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I've tried to use timers for animation with limited, but not complete, success. Small sample code was easy, but 16 Puzzle v3.0 is now 800+ lines of code. Somehow moving from simple Timer cases to more complex ones wasn't easy at all, something went wrong all the time!

Here I have documented all found errors you can make with Timer, how to identify each and fix them. There's a few correct ways, too, I hope.

My experiments continue, but it's beginning to look better. In general I very strongly propose not to both use asynchronous and synchronous timers in same application. More exactly I very strongly propose using only async timers (the ones with callback function). That might require a bit more complex application logic, but life with timers is much easier. Not to mention you have a possibility to save battery.

Usage: press keyboard numbers 1, 2, 3, 4, 5 and 6.

#
# Timer Sample
#
# Usage: This is for developers, so read the source code.
#        This is NOT a tutorial, but an error reference.
#
# I went through most possible errors anyone can do with Timers.
# After all the trouble I want to demonstrate how timers can be
# used and how to identify some of the mistakes. Hope it helps!
#
# Copyright (c) 2008-2009 Jouni Miettunen
# http://jouni.miettunen.googlepages.com/
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
 
'''
1.20 2009-05-05 Added time problems with class (timer_problem6)
1.10 2008_09-03 Added RunTimeError: negative number not allowed
1.00 2008-06-12 Initial release
'''
 
import e32
import appuifw
import key_codes
import random
 
g_count = 0
g_limit = 3
 
# You can use just one timer for whole program
# Just ALWAYS remember to cancel it before use
my_timer = e32.Ao_timer()
 
# Not a timer, but close (Active Object) relative
app_lock = e32.Ao_lock()
 
def get_color():
    ''' Generate random RGB color, used to proof that something happens '''
    return (random.choice(range(0,255)), random.choice(range(0,255)), random.choice(range(0,255)))
 
def timer_color1():
    ''' Timer callback without parameters '''
    global g_count
    if g_count < g_limit:
        g_count += 1
    else:
        g_count = 0
        return
 
    # Change screen color, show that something happens
    canvas.clear(get_color())
    # You might call this routine before timer has expired
    # Solution: always cancel timer beforehands
    my_timer.cancel()
    # Note: Timer interval is in SECONDS
    my_timer.after(1, timer_color1)
    # Watch closely and see pattern: color change + dialog
    # Program execution continued immediately after timer set
    # Timer trickers callback function after given interval
    appuifw.note(unicode('Count: ' + str(g_count)))
    # Screen goes white, because we're missing redraw callback
    # Dialog removal requests screen update --> blank by default
    # Not a defect as itself, redraw is missing on purpose
 
    # Error:
    #   Python: System error (-50)
    #   TypeError: bad argument type for built-in operation
    # Solution:
    #   Add time interval (SECONDS)
    # This is invalid sample code, do not use:
    #   my_timer.after(timer_color1)
 
    # Error:
    #   RuntimeError: negative number not allowed
    # Reason:
    #   Your time interval was a negative number
    # Solution:
    #   Add error checking to verify time interval is non-negative
    # This is invalid sample code, do not use:
    #   my_interval = -1
    #   my_timer.after(my_interval, timer_color1)
 
def timer_color2(a_color=(0, 0, 0)):
    ''' Timer callback with parameters '''
    global g_count
    if g_count < g_limit:
        g_count += 1
    else:
        g_count = 0
        return
 
    # If no color was given (EKeyRightArrow) then it's default (now BLACK)
    # Otherwise given color is used, see end of this function
    canvas.clear(a_color)
    # You might call this before timer has expired
    # Solution: always cancel timer beforehands
    my_timer.cancel()
    # If you want to give parameters to callback
    # you MUST use lambda == standard python expression
    # Note: Timer interval is in SECONDS
    my_timer.after(0.5, lambda:timer_color2(get_color()))
 
    # Error:
    #   Python: System error (-50)
    #   TypeError: callable expected for 2nd argument
    # Solution:
    #   Use lambda when callback has parameters
    # This is invalid sample code, do not use:
    #   my_timer.after(0.5, timer_color2(get_color()))
 
    # Error:
    #   Python: System error (-50)
    #   RuntimeError: maximum recursion depth exceeded
    # Reason:
    #   Your timer callback called itself in loop too many times
    #   Most likely because of invalid callback setup
    # Solution:
    #   Use something to limit and break recursion
    #   Check your code
    # This is invalid sample code, do not use:
    #   my_timer.after(0.01, timer_color2(get_color()))
 
def timer_crash3():
    ''' Sample how to crash without timer.cancel '''
    canvas.clear(get_color())
    # Error:
    #   Python: System error (-50)
    #   RunTimerError: Timer pending - cancel first
    # Reason:
    #   Crash if you call this before timer has expired
    #   Missing cancel on active timer
    # Solution:
    #   add my_timer.cancel() just before my_timer.after()
    # Note:
    #   Everything is ok, IF you call this AFTER timer has expired
    #   Always cancel timer beforehands, just in case
    my_timer.after(2, timer_crash3)
 
