我寫了程序本來想在NOkia 6108上比較用和不用的區(qū)別，結(jié)果這個手機自己就支持雙緩沖，所以根本沒有比較出來。不過了解這個技術(shù)還是有必要的，所以我決定還是寫出來。雙緩沖本來是在開發(fā)PC應(yīng)用程序的時候遇到的。在MIDP開發(fā)中同樣存在，當(dāng)你在屏幕上進行原始寫畫的時候，如果很復(fù)雜的話，用戶會發(fā)現(xiàn)界面在閃爍。因此你可以這樣做，首先在另一個圖片上進行paint()得操作，當(dāng)完成了以后就把它copy到屏幕上，由于通常copy得時候速度很快就不會出現(xiàn)閃爍了。這個技術(shù)就是雙緩沖。
Canvas類提供了isDoubleBuffered()方法來判斷設(shè)備是不是支持這個功能，如果返回true的話，那么我們就沒有必要使用雙緩沖了，如果false的話，我們可以這么做：
public DoubleCanvas(UIController uicontroller)
{
super();
this.uicontroller = uicontroller;
width = this.getWidth();
height = this.getHeight();
this.setCommandListener(this);
if(!isDoubleBuffered())
{
offImage = Image.createImage(width,height);
}
}
protected void paint(Graphics arg0)
{
arg0.drawString(isDoubleBuffered()+"",width/2,height/2,Graphics.HCENTER|Graphics.TOP);
Graphics saved = arg0;
if(offImage != null)
{
arg0 = offImage.getGraphics();
}
arg0.setColor(255,128,128);
for(int i = 2,j=2;i<width/2-6&&j<height/2-6;i=i+2,j=j+2)
{
arg0.drawRect(i,j,width-2*i,height-2*j);
}
if(arg0 != saved)
{
saved.drawImage(offImage,0,0,Graphics.LEFT|Graphics.TOP);
}
}
得到offImage得Graphics實例后，進行paint()得操作。這個部分通常比較復(fù)雜，我這里的不夠復(fù)雜：）
然后把offImage直接copy到屏幕上也就是執(zhí)行saved.drawImage(offImage,0,0,Graphics.LEFT|Graphics.TOP);
我寫了一個應(yīng)用程序來比較使用和不使用雙緩沖的效果，但是由于我的手機支持雙緩沖所以看不出效果，如果你有興趣，那么可以把paint()部分的代碼修改的復(fù)雜一些然后再移植到不支持的手機上也許可以看出效果，下面是我程序的代碼：
import javax.microedition.midlet.MIDlet;
import javax.microedition.midlet.MIDletStateChangeException;
public class DoubleBufferMIDlet extends MIDlet
{
private UIController uicontroller;
protected void startApp() throws MIDletStateChangeException
{
uicontroller = new UIController(this);
uicontroller.init();
}
protected void pauseApp()
{
}
protected void destroyApp(boolean arg0) throws MIDletStateChangeException
{
}
}
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.List;
public class MainListUI extends List implements CommandListener
{
private UIController uicontroller;
public MainListUI(UIController uicontroller)
{
super("Test",List.IMPLICIT);
this.uicontroller = uicontroller;
this.append("Non-buffer",null);
this.append("Double-buffer",null);
this.setCommandListener(this);
}
public void commandAction(Command arg0, Displayable arg1)
{
if(arg0 == List.SELECT_COMMAND)
{
if(this.getSelectedIndex() == 0)
{
uicontroller.handleEvent(UIController.EventID.DISPLAY_NON_BUFFER);
}
else
{
uicontroller.handleEvent(UIController.EventID.DISPLAY_BUFFER);
}
}
}
}
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
public class UIController implements CommandListener
{
private Display display;
private DoubleBufferMIDlet midlet;
private MainListUI mainList;
private NonDoubleCanvas noDoubleCanvas;
private DoubleCanvas doubleCanvas;
public static final Command backCommand = new Command("Back",Command.BACK,2);
public static class EventID
{
public static final int DISPLAY_NON_BUFFER = 0;
public static final int DISPLAY_BUFFER = 1;
}
public UIController(DoubleBufferMIDlet midlet)
{
this.midlet = midlet;
}
public void init()
{
display = Display.getDisplay(midlet);
mainList = new MainListUI(this);
noDoubleCanvas = new NonDoubleCanvas(this);
doubleCanvas = new DoubleCanvas(this);
addCommand();
display.setCurrent(mainList);
}
public void addCommand()
{
noDoubleCanvas.addCommand(backCommand);
doubleCanvas.addCommand(backCommand);
}
public Display getDisplay()
{
return display;
}
public void setCurrent(Displayable disp)
{
display.setCurrent(disp);
}
public void handleEvent(int eventID)
{
switch(eventID)
{
case EventID.DISPLAY_BUFFER:
{
setCurrent(doubleCanvas);
System.out.println(EventID.DISPLAY_BUFFER);
break;
}
case EventID.DISPLAY_NON_BUFFER:
{
setCurrent(noDoubleCanvas);
System.out.println(EventID.DISPLAY_NON_BUFFER);
break;
}
}
}
public void commandAction(Command arg0, Displayable arg1)
{
if(arg0 == backCommand)
{
display.setCurrent(mainList);
}
}
}
import javax.microedition.lcdui.Canvas;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Graphics;
public class NonDoubleCanvas extends Canvas implements CommandListener
{
private UIController uicontroller;
private int width;
private int height;
public NonDoubleCanvas(UIController uicontroller)
{
super();
this.uicontroller = uicontroller;
width = this.getWidth();
height = this.getHeight();
this.setCommandListener(this);
}
protected void paint(Graphics arg0)
{
arg0.setColor(100,100,100);
arg0.drawString(isDoubleBuffered()+"",width/2,height/2,Graphics.HCENTER|Graphics.TOP);
for(int i = 2,j=2;i<width/2-6&&j<height/2-6;i=i+2,j=j+2)
{
arg0.drawRect(i,j,width-2*i,height-2*j);
}
}
public void commandAction(Command arg0, Displayable arg1)
{
uicontroller.commandAction(arg0,arg1);
}
}
import javax.microedition.lcdui.Canvas;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Graphics;
import javax.microedition.lcdui.Image;
public class DoubleCanvas extends Canvas implements CommandListener
{
private UIController uicontroller;
private Image offImage;
private int width;
private int height;
public DoubleCanvas(UIController uicontroller)
{
super();
this.uicontroller = uicontroller;
width = this.getWidth();
height = this.getHeight();
this.setCommandListener(this);
if(!isDoubleBuffered())
{
offImage = Image.createImage(width,height);
}
}
protected void paint(Graphics arg0)
{
arg0.drawString(isDoubleBuffered()+"",width/2,height/2,Graphics.HCENTER|Graphics.TOP);
Graphics saved = arg0;
if(offImage != null)
{
arg0 = offImage.getGraphics();
}
arg0.setColor(255,128,128);
for(int i = 2,j=2;i<width/2-6&&j<height/2-6;i=i+2,j=j+2)
{
arg0.drawRect(i,j,width-2*i,height-2*j);
}
if(arg0 != saved)
{
saved.drawImage(offImage,0,0,Graphics.LEFT|Graphics.TOP);
}
}
public void commandAction(Command arg0, Displayable arg1)
{
uicontroller.commandAction(arg0,arg1);
}
}