當我們開發應用程序的時候，有的時候需要詢問用戶是不是要繼續下面的操作，比如刪除一個電話號碼，然后根據用戶的不同的動作進入不同的流程。這時候我們需要一個像樣的Dialog組件，很遺憾MIDP中并沒有提供，但是我們可以用Canvas自己寫一個。下面將簡單介紹這個組件的設計，然后給出測試的MIDlet的源代碼。希望對讀者有幫助！
首先我們寫一個抽象類Dialog，內容如下
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;
}
}
你需要覆蓋getDisplayable()方法返回一個Displayable的對象，當你調用dialog的display()方法的時候，你的YourDialog將會顯示在屏幕上，有的時候你可能要傳遞一個事件值給后面的對象，那么你應該調用方法display(int event)。Dialog可以注冊DialogListener，這個接口定義了一個方法，內容如下：
public interface DialogListener
{
void dialogDismissed(Dialog dialog, int code);
}
當Dialog顯示的時候，我們提供給用戶的界面是用WaitCanvas實現的，下面是他的代碼：
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);
}
}
}
過控制boolean run的值可以決定是不是讓這個畫面動起來。下面兩個例子是Dialog的子類，他們實現了它的抽象方法。我直接給出代碼：
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 = (amessage == null) ? "繼續操作？" : amessage;
this.yesLabel = (yesLabel == null) ? "確定" : 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)?"確定":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這兩個Dialog，你也可以擴展Dialog類實現自己的Dialog，下面是測試這兩個Dialog的MIDlet。首先我們看一下它的截圖然后給出源代碼！
這里只給出用戶選擇確定的界面，如果你選擇返回那么知識下面的文字改變。
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, "繼續操作嘛？");
confirm.setDialogListener(this);
confirm.display();
}
public void dialogDismissed(Dialog d, int code)
{
if (d == confirm)
{
if (code == ConfirmationDialog.YES)
{
message = new MessageDialog(display, "您選擇了確定");
} else
{
message = new MessageDialog(display, "您選擇了返回");
}
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);
}
}
}
上面的整個代碼是一個完整的Dialog組件（不包括DialogTest)如果需要使用直接放在你的project里面就可以了！