如何使用鼠标拖动事件吸取Java Applet的矩形 [英] how to draw rectangle on java applet using mouse drag event

问题描述

我使用的Java。
我想根据mousedrag事件绘制矩形。如果用户拖动鼠标，然后在小程序中的矩形应该增加或减少立足当前鼠标的坐标。
我有以下code。

i am using java. i want to draw rectangle based on mousedrag event. if user dragging the mouse, then the rectangle on the applet should increase or decrease basing on current mouse coordinates. i have the following code.

在以下code我用[B] SelectionArea其上对此我进行绘图操作扩展画布[/ B]类。我正在使用本类双缓冲[B]图片[/ B]变量减少闪烁并保存applet的previous状态（即绘图小程序的内容）

in the following code i am using [b]SelectionArea[/b] class which extends a canvas on which i am performing drawing operation. i am using [b]image[/b] variable in this class for double buffering to reduce flickering and to save the applet's previous state(i.e drawing content of applet)

但是，如果我画一个矩形的code是工作的罚款。如果我开始绘制第二个矩形的previously绘制的矩形正在消失。我想previously绘制的矩形要在屏幕上

but the code is working fine if i draw first rectangle. if i start to draw second rectangle the previously drawn rectangle is disappearing. i want the previously drawn rectangle to be on the screen

任何一个可以告诉我怎么解决这个问题。

can any one tell me how to solve this.

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，如果我运行在全屏模式，然后我看到了矩形appering在屏幕上后，才释放鼠标这个code。但我想的矩形在屏幕上，同时拖动鼠标，它应该根据当前的鼠标坐标改变它的尺寸。
任何一个可以帮我请。

also if i run this code on full screen mode then i am seeing that the rectangle is appering on screen only after i released the mouse. but i want the rectangle to be on the screen while dragging the mouse and it should change it's dimension according to the current mouse coordinates. can any one help me pls.

推荐答案

功课？

基本上你需要做的是：

1. 在鼠标按下保持鼠标按下坐标和重新绘制

在鼠标移动
2. 保持当前鼠标的坐标和重绘


3. 在鼠标了，废了鼠标按下坐标来表示没有RECT，并重绘。


4. 在油漆，画鼠标按下和CUR-鼠标坐标之间的背景，然后正确。

如果你不希望保留的背景图像，你可以用图形XOR函数一招，吸取同一矩形两次将擦除旧的矩形，所以你可以用它来恢复直上的旧形象图形对象。

if you don't want to keep a background image, you can do a trick with the Graphics xor function, drawing the same rect twice will erase the old rect, so you can use it to restore the old image straight on the graphics object.

编辑：code XOR用法示例：

code xor usage sample:

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
}

异或有一个有趣的属性：

Xor has the an interesting property:

xor(xor(x)) = x

所以异或相同的像素两次恢复它原来的颜色。

so xoring the same pixel twice restores it's original color.

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