Structure of a Palm Application, Part 3

Structure of a Palm Application, Part 3
Pages: 1, 2

Examples

There are other communications between the Palm OS and your application that you might want to handle in your code. Particular events, or overall access might be important for you to control. In any case, here are events some applications want to handle:

A particular pen-down event

The input of a graffiti character

How to take over the hard buttons for your own use within your application

How to take over the entire device and not allow any other applications to run

Let's take a look at the code required to handle each of these instances.

Handling a Pen-Down Event

Normally, you will not handle pen-down events directly, but instead handle higher-level events like ctlSelectEvent. However, occasionally, applications will want to be on the lookout for a penDown event. An example is the Address Book: tapping and releasing on the display of an address in the display view switches to the edit view.

The source code to the Address Book is part of the Palm OS 3.5 SDK. The RecordViewHandleEvent in Address.c from that example contains the following case:

case penDownEvent:
         handled = RecordViewHandlePen(event);
         break;

RecordViewHandlePen handles penDown events in the display-only view of the record (see Example 5-15).

Example 5-15: RecordViewHandlePen in the Address Book source


static Boolean RecordViewHandlePen (EventType * event) {
   Boolean      handled = false;
   FormPtr      frm;
   RectangleType r;
   Int16        x, y;
   Boolean      penDown;

   // If the user taps in the RecordViewDisplay take her to
the Edit View.
   frm = FrmGetActiveForm(  );
   FrmGetObjectBounds(frm, FrmGetObjectIndex(frm,
RecordViewDisplay), &r);
   if (RctPtInRectangle (event->screenX,
event->screenY, &r))
      {
      do 
         {
         PenGetPoint (&x, &y, &penDown);
         } while (penDown);
      
      if (RctPtInRectangle (x, y, &r))
         FrmGotoForm (EditView);
         
      handled = true;
      }
      
   return handled;
}

The calls to the Form Manager routines FrmGetActiveForm and FrmGetObjectBounds yield the rectangle containing the address display area. RctPtInRectangle checks to see whether the pen-tap location (event->screenX and event->screenY) are within those bounds. Note that it then enters a loop calling PenGetPoint until the user releases the stylus. If the user lets go within those same bounds, the code switches to the edit view.

Handling a Graffiti Character

To show you an example of handling a graffiti character, we turned to the Reptoids game (one of the samples in the Palm OS 3.5 SDK). While the game is running, entering a "t" character displays the amount of time spent playing on-screen.

The Rocks.c file has the following check in MainViewHandleEvent:

else if (event->eType == keyDownEvent)
    {
    // Time spent playing.       (Quick code at this
point.)
    if (event->data.keyDown.chr == 't')
        {
        // ... Code that displays game-time is here.    

        }
 
    return true;
    }

Note that the routine checks the incoming character by looking within the event: event->data.keyDown.chr.

Overriding Hard-Button Behavior

Some applications want to redirect the hard button presses (the Date Book key, for instance) for their own use. An obvious example would be games. Pressing a hard button generates a keyDownEvent and as such can be looked for by an application. Indeed, the keyDownEvent is sent for all of these:

The four application buttons

The Power button

The scroll buttons

The Find button

The Calculator button

The antenna being raised on a Palm VII

In all the instances, a special modifier bit (the commandKeyMask) is set in the modifiers associated with that key. This distinguishes it from a normal Graffiti character.

However, for all but the scroll buttons, the system handles the keyDownEvent, and doesn't allow the form's event handler to deal with it.

Now, let's see how we can use this information to modify our source code to our advantage. We will make an application where tapping on the Date Book key doesn't bring up the Date Book, but instead does something app-specific.

First off, we need to avoid calling SysHandleEvent when that key is pressed (see Example 5-16).

Example 5-16: AppEventLoop that doesn't call SysHandleEvent for taps on the Date Book key

static void AppEventLoop(void)
{
  Err error;
  EventType event;

  do {
    Boolean isDatebookKey;
    
    EvtGetEvent(&event, evtWaitForever);
    
    isDatebookKey = (event.eType == keyDownEvent)
      &&
(TxtCharIsHardKey(event.data.keyDown.modifiers, 
        event.data.keyDown.chr))
      && (event.data.keyDown.chr == vchrHard1);

    if (isDatebookKey || ! SysHandleEvent(&event))
      if (! MenuHandleEvent(0, &event, &error))
        if (! AppHandleEvent(&event))
          FrmDispatchEvent(&event);

  } while (event.eType != appStopEvent);
}

Note that we figure out whether it is a hard key, and then whether it is the Date Book key (vchrHard1). If it is, we don't call SysHandleEvent, and so the normal processing for that character won't happen. However, the event will be passed to FrmDispatchEvent (after being ignored by MenuHandleEvent and AppHandleEvent), and from there to our event handler. Here's code in MainFormHandleEvent that switches to the second form if the Date Book key is pressed:

switch (event->eType) 
  {
  case keyDownEvent:
    if (TxtCharIsHardKey(event->data.keyDown.modifiers,

        event->data.keyDown.chr)
      && (event->data.keyDown.chr == vchrHard1))
{
      FrmGotoForm(SecondForm);
      handled = true;
    }
    break;

Controlling the Exiting of an Application

Some turnkey applications take over the machine and don't allow any other applications to run. For example, a Palm OS device carried by delivery people might only run a delivery application; it's locked for any other purpose.

