在MIDP中没有提供Dialog组件，但是提供了一个Alert。Alert的功能有限，因此写一个Dialog组件是非常有必要的。本文将提供一个基于MIDP的Dialog组件，你可以在应用程序中使用它，功能强大且非常方便。
当我们开发应用程序的时候，有的时候需要询问用户是不是要继续下面的操作，比如删除一个电话号码，然后根据用户的不同的动作进入不同的流程。这时候我们需要一个像样的Dialog组件，很遗憾MIDP中并没有提供，但是我们可以用Canvas自己写一个。下面将简单介绍这个组件的设计，然后给出测试的MIDlet的源代码。希望对读者有帮助！
首先我们写一个抽象类Dialog，内容如下
import javax.microedition.lcdui.*;
public abstract class Dialog
{
PRotected Display display;
protected DialogListener listener;
protected Displayable restore;
private int eventID;
protected Dialog(Display display)
{
this.display = display;
}
public int getEventID()
{
return eventID;
}
public void setEventID(int eventID)
{
this.eventID = eventID;
}
public void dismiss(int code)
{
Displayable curr = display.getCurrent();
if (curr != getDisplayable())
return;
if (restore != null)
{
display.setCurrent(restore);
} else
{
display.setCurrent(new Form(""));
}
if (listener != null)
{
listener.dialogDismissed(this, code);
}
}
public void display()
{
Displayable curr = display.getCurrent();
Displayable dialog = getDisplayable();
if (curr != dialog)
{
restore = curr;
display.setCurrent(dialog);
}
}
public void display(int event)
{
Displayable curr = display.getCurrent();
Displayable dialog = getDisplayable();
this.eventID = event;
if (curr != dialog)
{
restore = curr;
display.setCurrent(dialog);
}
}
public DialogListener getDialogListener()
{
return listener;
}
protected abstract Displayable getDisplayable();
public void setDialogListener(DialogListener l)
{
listener = l;
}
}
你需要覆盖getDisplayable()方法返回一个Displayable的对象，当你调用dialog的display()方法的时候，你的YourDialog将会显示在屏幕上，有的时候你可能要传递一个事件值给后面的对象，那么你应该调用方法display(int event)。Dialog可以注册DialogListener，这个接口定义了一个方法，内容如下：
public interface DialogListener
{
void dialogDismissed(Dialog dialog, int code);
}
当Dialog显示的时候，我们提供给用户的界面是用WaitCanvas实现的，下面是他的代码：
import java.util.*;
import javax.microedition.lcdui.*;
public class WaitCanvas extends Canvas
{
private int mCount, mMaximum;
private int mInterval;
private int mWidth, mHeight, mX, mY, mRadius;
private String mMessage;
private boolean run = false;
public WaitCanvas(String message, boolean run)
{
this.mMessage = message;
mCount = 0;
mMaximum = 36;
mInterval = 100;
mWidth = getWidth();
mHeight = getHeight();
// Calculate the radius.
int halfWidth = (mWidth - mRadius) / 2;
int halfHeight = (mHeight - mRadius) / 2;
mRadius = Math.min(halfWidth, halfHeight);
// Calculate the location.
mX = halfWidth - mRadius / 2;
mY = halfHeight - mRadius / 2;
// Create a Timer to update the display.
if (run)
{
TimerTask task = new TimerTask()
{
public void run()
{
mCount = (mCount + 1) % mMaximum;
repaint();
}
};
Timer timer = new Timer();
timer.schedule(task, 0, mInterval);
}
}
public void paint(Graphics g)
{
int theta = -(mCount * 360 / mMaximum);
// Clear the whole screen.
g.setColor(255, 255, 255);
g.fillRect(0, 0, mWidth, mHeight);
// Now draw the pinwheel.
g.setColor(128, 128, 255);
g.drawArc(mX, mY, mRadius, mRadius, 0, 360);
g.fillArc(mX, mY, mRadius, mRadius, theta + 90, 90);
g.fillArc(mX, mY, mRadius, mRadius, theta + 270, 90);
// Draw the message, if there is a message.
