apr01_ericg.txt

TechTips j2me的常用技巧. 网络功能

After you've defined the alert, you display it by calling 
Display.setCurrent. You pass the method both a reference to the 
alert and a reference to a displayable element. A displayable 
element is a top-level user interface object that extends the 
Displayable class -- your application will have at least one
of these whether it's using the low-level or high-level UI APIs. 
The system immediately activates the displayable element when
the alert times out or is dismissed. To return to the 
displayable element that was active just before the alert was 
displayed, simply do the following:

Display display = ....; // assign on startup
Alert   msg = ....; // create an alert

display.setCurrent( alert, display.getCurrent() );

Notice that the setCurrent method is invoked on an instance of 
the Display class. You can obtain the class when the MIDlet 
starts by calling the static Display.getDisplay method, passing 
the MIDlet instance as its only parameter. For example:

public MyApp extends MIDlet
{
    private Display display;
      
    public MyApp(){
        display = Display.getDisplay( this );
    }
  
    public Display getDisplay(){
        return display;
    }
   
    // rest of MIDlet goes here
}

It's simplest to save the Display instance as a member of your 
main application class and make it available to the other classes
in your application.

Note that calling Display.setCurrent simply changes what is 
displayed on the screen -- it doesn't halt the current thread of 
execution, even if the alert is modal. You typically call it
in response to some event; you should exit the method you're in 
as soon as possible to let the system's event processing to
continue unhindered.

The sound associated with an alert must be one of five predefined 
instances of the AlertType class: INFO, WARNING, ERROR, ALARM, or 
CONFIRMATION. The device associates different sounds with each 
class, if possible, though some devices may not even support 
sound.  

You can play any sound at any time by calling the playSound 
method, as in:

    AlertType.ERROR.playSound( display );
    
The icon associated with an alert must be an instance of the Image 
class. The icon should be as small as possible and look good when 
mapped to monochrome.

Now what about tickers? A ticker is a user interface control that 
displays a single line of text, scrolling it onscreen at regular 
intervals just like an old-fashioned movie marquee. To use a 
ticker, you create an instance of the Ticker class, passing the 
string you want displayed into the constructor. For example:

import javax.microedition.ldcdui.*;

Ticker ticker = new Ticker( "Hello, world!" );

To display the ticker, you associate it with a top-level window 
created by the high-level UI, that is, any class that extends
the Screen class. You associate the ticker by calling the 
setTicker method, as in:

Form f = new Form( "A Title" );
f.setTicker( ticker );

The ticker is displayed in an appropriate area of the window. 
You can share the same ticker across different windows; the 
system then attempts to keep it in the same position.

The only thing you can do with a ticker is change its text by 
calling the setString method. To stop displaying a ticker, 
remove it from the top-level window by passing null to the 
setTicker method.

Here's a simple stock tracking MIDlet that demonstrates the use 
of alerts and tickers. Note that the stock values are generated 
randomly for example purposes. In the real world, you would use 
the HttpConnection class to obtain stock values from a web site.  
Note particularly how the alert text can be changed while the 
alert is still active.

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

public class StockWatcher extends MIDlet {

    Display      display;
    Ticker       ticker = new Ticker( "" );
    Command      exitCommand = new Command( 
                                     "Exit", Command.EXIT, 1 );
    Timer        timer = new Timer();
    StockChecker checker = new StockChecker();
    TickerForm   form = new TickerForm();
    Alert        alert = new Alert( "Stock Alert!" );

    public StockWatcher() {
        display = Display.getDisplay( this );
        alert.setTimeout( Alert.FOREVER );
    }

    protected void destroyApp( boolean unconditional ) {
    }

    protected void startApp() {
        display.setCurrent( form );
        timer.schedule( checker, 0, 30000 );
    }

    protected void pauseApp() {
    }

    public void exit(){
        timer.cancel();
        destroyApp( true );
        notifyDestroyed();
    }

    // Display a simple form to hold the ticker

    class TickerForm extends Form implements CommandListener {
        public TickerForm(){
            super( "Stock Watch" );
            setTicker( ticker );
            addCommand( exitCommand );
            setCommandListener( this );
        }

        public void commandAction( Command c, Displayable d ){
            exit();
        }
    }
  
    // Check the stock values and put up an alert if
    // they're beyond certain limits....

    class StockChecker extends TimerTask {
        Random       generator = new Random();
        int          sybsValue = 20000;
        int          sunwValue = 30000;
        int          ibmValue = 40000;
        StringBuffer buf = new StringBuffer();

        public void run(){
            String values = getStockValues();

            ticker.setString( values );

            if( sybsValue < 18000 || sybsValue > 22000 ||
                sunwValue < 28000 || sunwValue > 32000 ||
                ibmValue < 38000 || ibmValue > 42000 ){
                alert.setString( values );
            }

            if( !alert.isShown() ){
              display.setCurrent( alert, form );
            }
        }

        private String getStockValues(){
            sybsValue = randomStockValue( sybsValue );
            sunwValue = randomStockValue( sunwValue );
            ibmValue = randomStockValue( ibmValue );

            buf.setLength( 0 );
            appendValue( "SYBS", sybsValue );
            appendValue( "SUNW", sunwValue );
            appendValue( "IBM", ibmValue );

            return buf.toString();
        }

        // Generate a random stock value... in the
        // real world you'd use HTTP to obtain the
        // stock value from a broker's website.

        private int randomStockValue( int oldVal ){
            int incr1 = ( generator.nextInt() % 2 );
            int incr2 = ( generator.nextInt() % 16 );

            if( incr1 < 1 ){
                oldVal -= incr1 * 1000;
            } else {
                oldVal += ( incr1 - 2 ) * 1000;
            }

            if( incr2 < 8 ){
                oldVal -= incr2 * 250;
            } else {
                oldVal += incr2 * 250;
            }

            return oldVal;
        }

        private void appendValue( String stock, int val ){
            buf.append( stock );
            buf.append( ' ' );
            buf.append( Integer.toString( val / 1000 ) );
            buf.append( '.' );
            buf.append( Integer.toString( val % 1000 ) );
            buf.append( ' ' );
        }
    }
}

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J2ME Tech Tips 
April 16, 2001

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