscreen();  
    repaint();
}

public void update(Graphics g){
    paint(g);
}

public void paint(Graphics g) {
    g.drawImage(image, 0, 0, this);
}

Rectangle getDrawableRect(Rectangle originalRect, Dimension drawingArea) {
    int x = originalRect.x;
    int y = originalRect.y;
    int width = originalRect.width;
    int height = originalRect.height;

    //Make sure rectangle width and height are positive.
    if (width < 0) {
        width = 0 - width;
        x = x - width + 1;
        if (x < 0) {
            width += x;
            x = 0;
        }
    }
    if (height < 0) {
        height = 0 - height;
        y = y - height + 1;
        if (y < 0) {
            height += y;
            y = 0;
        }
    }

    //The rectangle shouldn't extend past the drawing area.
    if ((x + width) > drawingArea.width) {
        width = drawingArea.width - x;
    }
    if ((y + height) > drawingArea.height) {
        height = drawingArea.height - y;
    }

    return new Rectangle(x, y, width, height);
}

}

另外，如果我在全屏模式下运行此代码，那么我会看到只有在释放鼠标后矩形才会出现在屏幕上。但是我希望矩形在拖动鼠标时出现在屏幕上，并且应该根据当前鼠标坐标更改其尺寸。谁能帮助我。

Answer 1

家庭作业？

基本上，您需要做的是：

1. 按下鼠标时保持鼠标按下坐标并重新绘制
2. 鼠标移动时保持当前鼠标坐标并重新绘制
3. 在鼠标上移时，使鼠标下移坐标无效，以指示没有矩形，然后重新绘制。
4. 在油漆上绘制背景，然后在鼠标按下和鼠标坐标之间进行调整。

如果您不想保留背景图像，则可以使用Graphics
xor函数来完成技巧，两次绘制相同的rect会擦除旧的rect，因此您可以使用它在图形对象上直接还原旧的图像。

编辑：代码xor用法示例：

public void paint(Graphics g)
{
   g.setXORMode(Color.black);
   // draw old rect if there is one. this will erase it
   // draw new rect, this will draw xored
   g.setDrawMode(); // restore normal draw mode
}

Xor具有一个有趣的属性：

xor(xor(x)) = x

因此对同一个像素进行两次异或处理可恢复其原始颜色。