if (mMessage != null)
{
g.drawString(mMessage, mWidth / 2, mHeight, Graphics.BOTTOM
Graphics.HCENTER);
}
}
}
过控制boolean run的值可以决定是不是让这个画面动起来。下面两个例子是Dialog的子类，他们实现了它的抽象方法。我直接给出代码：
import javax.microedition.lcdui.*;
public class ConfirmationDialog extends Dialog implements CommandListener
{
public static final int YES = 0;
public static final int NO = 1;
protected Canvas canvas;
protected Command noCommand;
protected Command yesCommand;
private String message;
private String yesLabel;
private String noLabel;
public ConfirmationDialog(Display display, String message)
{
this(display, message, null, null);
}
public ConfirmationDialog(Display display, String amessage,
String ayesLabel, String anoLabel)
{
super(display);
this.message = (amessa
ge == null) ? "继续操作？" : amessage;
this.yesLabel = (yesLabel == null) ? "确定" : ayesLabel;
this.noLabel = (noLabel == null) ? "返回" : anoLabel;
yesCommand = new Command(yesLabel, Command.OK, 1);
noCommand = new Command(noLabel, Command.CANCEL, 1);
canvas = new WaitCanvas(message, true);
canvas.addCommand(yesCommand);
canvas.addCommand(noCommand);
canvas.setCommandListener(this);
}
public String getMessage()
{
return message;
}
public void setMessage(String message)
{
this.message = message;
}
public void commandAction(Command c, Displayable d)
{
if (c == yesCommand)
{
dismiss(YES);
} else if (c == noCommand)
{
dismiss(NO);
}
}
protected Displayable getDisplayable()
{
return canvas;
}
}
import javax.microedition.lcdui.*;
public class MessageDialog extends Dialog implements CommandListener
{
public static final int OK = 0;
protected Command command;
protected Canvas canvas;
private String message;
private String label;
public MessageDialog(Display display, String message)
{
this(display, message, null);
}
public MessageDialog(Display display, String amessage, String alabel)
{
super(display);
this.message = (amessage == null)?"完成":amessage;
this.label = (alabel == null)?"确定":alabel;
command = new Command(label, Command.OK, 1);
canvas = new WaitCanvas(message, true);
canvas.addCommand(command);
canvas.setCommandListener(this);
}
public void commandAction(Command c, Displayable d)
{
if (c == command)
{
dismiss(OK);
}
}
protected Displayable getDisplayable()
{
return canvas;
}
}
你可以方便的在自己的程序中使用这两个Dialog，你也可以扩展Dialog类实现自己的Dialog，下面是测试这两个Dialog的MIDlet。首先我们看一下它的截图然后给出源代码！
这里只给出用户选择确定的界面，如果你选择返回那么知识下面的文字改变。
import javax.microedition.lcdui.*;
import javax.microedition.midlet.*;
public class DialogTest extends MIDlet implements CommandListener,
DialogListener
{
private Display display;
private Form mainForm;
private ConfirmationDialog confirm;
private MessageDialog message;
public static final Command exitCommand = new Command("Exit", Command.EXIT,
1);
public DialogTest()
{
}
public void commandAction(Command c, Displayable d)
{
if (c == exitCommand)
{
exitMIDlet();
}
}
protected void destroyApp(boolean unconditional)
throws MIDletStateChangeException
{
exitMIDlet();
}
public void exitMIDlet()
{
notifyDestroyed();
}
public Display getDisplay()
{
return display;
}
protected void initMIDlet()
{
}
protected void pauseApp()
{
}
protected void startApp() throws MIDletStateChangeException
{
if (display == null)
{
display = Display.getDisplay(this);
initMIDlet();
}
confirm = new ConfirmationDialog(display, "继续操作嘛？");
confirm.setDialogListener(this);
confirm.display();
}
public void dialogDismissed(Dialog d, int code)
{
if (d == confirm)
{
if (code == ConfirmationDialog.YES)
{
message = new MessageDialog(display, "您选择了确定");
} else
{
message = new MessageDialog(display, "您选择了返回");
}
message.display();
message.setDialogListener(this);
} else if (d == message)
{
Form f = new Form(null);
f.append("退出程序");
f.addCommand(exitCommand);
f.setCommandListener(this);
display.setCurrent(f);
}
}
}
上面的整个代码是一个完整的Dialog组件（不包括DialogTest)如果需要使用直接放在你的project里面就可以了！
（出处：http://www.knowsky.com）
