1.用雙緩衝解決畫板程序中的刷新問題
我們用Java編制畫板程序的時候,總是存在一個刷新的問題:當Canvas所在的窗口最小化或者被其他應用程序遮擋後,再次恢復,Canvas上的圖形數據將被部分或者完全擦除掉.
通常解決這個問題的方法是在Canvas的paint()函數中重繪圖形,但是由於在繪圖的過程中產生了大量的數據,重新在Canvas上繪製這些數據將導致大量的系統開銷,還會產生閃爍,故該方法可行但並不可取.
利用雙緩衝技術可以很好的解決這個問題,其主要原理是開闢兩個圖形緩衝區,一個是前臺的顯示緩衝(也就是Canvas),一個是後臺的圖形緩衝(通常是Image).用戶在繪製圖形時,我們對這兩個緩衝區進行同步更新,相當於爲前臺的數據作了一個後臺備份.當前臺的圖形被遮蓋需要恢復的時候,我們就可以用這個後臺備份來恢復,具體方法是重寫paint()函數,將備份好的圖像一次性的畫到屏幕上去.
爲什麼是paint()?這裏需要先了解一下有關Java處理AWT繪圖的基礎知識:Java的顯示更新是由一個AWT線程來控制完成的.該線程主要負責兩種與顯示更新相關的情況.第一種情況稱爲曝光,表示部分顯示區域毀壞或需要清除.這種情況發生時,系統直接調用paint()方法;第二種情況是程序決定重畫顯示區域,添加一些新的內容,此時需要程序調用成員的repaint()方法,repaint()方法調用成員的update(),update()再調用paint()方法.顯然我們所說的就是第一種.只要Canvas組件所在的窗口最小化或者被其他應用程序遮擋住,系統就會調用paint()對畫布進行重繪.如果我們在paint()方法中什麼都不做,就只能眼睜睜的看着辛辛苦苦畫的線條一旦被覆蓋就再也看不見了.
作爲起點,請先看一個最簡單的畫板程序,請注意,以下程序使用的是j2sdk1.4.1版本,在Win98的IE下(不是Tencent Explorer)全部測試通過:
//:WBApplet.java
import java.applet.*;
public class WBApplet extends Applet{
final static int DEFAULT_BOARDWIDTH=700;
final static int DEFAULT_BOARDHEIGHT=400;
public void init(){
super.init();
setLayout(null);
whiteBoard = new WhiteBoard(this);
whiteBoard.reshape(0,0,DEFAULT_BOARDWIDTH,DEFAULT_BOARDHEIGHT);
add(whiteBoard);
}
WhiteBoard whiteBoard;
}
///:~
//:WhiteBoard.java
java.awt.*;
import java.awt.event.*;
public class WhiteBoard extends Canvas implements MouseMotionListener,MouseListener{
final static int DEFAULT_BOARDWIDTH=700;
final static int DEFAULT_BOARDHEIGHT=400;
int x0,y0,x1,y1;
WhiteBoard(WBApplet WBApplet1){
parent = WBApplet1;
addMouseMotionListener(this);
addMouseListener(this);
}
synchronized public void update_buffer(Graphics g,DrawItem data) {
g.drawLine(data.x0,data.y0,data.x1,data.y1);
}
public void mouseReleased(MouseEvent e){}
public void mouseEntered(MouseEvent e){}
public void mouseExited(MouseEvent e){}
public void mouseClicked(MouseEvent e){}
public void mouseMoved(MouseEvent e){}
public void mouseDragged(MouseEvent e){
x1=e.getX();
y1=e.getY();
Graphics g = getGraphics();
update_buffer(g,new DrawItem(x0,y0,x1,y1));
g.dispose();
x0=x1;
y0=y1;
}
public void mousePressed(MouseEvent e){
x0 =e.getX();
y0 =e.getY();
}
WBApplet parent;
}
class DrawItem{
DrawItem(int x0,int y0,int x1,int y1){
this.x0=x0;
this.y0=y0;
this.x1=x1;
this.y1=y1;
}
int x0;
int y0;
int x1;
int y1;
}
///:~
import java.applet.*;
public class WBApplet extends Applet{
final static int DEFAULT_BOARDWIDTH=700;
final static int DEFAULT_BOARDHEIGHT=400;
public void init(){
super.init();
setLayout(null);
