在棋盘的两个方格之间拖动棋子
摘要
编写一个跳棋游戏会碰到许多挑战，在棋盘上拖动棋子就是其中之一，本期的Java Fun and Games讨论了这个方面，并着重介绍了如何消除拖动棋子时产生的屏幕闪烁。
你有没有想过要编写一个跳棋游戏——可以是双人对战，也可以是人机对战？编写一个跳棋游戏会碰到许多挑战，在棋盘上把棋子从一个方格拖动到另外一个方格就是其中之一。本期的Java Fun and Games通过一个名为CheckerDrag的applet演示了棋子的拖动。
CheckerDrag applet在屏幕上显示了一个棋盘和一个棋子，你可以在棋盘的边界内任意拖动棋子。CheckerDrag不考虑棋子会落在哪里——也许是在不同颜色的格子上，在两个格子之间，或者是离开几个格子远。该程序的目的是演示如何拖动棋子。表1展示了applet的源代码：
表1. CheckerDrag.java
// CheckerDrag.java
import java.awt.*;
import java.awt.event.*;
public class CheckerDrag extends java.applet.Applet
{
// Dimension of checkerboard square.
// 棋盘上每个小方格的尺寸
final static int SQUAREDIM = 40;
// Dimension of checkerboard -- includes black outline.
// 棋盘的尺寸 – 包括黑色的轮廓线
final static int BOARDDIM = 8 * SQUAREDIM + 2;
// Dimension of checker -- 3/4 the dimension of a square.
// 棋子的尺寸 – 方格尺寸的3/4
final static int CHECKERDIM = 3 * SQUAREDIM / 4;
// Square colors are dark green or white.
// 方格的颜色为深绿色或者白色
final static Color darkGreen = new Color (0, 128, 0);
// Dragging flag -- set to true when user presses mouse button over checker
// and cleared to false when user releases mouse button.
// 拖动标记 --当用户在棋子上按下鼠标按键时设为true，
// 释放鼠标按键时设为false
boolean inDrag = false;
// Left coordinate of checkerboard's upper-left corner.
// 棋盘左上角的左方向坐标
int boardx;
// Top coordinate of checkerboard's upper-left corner.
//棋盘左上角的上方向坐标
int boardy;
// Left coordinate of checker rectangle origin (upper-left corner).
// 棋子矩形原点(左上角)的左方向坐标
int ox;
// Top coordinate of checker rectangle origin (upper-left corner).
// 棋子矩形原点(左上角)的上方向坐标
int oy;
// Left displacement between mouse coordinates at time of press and checker
// rectangle origin.
// 在按键时的鼠标坐标与棋子矩形原点之间的左方向位移
int relx;
// Top displacement between mouse coordinates at time of press and checker
// rectangle origin.
// 在按键时的鼠标坐标与棋子矩形原点之间的上方向位移
int rely;
// Width of applet drawing area.
// applet绘图区域的宽度
int width;
// Height of applet drawing area.
// applet绘图区域的高度
int height;
// Image buffer.
// 图像缓冲
Image imBuffer;
// Graphics context associated with image buffer.
// 图像缓冲相关联的图形背景
Graphics imG;
public void init ()
{
// OBTain the size of the applet's drawing area.
// 获取applet绘图区域的尺寸
width = getSize ().width;
height = getSize ().height;
// Create image buffer.
// 创建图像缓冲
imBuffer = createImage (width, height);
// Retrieve graphics context associated with image buffer.
// 取出图像缓冲相关联的图形背景
imG = imBuffer.getGraphics ();
// Initialize checkerboard's origin, so that board is centered.
// 初始化棋盘的原点，使棋盘在屏幕上居中
boardx = (width - BOARDDIM) / 2 + 1;
boardy = (height - BOARDDIM) / 2 + 1;
// Initialize checker's rectangle's starting origin so that checker is
// centered in the square located in the top row and second column from
// the left.
// 初始化棋子矩形的起始原点，使得棋子在第一行左数第二列的方格里居中
ox = boardx + SQUAREDIM + (SQUAREDIM - CHECKERDIM) / 2 + 1;
oy = boardy + (SQUAREDIM - CHECKERDIM) / 2 + 1;
// Attach a mouse listener to the applet. That listener listens for
// mouse-button press and mouse-button release events.
// 向applet添加一个用来监听鼠标按键的按下和释放事件的鼠标监听器
addMouseListener (new MouseAdapter ()
{
public void mousePressed (MouseEvent e)
{
// Obtain mouse coordinates at time of press.
// 获取按键时的鼠标坐标
int x = e.getX ();
int y = e.getY ();
// If mouse is over draggable checker at time
// of press (i.e., contains (x, y) returns
// true), save distance between current mouse
// coordinates and draggable checker origin
// (which will always be positive) and set drag
// flag to true (to indicate drag in progress).
