基于MIDP实现Dialog组件

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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/* * Created on 2004-7-10 * * TODO To change the template for this generated file go to * Window - Preferences - Java - Code Style - Code Templates */

/** * @author P2800 * * TODO To change the template for this generated type comment go to Window - * Preferences - Java - Code Style - Code Templates */

import javax.microedition.lcdui.*;

//The base class for a set of general-purpose//Dialogs. To use, create a concrete instance//of this class, set the Dialog listener,//and then call display.

public abstract class Dialog{ protected Display display; protected DialogListener listener; protected Displayable restore; private int eventID;

protected Dialog(Display display) { this.display = display; }

/** * @return Returns the eventID. */ public int getEventID() { return eventID; }

/** * @param eventID * The eventID to set. */ public void setEventID(int eventID) { this.eventID = eventID; }

// Dismisses the Dialog, restoring the old // displayable, if any. This is normally done // by the Dialog itself in response to commands, // but can also be called by the application // in response to some other event, such as a // timer expiration. The code is passed directly // to the Dialog listener, if any.

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); } }

// Displays the Dialog, saving the current // displayable for later restoration.

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); } }

// Returns the registered Dialog listener.

public DialogListener getDialogListener() { return listener; }

// Subclasses implement this method to return // the actual object to display on the screen.

protected abstract Displayable getDisplayable();

// Registers a Dialog listener.

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：/* * Created on 2004-7-10 * * TODO To change the template for this generated file go to * Window - Preferences - Java - Code Style - Code Templates */

/** * @author P2800 * * TODO To change the template for this generated type comment go to Window - * Preferences - Java - Code Style - Code Templates */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：/* * Created on 2004-3-14 * * To change the template for this generated file go to * Window - Preferences - Java - Code Generation - Code and Comments */

import java.util.*;import javax.microedition.lcdui.*;

/** * @author p2800 * * To change the template for this generated type comment go to Window - * Preferences - Java - Code Generation - Code and Comments */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); } }

}

通过控制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.*;

//Defines a Dialog that displays a text//message and "Yes" and "No" buttons.

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) ? "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); }

/** * @return Returns the message. */ public String getMessage() { return message; }

/** * @param message * The message to set. */ 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(); }

// First ask the user a question

confirm = new ConfirmationDialog(display, "继续操作嘛？"); confirm.setDialogListener(this); confirm.display(); }

public void DialogDismissed(Dialog d, int code) { if (d == confirm) { // Then give the user a response

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) { // Now let the user quit

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！