def timer_color4():
    ''' Blocking timer sample '''
    # Change screen color, show that something happens
    canvas.clear(get_color())
    # You might call this before timer has expired
    # Solution: always cancel timer beforehands
    my_timer.cancel()
    # Note: Timer interval is in SECONDS
    #my_timer.after(2, timer_color1)
    my_timer.after(2)
    # Watch closely and see pattern: color change, wait, dialog
    # Program execution was blocked and continued only after timer expired
    appuifw.note(u'After timer expired')
    # Screen goes white, because we're missing redraw callback
    # Dialog removal requests screen update --> blank by default
    # Not a defect as itself, redraw is missing on purpose
 
def timer_crash5():
    ''' Sample how to crash without timer.cancel '''
    canvas.clear(get_color())
    my_timer.cancel()
    # Error:
    #   Application closed:
    #   Python
    #   E32USER-CBase 91
    # Reason:
    #   Timer is active and blocking function execution
    #   User started same function and tried to cancel active timer
    #   In this case function was started via keyboard event
    # SDK docs say:
    #   "This panic is raised by CActiveSchedulerWait::Start() when
    #   the CActiveSchedulerWait object has already been started."
    # Solution:
    #   Avoid this situation
    # Note:
    #   Everything is ok, IF you call this AFTER timer has expired
    my_timer.after(2)
 
def timer_problem6():
    ''' Timer callback inside class '''
    my_timer.cancel()
    tc = TimerClass()
 
    # WARNING: this is wrong, callback is never called
    appuifw.note(u'TimerClass 1 Before')
    my_timer.after(0.1, lambda:tc.timer_call1)
    appuifw.note(u'TimerClass 1 After')
    # Error:
    #   Nothing happens
    # Reason:
    #   Missing parenthesis in callback with lambda
    # Solution:
    #   add parenthesis
    # Note:
    #   No error note, search trace output for something like
    #   <bound method TimerClass.timer_call1 of <__main__.TimerClass object at 0x31f08534>>
 
    # Wait a moment to make these different cases visible
    my_timer.after(1)
 
    # This is right, no lambda and no parenthesis
    appuifw.note(u'TimerClass 2 Before')
    my_timer.after(0.1, tc.timer_call2)
    appuifw.note(u'TimerClass 2 After')
 
    # Wait a moment to make these different cases visible
    my_timer.after(1)
 
    # This is right, lambda and parenthesis
    appuifw.note(u'TimerClass 3 Before')
    my_timer.after(0.1, lambda:tc.timer_call3())
    appuifw.note(u'TimerClass 3 After')
 
    # Wait a moment to make these different cases visible
    # my_timer.after(1)
 
    # WARNING: this is wrong, parenthesis without lambda
    # Note: commented away, since this stops/crashes function execution
    # appuifw.note(u'TimerClass 4 Before')
    # my_timer.after(0.1, tc.timer_call4())
    # appuifw.note(u'TimerClass 4 After')
 
    # Error:
    #   Python: System error (-50)
    #   TypeError: callable expected for 2nd argument
    # Reason:
    #   Missing lambda on callback with parenthesis
    # Solution:
    #   add lambda
    # Note:
    #   Traceback gives filename and source code line number for error
 
def cb_quit():
    # There is possible crash if exit while timer is active
    # Cancel it always before exit, just in case
    my_timer.cancel()
    app_lock.signal()
 
class TimerClass(object):
    ''' Minimal sample class '''
    def show(self, a_value):
        s = "TimerClass %d Inside" % a_value
        appuifw.note(unicode(s))
    def timer_call1(self):
        self.show(1)
    def timer_call2(self):
        self.show(2)
    def timer_call3(self):
        self.show(3)
    def timer_call4(self):
        self.show(4)
 
appuifw.app.orientation = 'portrait'
appuifw.app.screen = 'full'
appuifw.app.exit_key_handler = cb_quit
appuifw.app.menu = [(u"Exit", cb_quit)]
appuifw.app.body = canvas = appuifw.Canvas()
 
canvas.bind(key_codes.EKey1, timer_color1)
canvas.bind(key_codes.EKey2, lambda:timer_color2())
 
# Error:
#   Application closed:
#   Python
#   KERN-EXEC 3
# Reason:
#   Missing lambda on callback with parenthesis
# Solution:
#   add lambda
# This is invalid sample code, do not use:
#   canvas.bind(key_codes.EKey2, timer_color2())
 
# Error:
#   Nothing happens
# Reason:
#   Missing parenthesis in callback with lambda
# Solution:
#   add parenthesis
# This is invalid sample code, do not use:
#   canvas.bind(key_codes.EKey2, lambda:timer_color2)
 
canvas.bind(key_codes.EKey3, timer_crash3)
canvas.bind(key_codes.EKey4, timer_color4)
canvas.bind(key_codes.EKey5, timer_crash5)
canvas.bind(key_codes.EKey6, timer_problem6)
 
app_lock.wait()

Here are some screenshots