whiteBoard = new WhiteBoard(this);
whiteBoard.reshape(0,0,DEFAULT_BOARDWIDTH,DEFAULT_BOARDHEIGHT);
add(whiteBoard);
}
WhiteBoard whiteBoard;
}
///:~
//:WhiteBoard.java
java.awt.*;
import java.awt.event.*;
public class WhiteBoard extends Canvas implements MouseMotionListener,MouseListener{
final static int DEFAULT_BOARDWIDTH=700;
final static int DEFAULT_BOARDHEIGHT=400;
int x0,y0,x1,y1;
WhiteBoard(WBApplet WBApplet1){
parent = WBApplet1;
addMouseMotionListener(this);
addMouseListener(this);
}
synchronized public void update_buffer(Graphics g,DrawItem data) {
g.drawLine(data.x0,data.y0,data.x1,data.y1);
}
public void mouseReleased(MouseEvent e){}
public void mouseEntered(MouseEvent e){}
public void mouseExited(MouseEvent e){}
public void mouseClicked(MouseEvent e){}
public void mouseMoved(MouseEvent e){}
public void mouseDragged(MouseEvent e){
x1=e.getX();
y1=e.getY();
Graphics g = getGraphics();
update_buffer(g,new DrawItem(x0,y0,x1,y1));
g.dispose();
x0=x1;
y0=y1;
}
public void mousePressed(MouseEvent e){
x0 =e.getX();
y0 =e.getY();
}
WBApplet parent;
}
class DrawItem{
DrawItem(int x0,int y0,int x1,int y1){
this.x0=x0;
this.y0=y0;
this.x1=x1;
this.y1=y1;
}
int x0;
int y0;
int x1;
int y1;
}
///:~
我們將白板需完成的所有邏輯操作封裝在了一個WhiteBoard類中,以方便主程序的Applet調用.同時,定義了一個繪圖的數據類DrawItem,用來封裝圖形數據.繪圖的操作都寫在update_buffer中.顯然,這個程序無法實現刷新後的恢復,我們需要使用雙緩衝技術.
爲了實現雙緩衝,首先定義圖形緩衝區如下
private Image off_screen_buf;
private Graphics off_screen_gc;
private Graphics off_screen_gc;
並在WhiteBoard類的構造函數中對其進行初始化
off_screen_buf =parent.createImage(DEFAULT_BOARDWIDTH,DEFAULT_BOARDHEIGHT);
off_screen_gc = off_screen_buf.getGraphics();
off_screen_gc = off_screen_buf.getGraphics();
在處理用戶繪製圖形的函數中,我們使用update_buffer對顯示緩衝和圖形緩衝同時進行更新
Graphics g = getGraphics();
update_buffer(g,new DrawItem(x0,y0,x1,y1));//前臺更新畫布
update_buffer(off_screen_gc,new DrawItem(x0,y0,x1,y1));//後臺更新緩衝
g.dispose();
update_buffer(g,new DrawItem(x0,y0,x1,y1));//前臺更新畫布
update_buffer(off_screen_gc,new DrawItem(x0,y0,x1,y1));//後臺更新緩衝
g.dispose();
顯然,後臺的更新操作是不可視的,所以是off-screen.
最後,重寫paint()方法,調用copy_from_offscreen_buf(g)將圖形緩衝區的圖像畫到屏幕上去.
public void paint(Graphics g){
copy_from_offscreen_buf(g);
}
void copy_from_offscreen_buf(Graphics g){
if(g != null)
g.drawImage(off_screen_buf, 0, 0, null);
}
copy_from_offscreen_buf(g);
}
void copy_from_offscreen_buf(Graphics g){
if(g != null)
g.drawImage(off_screen_buf, 0, 0, null);
}
就是這麼簡單的幾行代碼,就可以讓我們完全的避免圖形不能恢復的問題.下面是WhiteBoard經改進後的完整代碼.
