ITCamp 2011 - Catalin Zima - Common pitfalls in Windows Phone 7 game development

Common Pitfalls in
Windows Phone 7
Game Development
Catalin Zima-Zegreanu
Evozon Systems
email: catalin.zima@evozon.com
blog: http://catalinzima.com
twitter: CatalinZima
Microsoft XNA MVP

Premium conference on Microsoft’s Dev and ITPro technologies @itcampro / #itcampro

IT Camp 2011
• Thanks for coming!
• ITCamp is made possible by our sponsors:


Session agenda
• Introduction
• Performance
• Certification
• Devices
• Isolated Storage
• Miscellaneous
• Q&A


What? Who? Why?

INTRODUCTION


WP7 game development
• Windows Phone 7
– One of the three major players in the smartphone
world
– Standard chassis specifications
– 10 models from 4 manufacturers (more coming soon)
– New programming model
• XNA Game Studio 4.0
– One of the technologies available for WP7
development
– Recommended for Games
– Cross platform game development using C#


The questions
• What does this session contain?
– Common „gotchas‟
– Tips and advice for avoiding them
– NOT an introduction to XNA and WP7
(but don‟t go! This is still useful!)
• Who is the target audience?
– Game developers using XNA
– Game developers targeting WP7
• Why should you care?
– Learn from our mistakes
– Repairing is harder than preventing


Where we see all sort of things that can slow down an XNA game

PERFORMANCE


Loading Times
• Users like games that load fast
• Also, the watchdog watches you
– If the game does not show something to the
user in 10 seconds, it is closed
• Proper techniques can reduce loading
times significantly
• Example: Chickens Can Dream
– From 14 seconds to ~4 seconds


Techniques: Compression
• Use DXT compression for images
– Requires power-of-two size
– Unlike PNG or JPEG, the texture is actually
compressed in the GPU memory
– 1/8 compression for opaque images
– ¼ compression for images with transparency
• Precompile XML using the Content
Pipeline -> much smaller sizes


Techniques: Spritesheets
• Pack multiple sprites in a single texture
– http://create.msdn.com/en-US/education/catalog/sample/sprite_sheet
– http://spritesheetpacker.codeplex.com/
• Use sourceRectangle parameter of
SpriteBatch.Draw to draw individual sprites

• Benefits:
– Faster Loading (one big file instead of multiple small
ones)
– Avoids DXT power-of-two restrictions
– Bonus: faster drawing (fewer texture switches)


Spritesheet example


Loading Times

DEMO


Techniques: Blame someone else
• Phones without the NoDo update have
significantly slower loading times
– Tell everyone to apply the update

• Some HTC phones have loading times
much slower than all other phones
– Suffer the wrath of HTC owners, or just go
back to the two previous techniques


Garbage Collection
• .Net CF uses a non-generational GC
– At each collection, the whole heap is „swept‟
– Much slower that a generational GC
• If you generate garbage each frame =>
hiccups
• Mango Update– good news
– .Net CF receives generational GC
– This still doesn‟t solve the problem, just hides it
– Garbage is the #1 enemy for your performance


Garbage Collection
• If you think you have garbage issues, profile and
see who‟s allocating
– Use a Windows build of the project and the .Net CLR
Profiler
– Mango adds more profiling tools for the WP7 build

• Common sources
– String manipulations
– Linq extension methods
– Boxing
– foreach – don‟t use it for arrays
• Use Object Pools


Strings
• Strings are immutable
• String manipulation => Garbage creation
– .ToString()
– String.Format()

• Use StringBuilder
– Mutable
– SpriteBatch.DrawString has an overload that
accepts StringBuilder


LINQ Extension methods
• They all create garbage.
– use of delegates
– use of IEnumerable

• Intellisense makes these easy to use by accident.
• Removing “using System.Linq” will prevent accidents


Boxing
• Value types used as reference type
– structs that implement interfaces
– Dictionaries that use Enums as keys
• IEqualityComparer<T>.Default uses object
• See http://bit.ly/nick_enums for solution
– Methods working wit the object class
• Use Generics as often as possible


Using Object Pools
• Use a class to manage a list of instances for
reference types
– When you need new instance, instead of using
new, request an instance from the pool
– When done with an instance, release it back to the
pool
• Objects are pre-allocated
• Management of objects has to be done
manually (back to the dark ages?)
• Heap complexity is increased
– Occasional GCs are slower


Other Performance Tips
• Guide.IsTrial – Slow: cache and reuse
• Manual Inline sensitive code - Nasty, but efficient
• Use public fields instead of Properties
– Please hold the tomatoes back!
• Use „out‟ and „ref‟ methods when dealing with large
value types (Matrix, Vector2, Vector3)
• Use jagged arrays instead of 2D arrays
• GPU Fillrate - use a smaller resolution to investigate
• GPU batching –use spritebatch to batch more Draw
calls together

• Profile, profile, profile!


Where we try to pass certification in the first try

CERTIFICATION


Common certification fails
• Publisher name and version number
• The Back Button
• Background music
• Memory Limits

• Phone capabilities


Publisher name and version
number
• Rule 5.6 - Technical Support Information
– An application must include the application name,
version information, and technical support contact
information that are easily discoverable.
• Before November 1st 2011, this was only
„recommended‟
• One of the most common cert. failures
• Starting with June 3, this will return to
„recommendation‟ status.


