應(yīng)用雙緩沖技術(shù)解決畫面閃爍的問題

2019-11-18 16:08:07

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供稿：網(wǎng)友

有些時(shí)候我們可能在J2ME中遇到畫面閃爍的問題，這時(shí)候我們可以應(yīng)用雙緩沖的技術(shù)來解決，不過由于現(xiàn)在越來越多的手機(jī)自身就支持雙緩沖了。所以這里只做個(gè)簡單的介紹。

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(gè)手機(jī)自己就支持雙緩沖，所以根本沒有比較出來。不過了解這個(gè)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(shí)候遇到的。在MIDP開發(fā)中同樣存在，當(dāng)你在屏幕上進(jìn)行原始寫畫的時(shí)候，如果很復(fù)雜的話，用戶會(huì)發(fā)現(xiàn)界面在閃爍。因此你可以這樣做，首先在另一個(gè)圖片上進(jìn)行paint()得操作，當(dāng)完成了以后就把它c(diǎn)opy到屏幕上，由于通常copy得時(shí)候速度很快就不會(huì)出現(xiàn)閃爍了。這個(gè)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(gè)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.HCENTERGraphics.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.LEFTGraphics.TOP);
        }

}

得到offImage得Graphics實(shí)例后，進(jìn)行paint()得操作。這個(gè)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.LEFTGraphics.TOP);

我寫了一個(gè)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(jī)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(jī)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.HCENTERGraphics.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);
    }

}

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.HCENTERGraphics.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.LEFTGraphics.TOP);
        }