Palm OS Programming - Gotchyas

This is the text, extracted from a presentation for Palm OS Programming Gotchyas and Pitfalls. I don't use Adobe and I wanted a textual reference instead of printing out slides, so I did this. View the original information at PalmSource, number 106.


Confusing Form Object ID with Object Index
pBtn = FrmGetObjectPtr(MainOKButton);

  • Most Form Manager routines need object index
  • Compiler can't catch: both are type UInt16
  • Typical error messages:
    • Object not found in form
    • Index out of range
  • Solution: check API parameters carefully and use FrmGetObjectIndex()

Correct ID, Wrong Resource
pLogo = DmGetResource(bitmapRsc, 10004);

  • If DmGetResource() can't find match in the last opened resource database, continues search through resource chain
  • Solution
    • Use Console opened command to display theresource chain
    • Don't use resource IDs <= 128 or >= 10000
    • Use DmGet1Resource()

Apps Must Use appl as Database Type

  • Runs when you debug, but does not show up in Launcher
  • Launcher only displays icons for resource databases of type appl
  • App built with a different type can run, but must be launched via code
  • Set in PalmRez post linker panel

Popup Lists

  • Assign list object ID to trigger
  • Check for popSelectEvent
  • Override default behavior of popSelectEvent if list items are not defined
    • In resources
    • By using LstSetListItems()
  • Return true from event handler

FrmCopyTitle & FrmCopyLabel

  • These calls copy a string to a buffer
  • Buffer size is determined by size of label or title defined in resources
  • Solution 1: Make placeholder label/title "big enough" to hold anticipated strings
  • Solution 2: Use FrmSetTitle() for forms
  • Solution 3: Use text fields instead of labels if you need to change at runtime

Common C/C++ Problems

  • Learn C before learning Palm OS APIs
  • Pointers and data type promotion
  • Example
    • Char *p;
      StrCopy(p, "Ooops");
      P = MemPtrNew(60 * 1024);

Standard C Libraries vs. Palm OS APIs

  • Use String and Memory Managers
    • Smaller app footprint
    • Support for multibyte character sets
  • However, check API parameters carefully
    • StrPrintF is almost the same as sprintf
    • MemSet parameters switched from memset
  • Exception handling
    • Use Error Manager instead of C++ exceptions
    • ErrTry, ErrCatch, etc.

Form Drawing

  • Call FrmDrawForm before drawing on form
    • Problems with modal dialogs and "save behind" bit
    • Debug ROMs will catch
    • Can use WinScreenLock to improve performance
  • Beware functions that draw immediately
    • FldDrawField, LstDrawList, etc.

Form Updating

  • Default frmUpdateEvent calls FrmDrawForm()
    • Erases form
    • Draws form border
    • Draws form objects
    • But not old-style gadgets
  • Custom drawing not updated unless you handle the event yourself
  • Use extended gadgets to automate
  • If you do handle frmUpdateEvent return true to stop system from handling it after you

Text Fields

  • Don't use FldGrabFocus()
    • Use FrmSetFocus()
    • Call FrmSetFocus() after FrmDrawForm()
  • Use PrvSetFieldText() from NetSample sample project
    • Does all the right things
    • Avoids all the standard difficulties
    • Supports immediate/delayed update and appending

Database Names and Creator Codes

  • Creator codes should be unique
    • Benefits your customers if unique
    • Automates clean-up of device when deleting
    • Register code before coding starts
    • www.palmos.com/dev web form
  • Database names must be unique
    • Remember, apps are databases also
    • Use form "AppName-Code"

Palm OS Data Structure Access

  • Don't access UI data structure fields directly
  • Use accessor functions
    • FrmGetFormId(), LstGetSelection(), etc.
    • New accessors in SDK 4.0 Update 1
  • Good way to prepare for Palm OSŪ Version 5
  • Palm OS Emulator reports (non-fatal error)
  • Catch in compiler
    • #define DO_NOT_ALLOW_ACCESS_TO_INTERNALS_OF_STRUCTS
      #include <PalmOS.h>

Database Searching

  • DmFindSortPosition()
    • Design: maintain sort order when adding records
    • Returns index after any matching records
  • When used to find records:
    • If index > 0, compare previous record
    • If record matches, you found search target
    • If record does not match, search target is not in database

Global Variables

  • Not accessible if app is not active
    • Find, receive beam, alarms, etc.
  • Global data not always obvious
    • String constants
    • Smart code model intra-segment jumps
    • Access at end of long function call chain
  • Use #pragma warn_a5_access on
  • Use "PC-Relative strings" options

Error Avoidance

  • Require function prototypes
  • Turn on all warnings and errors
  • Use DO_NOT_ALLOW_ACCESS_TO_INTERNALS_OF_STRUCTS
  • Use assertions
  • Use POSE and debug ROMs

POSE and Debug ROMs

  • Palm OS Emulator will catch numerous errors
    • Memory leaks, data structure access, stacko verflow, low memory access, ...
    • Gremlins and new minimize feature automate the hunt
  • Debug ROMs check for invalid parameters
    • Range checking, forced update events, etc.
  • Always use the latest POSE build
  • Test using ROMs for minimum and latest versions of Palm OS

Suggestions

  • Palm OS® Essentials course
    • Lots of lab time
    • Developer instructors
    • Terrific sample applications with source
  • Palm OS Recipes
    • Step-by-step solutions to common problems
    • Under Documentation link on www.palmos.com/dev
  • Sample code
    • Use CodeWarrior Find feature, grep, etc.
  • Third-party books
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