Building WML Gadgets: Phone Message Application

January 31, 2003
By Steve Schafer

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Coding the Application

Let's break down the individual processes and then code foreach.

Database Design

Our "database" will be a simple delimited flatfile. Although we could go the fancy route with an actual database format, thedelimited format will work well for our simple application.

The database will contain the following fields:

Date and time the message was taken
Caller's name
Caller's message
Caller's phone number

We'll use a double vertical bar for our delimiter. We coulduse a more standard delimiter, such as a comma, but we need something thatwouldn't end up in the middle of the message field. In short, our databaserecords will resemble the following:

HTML Form for Data Entry

We'll use a simple HTML form for entering the data:

Listing: msgform.html

<html><body><form name="msgform" method="post" action="phonemsg.pl"><center>Telephone Message</center><br><table><tr><td><input type="hidden" name="cmd" value="save">From:</td><td><input type="text" name="from" size="100" maxlength="100"></td></tr><tr><td>Message:</td><td><textarea name="message" cols="100" rows="5" wrap="virtual"></textarea></td></tr><tr><td>Callback #:</td><td><input type="text" name="number" size="12" maxlength="12"></td></tr><tr><td colspan="2"><center><input type="submit" value="Submit"></center></td></tr></table></form></body></html>

Note that we include an extra field, "cmd." Thishidden field will be passed to our CGI script to tell it what to do; namely,"save" the data.

The CGI Script

Our CGI script will be one multipurpose script, performingthe following functions:

Saving the data (caller info)
Listing the caller record(s)
Displaying a selected record's details
Automatically dialing the caller's number
Optionally deleting the record and then calling the number

The script could also display the input form. However, formaximum portability, we'll use a simple HTML file. With this method, our formcan easily be included in almost any Web page template, simply by applying astyle sheet or by cutting-and-pasting the "guts" of the form intoanother page.

Saving the Data

The following code fragment saves the data entered into theform:

Listing: phonemsg.pl - Save data fragment

  # Start response page  print header;  print "<html><head>";  print "<META HTTP-EQUIV=\"Refresh\" CONTENT=\"2; URL='msgform.html'\">";  print "</head><body>Please wait...<p>";    # Grab the parameters  $from = param('from');  $message = param('message');  $callback = param('number');  # Remove any vertical bars   $message =~tr/|/ /;  # Check file lock  $count = 0;  if (-e "msgfile.lock") {    select(undef,undef,undef,0.1);    $count++;    if ($count = 10) { die "Can't open message file! </body></html>"; }  }  open LOCK, ">msgfile.lock";  # Write new record to end of file  open FILE, ">>msgfile.txt";  print FILE $date ." ". $time ."||";  print FILE $from ."||". $message ."||". $callback;  print FILE "\n";  close FILE;    close LOCK;  unlink "msgfile.lock";    # All done, close response page  print "</body></html>";

Because the form and the mobile device could both beutilizing the script (and hence, the database) simultaneously, we must use filelockingto avoid having two processes accessing the database and corrupting ourdata. We'll use a simple method: creating a file to lock the databaseif thefile exists, the process waits for it to be deleted before accessing thedatabase. When a process is done with the database, the lock file is deletedand other processes are allowed to access the database. Instead of failingright away if the file is locked, or waiting forever for the file to beunlocked, the code loops 10 times, waiting a tenth of a second between theiterations. If the file is still locked, we assume that something has gonewrong and exit with an appropriate error message.

Windows users: Some implementations of Perl on the Windowsplatform don't support the four-argument call of "select" used in ourfile-locking loop. If this code generates errors on a Windows system,substitute another delay function.

Listing the Records on the Mobile Device

The following code fragment comprises the code to list fiverecords on the mobile device, along with an option to move to the next fiverecords:

Listing: phonemsg.pl - List records fragment

  #Pass WML header  print header(-type=>'text/vnd.wap.wml');# Print WML header and beginning tagsprint <<ENDHEADER;<?xml version="1.0"?><!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN"  "http://www.wapforum.org/DTD/wml_1.1.xml"><wml><card>ENDHEADER  # Print start of cardprint <<BEGINCARD;<p mode="nowrap">Call List<br/><select name="Display" title="Display">BEGINCARD    # Check file lock  $count = 0;  if (-e "msgfile.lock") {    select(undef,undef,undef,0.1);    $count++;    if ($count = 10) {       die "Can't open message file! </select></p></card></wml>";    }  }  open LOCK, ">msgfile.lock";  # Read records into array  open FILE,"msgfile.txt";  push(@lines,$_) while (<FILE>);  close FILE;  close LOCK;  unlink "msgfile.lock";  # Nextitem = first item to display  $nextitem = param('nextitem');  if ("$nextitem" eq "") { $nextitem = 0; }  # Lastitem = last item to display  #  (nextitem + 4 or end of file)  if ($nextitem + 4 <= @lines) {    $lastitem = $nextitem + 4;  } else {    $lastitem = @lines - 1;  }   # Print first item through last item  for ($i = $nextitem; $i <= $lastitem; $i++) {    ($datetime,$from,$message,$callback) = split /\|\|/,$lines[$i];    print "<option onpick='phonemsg.pl?cmd=display&rec=$i'>";    print $from ." (". $datetime .") \n";    print "</option> \n";  }    # Set nextitem for NEXT function  $nextitem = $lastitem + 1;    # Display NEXT option  print "<option onpick='phonemsg.pl?cmd=list&nextitem=$nextitem'>";  print "Next</option> \n";  print "</select> \n</p> \n";  print "</card> \n</wml> \n";

Most of the work for this fragment happens in the lastquarter of the code. After the stage has been set (WML card defined), thedatabase is read into an array and then the correct set of records isdisplayed, five records at a time. Initially, the first five records aredisplayed and an option is set for the next five to be selected (through the"nextitem" variable). If the user picks the "Next" option,the script is called again, with the name/value pair "nextitem" setto the "next item" to display.

Each record is displayed as an <option> whose "onpick"attribute results in the script being called with the record number to display.(See the next section for the record-display format and the whole scriptsection for details on the format and use of the $cmd variable.)

The file-locking routine for the database differs only inthe message that's displayed if a lock cannot be achieved. The messageincorporates enough WML code to complete the card, ensuring that the user getsthe failure message instead of a WML compile error.

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