当前页面: 开发资料首页 → J2ME 专题 → 我的J2ME编程练习（7）——Canvas2

我的J2ME编程练习（7）——Canvas2

摘要: 我的J2ME编程练习（7）——Canvas2

/*
* Canvas2let.java
*
* Created on 2005年4月19日, 下午5:27
*/

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

/**
*
* @author Administrator
* @version
*/
public class Canvas2let extends MIDlet implements CommandListener {
private ChoiceGroup faceChoice;
private ChoiceGroup styleChoice;
private ChoiceGroup sizeChoice;

private Command okCommand;
private Command exitCommand;
private Command backCommand;

private TextBox faceTextBox;
private TextBox styleTextBox;
private TextBox sizeTextBox;

private Form choiceForm;

private Display aDisplay;

private MyCanvas canvas;

int face;
int size;
int style;

public Canvas2let(){
choiceForm=new Form("Select Font");

faceChoice=new ChoiceGroup("外观",Choice.EXCLUSIVE);
faceChoice.append("PROPORTIONAL",null);
faceChoice.append("MONOSPACE",null);
faceChoice.append("SYSTEM",null);

styleChoice=new ChoiceGroup("风格",Choice.MULTIPLE);
styleChoice.append("PLAIN",null);
styleChoice.append("BOLD",null);
styleChoice.append("ITALIC",null);
styleChoice.append("UNDERLINED",null);

sizeChoice=new ChoiceGroup("大小",Choice.EXCLUSIVE);
sizeChoice.append("SMALL",null);
sizeChoice.append("MEDIUM",null);
sizeChoice.append("LARGE",null);

exitCommand=new Command("退出",Command.EXIT,1);
okCommand=new Command("确定",Command.OK,1);
backCommand=new Command("后退",Command.BACK,2);

choiceForm.append(faceChoice);
choiceForm.append(styleChoice);
choiceForm.append(sizeChoice);

choiceForm.addCommand(exitCommand);
choiceForm.addCommand(okCommand);
choiceForm.setCommandListener(this);

canvas=new MyCanvas();
}
public void startApp() {
aDisplay=Display.getDisplay(this);
aDisplay.setCurrent(choiceForm);
}

public void pauseApp() {
}

public void destroyApp(boolean unconditional) {
}

public void commandAction(Command c ,Displayable d){
if (c==exitCommand){
destroyApp(false);
notifyDestroyed();
}
else {
//select the face and size
face=faceChoice.getSelectedIndex();
switch(face){
case 0: face=Font.FACE_PROPORTIONAL;break;
case 1: face=Font.FACE_MONOSPACE;break;
case 2: face=Font.FACE_SYSTEM;break;
}

size=sizeChoice.getSelectedIndex();
switch(size){
case 0:size=Font.SIZE_SMALL;break;
case 1:size=Font.SIZE_MEDIUM;break;
case 2:size=Font.SIZE_LARGE;break;
}

//select the style
boolean[] styleSelect =new boolean[4];
for(int i=0;i<4;i++){
if (styleChoice.isSelected(i)&&i==0){
style |= Font.STYLE_PLAIN;
}
else if (styleChoice.isSelected(i)&&i==1){
style |=Font.STYLE_BOLD;
}
else if (styleChoice.isSelected(i)&&i==2){
style |=Font.STYLE_ITALIC;
}
else if (styleChoice.isSelected(i)&&i==3){
style |=Font.STYLE_UNDERLINED;
}

}

//canvas=new MyCanvas();
aDisplay.setCurrent(canvas);
}
}

class MyCanvas extends Canvas implements CommandListener{

public MyCanvas(){

addCommand(backCommand);
setCommandListener(this);
}

public void paint(Graphics g){
g.setColor(0xFFFFFF);
g.fillRect(0,0,getWidth(),getHeight());

g.setColor(0);
String s="Hello World";
Font f=Font.getFont(face,style,size);
g.setFont(f);
g.drawString(s,150,250,Graphics.RIGHT|Graphics.BOTTOM);
}

public void commandAction(Command c ,Displayable d){
if (c==backCommand){
aDisplay.setCurrent(choiceForm);
}
}



}
}
这个程序主要是加深对Canvas的Font的认识。

1.MIDlet类的构造函数应该是public Canvas2let()而不应该是public void Cancas2let()。因为加了void就不会返回该类的对象，导致程序运行后没有反映。这是一个教训。

2.在Canvas中，需要使用代码

g.setColor(0xFFFFFF);
g.fillRect(0,0,getWidth(),getHeight());

来清除上一个界面。然后再使用 g.setColor(0);来设置绘画颜色。

3.关于这一点，应该这样理解：

既然使用了低级界面，那么就意味着屏幕上的一切都归你管，你有最大的权利。但你也有相应的义务，清除屏幕的工作自然也要你来做