The Back Button
• Rule 5.2.4
• Common failures: back button exits the
application
• Why? Default XNA template:
protected override void Update(GameTime gameTime)
{
// Allows the game to exit
if (GamePad.GetState(PlayerIndex.One).Buttons.Back ==
ButtonState.Pressed)
this.Exit();

// TODO: Add your update logic here
base.Update(gameTime);
}


User Music
• To summarize rule 6.5:
– Don‟t mess with the user‟s background music
– Don‟t crash when user is playing music

• Simple solution:
– Use MediaPlayer.GameHasControl


Memory Limits
• Phones have a minimum of 256MB
• Your game has a maximum of 90MB
• The game simply exits without having the
chance to do anything.

• Use DeviceExtendedProperties to query
memory while debugging and keep usage
low
• Don‟t use it too often, as it generates garbage


Phone Capabilities
• Not a Fail reason, but still good to know
• During certification, your manifest file gets regenerated
– May result in detecting capabilities you don‟t actually use
– Ex: Having a Reference to System.Net => Game uses data
connection
• Users may chose not to install the game based on the
shown capabilities
• You just lost a customer 

• Solution: Use the Windows Phone Capability Detection
Tool before submitting your game


Where we see that not all devices are created equal

DEVICES


Device Discrepancies
• Samsung Phones with AMOLED screens
– Color banding in gradients is more visible
• HTC Devices
– Experience with HD7 and 7 Pro, all reported
– SoundEffectInstance.Volume doesn‟t work
– If application is interrupted without
tombstoning (eg. phone call), the
Accelerometer class doesn‟t work anymore,
and a new instance needs to be created.


Remember the QWERTY
• Common scenario
– In your Windows build, you bind something to
keyboard keys (like invincibility, etc)
– You only have non-QWERTY devices to test
– You ship with the „forgotten‟ code in
– Some users discover the „cheat‟
• We shipped with such code in and it made us
doubt the users‟ highscores 
• Tip: Use compile #if directives for your „cheats‟


Where we see what bad things can happen in isolation

ISOLATED STORAGE


Program Flow
• Game is launched or reactivated
• Load content to show on the screen
– To make sure the watchdog doesn‟t get us
• Load saved data from IsolatedStorage

• … game is played …

• On Deactivated is called
• Save user data to IsolatedStorage


Program Flow
• Game is launched or reactivated
• Load content to show on the screen
• User hits the Back button before loading is
done
• Load saved data from IsolatedStorage

• … game is played … 

• On Deactivated is called
• Save user data (null) to IsolatedStorage


Accidentally deleting data
• Data is overwritten => User settings and data are
now gone
• Progress is erased
• User is angry -> bad review

• What to do ?
– Check for null before overwriting
– Save data directly to disk whenever it changes, not on
Deactivated event
– Keep a backup of the save file
– Use different files for different data


Game Updates - Scenario
• Use a class, let‟s call it… UserData to store
all the user settings and progress
• On save, serialize class to disk using XML
• On load, deserialize from XML
• On first run, if no data exists, create default
UserTemplate instance and save it to disk
• All works ok, QA approves, data is saved!


Game Updates - Scenario
• Add new functionality to the game
• New data added to the UserData class
• Added code to the code that properly
creates the default instance
• QA tests the new game version, all is well
• Everyone happy, game update is released
• The Nightmare begins!!!


What went wrong?
• Updating user has an older version of the
save file in his IsolatedStorage
• Deserialization failes because the class is
changed
• QA can‟t test this scenario
• Possible changes
– Add new value-type fields to UserData
– Add new reference type field to UserData
– Change the type of a member of UserData
– Remove any member of the UserData


True story, bro!
• Our first update to Chickens Can Dream faced this
problem
• We simply added a List<Boolean>
• 15.000 users who updated the game now had the
game crashing on startup
• Fixed the bug, updated, problem solved!

• However:
– Average ratings dropped by almost a full star
– We needed a full month to get back to the ratings before
the update
– Lots of reviews in the Marketplace saying that the game
doesn‟t work after the update


How to avoid?
• Be careful
– Pray someone doesn‟t miss something
– Check for nulls all over the place
– Use post-its to remind yourself!

• Use versions for your save files
– Tied to your game‟s version
– Code that reads an older version and converts
to a new version


Where we talk about stuff that didn‟t fit anywhere else

MISCELLANEOUS


User Input
• Unsupported Characters
– You ask for a name from the player
– Player uses non-ASCII characters
– When trying to draw the name => CRASH
• Global Highscores? That‟s just bad luck!
– Crisis averted in CCD by sanitizing text server-side
• Why it happens?
– Fonts are used as images in XNA
– The default SpriteFont only supports ASCII
• Fix: uncomment the DefaultCharacter tag


<DefaultCharacter>*</DefaultCharacter>

• (or use Mango and Silverlight/XNA integration)

Tombstoning and resources
• There is a tendency to dispose and unload
resources when OnDeactivated is called
• DON‟T!
• Currently, for very short periods of time the
game might not be evicted from memory =>
ObjectDisposedException
• After MANGO, it may never be evicted
• TODO: just save your state, don‟t try to clear or
dispose anything


Challenge Accepted!

Q&A


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