TIP: Admittedly, this is a very rare UI. A standard Palm application should always quit when the user requests another application (by pressing one of the hard buttons, for instance). Standard applications should not have an explicit UI for quit, however. Users implicitly quit an application by starting another.

How do you write an application that takes control of the Palm OS unit and won't let go? It's simple. In order to keep other applications from running, you rudely refuse to exit from your main event loop. As a result, the application never quits (except on a reset). All the OS can do is post an appStopEvent to the event queue, requesting (nay, strongly urging!) your application to quit. It's up to the application itself to actually quit, though.

In the simplest case, you might just code your event loop so that it never exits (see Example 5-17).

Example 5-17: AppEventLoop that never exits

static void AppEventLoop(void)
{
  Err error;
  EventType event;

  do {
    EvtGetEvent(&event, evtWaitForever);
    
    if (! SysHandleEvent(&event))
      if (! MenuHandleEvent(0, &event,&error))
        if (! AppHandleEvent(&event))
          FrmDispatchEvent(&event);

  } while (true);  // Don't ever quit.
}

This will ensure that your application, once running, will never exit. It'll silently ignore an appStopEvent that is posted to the event queue.

Another scenario is an application that won't quit unless given a specific command. Some turnkey applications provide a mechanism (perhaps a password-protected button or menu item) to quit the application and open up the unit to the rest of the Palm UI. The easiest way to implement this is to have a global variable specifying whether appStopEvents should be ignored. When the button is pressed, the application can post an appStopEvent, and set the variable. Here's how the event loop would change:

static  void AppEventLoop(void)
{
  Err error;
  EventType event;

  do {
    EvtGetEvent(&event, evtWaitForever);
    
    if (! SysHandleEvent(&event))
      if (! MenuHandleEvent(0, &event, &error))
        if (! AppHandleEvent(&event))
          FrmDispatchEvent(&event);

  } while (!(event.eType == appStopEvent && AppShouldStop(  )));
}

AppShouldStop just returns the value of the global:

Boolean gShouldStop = false;
 
Boolean AppShouldStop(  )
{
  return gShouldStop;
}

Now, when the specific action occurs (like pressing a button or choosing a menu item), a routine MakeAppStop would be called to set the global, and post an appStopEvent to the event queue (see Example 5-18).

Example 5-18: MakeAppStop that causes the application to quit

void MakeAppStop(  )
{
  UInt32 romVersion;
 
  gShouldStop = true;
  FtrGet(sysFtrCreator, sysFtrNumROMVersion,
&romVersion);
  if (romVersion <
sysMakeROMVersion(2,0,0,sysROMStageRelease,0)) {
    AppLaunchWithCommand(sysFileCDefaultApp, 
      sysAppLaunchCmdNormalLaunch, NULL);
  } else
    PostAppStopEvent(  ); // Launch previous app.
}

WARNING: Palm OS 1.0 relaunches the last application, so the code must explicitly launch an application in this case. (The default application is the one that is shown after a reset.) Any post-1.0 OS launches the previous application, so the code just posts an appStopEvent.

Here's the code that posts the appStopEvent. Note that whenever you create an event yourself, you should zero out the entire structure so that unused fields are zero:

static void PostAppStopEvent(  )
{
  EventType event;
  
  // Set all unused fields to 0.
  MemSet(&event, sizeof(event), 0);
	
  event.eType = appStopEvent;
  EvtAddEventToQueue(&event);
}

What to Remember

In this chapter, we have given you a description of important terminology, standards, and a description of how an application interacts with the Palm OS on a device. Most importantly, you should remember the following:

A Palm application is an event-driven system. The system's event queue feeds a constant flow of events to your application and it is up to you to handle them.

Events are fed to four different routines (SysHandleEvent, MenuHandleEvent, AppHandleEvent, FrmDispatchEvent) in your event loop. Each routine has responsibilities for various events.

A standard Palm application has a main routine, PilotMain, from which everything is called. We showed you a sample program that contains a typical PilotMain routine that you will use in your own applications.

There are times, other than when opened by the user, when your application may be called by the system. Launch codes and the various launch flags tell your application how it was called by the system and what is expected.

From all of this information, you should now be well on your way to understanding this application architecture. In the following chapters, you will be using this information to move beyond our simple OReilly Starter application to create a full-featured application.

Neil Rhodes and Julie McKeehan are experienced authors who, through their company, Calliope Enterprises, work closely with Palm Computing to develop new training materials, materials that are based on this book.