// 在按键时假如鼠标位于可拖动的棋子上方
// （也就是contains (x, y)返回true），则保存当前
// 鼠标坐标与棋子的原点之间的距离（始终为正值）并且
// 将拖动标志设为true（用来表明正处在拖动过程中）
if (contains (x, y))
{
relx = x - ox;
rely = y - oy;
inDrag = true;
}
}
boolean contains (int x, int y)
{
// Calculate center of draggable checker.
// 计算棋子的中心位置
int cox = ox + CHECKERDIM / 2;
int coy = oy + CHECKERDIM / 2;
// Return true if (x, y) locates with bounds
// of draggable checker. CHECKERDIM / 2 is the
// radius.
// 假如(x, y)仍处于棋子范围内则返回true
// CHECKERDIM / 2为半径
return (cox - x) * (cox - x) +
(coy - y) * (coy - y) <
CHECKERDIM / 2 * CHECKERDIM / 2;
}
public void mouseReleased (MouseEvent e)
{
// When mouse is released, clear inDrag (to
// indicate no drag in progress) if inDrag is
// already set.
// 当鼠标按键被释放时，假如inDrag已经为true，
// 则将其置为false（用来表明不在拖动过程中）
if (inDrag)
inDrag = false;
}
});
// Attach a mouse motion listener to the applet. That listener listens
// for mouse drag events.
//向applet添加一个用来监听鼠标拖动事件的鼠标运动监听器
addMouseMotionListener (new MouseMotionAdapter ()
{
public void mouseDragged (MouseEvent e)
{
if (inDrag)
{
// Calculate draggable checker's new
// origin (the upper-left corner of
// the checker rectangle).
// 计算棋子新的原点（棋子矩形的左上角）
int tmpox = e.getX () - relx;
int tmpoy = e.getY () - rely;
// If the checker is not being moved
// (at least partly) off board,
// assign the previously calculated
// origin (tmpox, tmpoy) as the
// permanent origin (ox, oy), and
// redraw the display area (with the
// draggable checker at the new
// coordinates).
// 假如棋子（至少是棋子的一部分）没有被
// 移出棋盘，则将之前计算的原点
// (tmpox, tmpoy)赋值给永久性的原点(ox, oy)，
// 并且刷新显示区域（此时的棋子已经位于新坐标上）
if (tmpox > boardx &&
tmpoy > boardy &&
tmpox + CHECKERDIM
< boardx + BOARDDIM &&
tmpoy + CHECKERDIM
< boardy + BOARDDIM)
{
ox = tmpox;
oy = tmpoy;
repaint ();
}
}
}
});
}
public void paint (Graphics g)
{
// Paint the checkerboard over which the checker will be dragged.
// 在棋子将要被拖动的位置上绘制棋盘
paintCheckerBoard (imG, boardx, boardy);
// Paint the checker that will be dragged.
// 绘制即将被拖动的棋子
paintChecker (imG, ox, oy);
// Draw contents of image buffer.
// 绘制图像缓冲的内容
g.drawImage (imBuffer, 0, 0, this);
}
void paintChecker (Graphics g, int x, int y)
{
// Set checker shadow color.
// 设置棋子阴影的颜色
g.setColor (Color.black);
// Paint checker shadow.
// 绘制棋子的阴影
g.fillOval (x, y, CHECKERDIM, CHECKERDIM);
// Set checker color.
// 设置棋子颜色
g.setColor (Color.red);
// Paint checker.
// 绘制棋子
g.fillOval (x, y, CHECKERDIM - CHECKERDIM / 13, CHECKERDIM - CHECKERDIM / 13);
}
void paintCheckerBoard (Graphics g, int x, int y)
{
// Paint checkerboard outline.
// 绘制棋盘轮廓线
g.setColor (Color.black);
g.drawRect (x, y, 8 * SQUAREDIM + 1, 8 * SQUAREDIM + 1);
// Paint checkerboard.
// 绘制棋盘
for (int row = 0; row < 8; row++)
{
g.setColor (((row & 1) != 0) ? darkGreen : Color.white);
for (int col = 0; col < 8; col++)
{
g.fillRect (x + 1 + col * SQUAREDIM, y + 1 + row * SQUAREDIM,
SQUAREDIM, SQUAREDIM);
g.setColor ((g.getColor () == darkGreen) ? Color.white :
darkGreen);
}
}
}
// The AWT invokes the update() method in response to the repaint() method
// calls that are made as a checker is dragged. The default implementation
// of this method, which is inherited from the Container class, clears the
// applet's drawing area to the background color prior to calling paint().
// This clearing followed by drawing causes flicker. CheckerDrag overrides
// update() to prevent the background from being cleared, which eliminates
// the flicker.
// AWT调用了update()方法来响应拖动棋子时所调用的repaint()方法。