//:WhiteBoard.java
import java.awt.*;
import java.awt.event.*;
public class WhiteBoard extends Canvas implements MouseMotionListener,MouseListener{
final static int DEFAULT_BOARDWIDTH=700;
final static int DEFAULT_BOARDHEIGHT=400;
int x0,y0,x1,y1;
WhiteBoard(WBApplet WBApplet1){
parent = WBApplet1;
off_screen_buf =parent.createImage(DEFAULT_BOARDWIDTH,DEFAULT_BOARDHEIGHT);
off_screen_gc = off_screen_buf.getGraphics();
addMouseMotionListener(this);
addMouseListener(this);
}
synchronized public void update_buffer(Graphics g,DrawItem data) {
g.drawLine(data.x0,data.y0,data.x1,data.y1);
}
public void mouseMoved(MouseEvent e){}
public void mouseReleased(MouseEvent e){}
public void mouseEntered(MouseEvent e){}
public void mouseExited(MouseEvent e){}
public void mouseClicked(MouseEvent e){}
public void mouseDragged(MouseEvent e){
x1=e.getX();
y1=e.getY();
Graphics g = getGraphics();
update_buffer(g,new DrawItem(x0,y0,x1,y1));
update_buffer(off_screen_gc,new DrawItem(x0,y0,x1,y1));
g.dispose();
x0=x1;
y0=y1;
}
public void mousePressed(MouseEvent e){
x0 =e.getX();
y0 =e.getY();
}
public void paint(Graphics g){
copy_from_offscreen_buf(g);//把這句話屏蔽掉,就不能恢復用戶繪製的圖形了
}
void copy_from_offscreen_buf(Graphics g){
if(g != null)
g.drawImage(off_screen_buf, 0, 0, null);
}
private Image off_screen_buf;
private Graphics off_screen_gc;
WBApplet parent;
}
class DrawItem{
DrawItem(int x0,int y0,int x1,int y1){
this.x0=x0;
this.y0=y0;
this.x1=x1;
this.y1=y1;
}
int x0;
int y0;
int x1;
int y1;
}
///:~
import java.awt.*;
import java.awt.event.*;
public class WhiteBoard extends Canvas implements MouseMotionListener,MouseListener{
final static int DEFAULT_BOARDWIDTH=700;
final static int DEFAULT_BOARDHEIGHT=400;
int x0,y0,x1,y1;
WhiteBoard(WBApplet WBApplet1){
parent = WBApplet1;
off_screen_buf =parent.createImage(DEFAULT_BOARDWIDTH,DEFAULT_BOARDHEIGHT);
off_screen_gc = off_screen_buf.getGraphics();
addMouseMotionListener(this);
addMouseListener(this);
}
synchronized public void update_buffer(Graphics g,DrawItem data) {
g.drawLine(data.x0,data.y0,data.x1,data.y1);
}
public void mouseMoved(MouseEvent e){}
public void mouseReleased(MouseEvent e){}
public void mouseEntered(MouseEvent e){}
public void mouseExited(MouseEvent e){}
public void mouseClicked(MouseEvent e){}
public void mouseDragged(MouseEvent e){
x1=e.getX();
y1=e.getY();
Graphics g = getGraphics();
update_buffer(g,new DrawItem(x0,y0,x1,y1));
update_buffer(off_screen_gc,new DrawItem(x0,y0,x1,y1));
g.dispose();
x0=x1;
y0=y1;
}
public void mousePressed(MouseEvent e){
x0 =e.getX();
y0 =e.getY();
}
public void paint(Graphics g){
copy_from_offscreen_buf(g);//把這句話屏蔽掉,就不能恢復用戶繪製的圖形了
}
void copy_from_offscreen_buf(Graphics g){
if(g != null)
g.drawImage(off_screen_buf, 0, 0, null);
}
private Image off_screen_buf;
private Graphics off_screen_gc;
WBApplet parent;
}
class DrawItem{
DrawItem(int x0,int y0,int x1,int y1){
this.x0=x0;
this.y0=y0;
this.x1=x1;
this.y1=y1;
}
int x0;
int y0;
int x1;
int y1;
}
///:~
運行一下,看是不是不一樣了.這一次你想讓你畫的東西消失都不可能了.爲了將這個原理說清楚,以上的代碼我都沒有編寫的太複雜,下一次我們會創建更加複雜,更加完善的畫板程序.