iPhone Tech Talk at HU-Berlin

I visited the iPhone Tech Talk at HU Berlin yesterday. It was quite interesting, altough I would have liked the speakers to go into more detail about certain subjects (i.e. layer manipulation). For a (somewhat) experienced iPhone developer like me, the event rather much of a promotional, commercial character and was less serious about conveying in-depth technical information.

Anyhow, for those who are interested, here are my rough notes covering some important things that were said:

== Session: Iphone Programming ==

- Don't implement a quit button (pretty obvious).
- Sandbox (limited directory access):
/Documents
/tmp
/Library/Caches
- External Data:
- contacts
- system settings
- voicemail
- photos
- other app data
- Exceptions in Objective-C++
- unified exception model (@try...@catch) works both with c++ and objective-c
- Objective-C memory management:
- retain / release pattern (reference counting)
- first initialisation sets reference counter to 1 (no extra retain needed)
- -autorelease message: sends -release at a later moment (using autoreleasepool)

myString = [[NSString alloc] initWith...] -> retain count 1
myString = [NSString stringWith...] -> utility method, retain count 0 (gets autoreleased at end of method)
- Objective-C setters / getters in Objective-C 2.0
- "properties" control storage semantics
- e.g. @property(readwrite) float speed;
- "readwrite" only one of many keywords, e.g. "atomic, nonatomic, readonly..."
- in implementation file: @synthesize p1,p2,... ; generates getter/setter code implicitly
- Objective-C categories: expand functionality at runtime (similar to protocols)
- @implementation NSString (myExtensions) ...
- Foundation Bundles -> special folders containing the data associated with your app
- special apis provide access to the bundle (NSBundle class)
- bundles are read-only on iPhone (because of code-signing)
- Application Design Patterns
- MVC
- Model: Foundation
- View: UIKit
- Controller: UIKit View Controllers (provide basic functionality)
- Target-Action: (controls can be re-used for different actions)
Control -> Target -> Object
-> Action -> Selector
- Notifications: Notification -> Notification Center --> Observers
- i.e. finishedPlaying Notification when movie terminated
- Delegation: Application Life Cycle:
- UIApplication instantiation, Main NIB loaded, Main Window created
- Application Delegate: this is the user's code that gets called after UIapplication has started (your app)
- benefit: no subclassing necessary, as delegate extends functionality
- app delegate hooked up in interface builder
- other UI classes relying on delegates: pickers, table views, etc...


== Session: UIKit App Development ==
- View controllers:
- manage currenct screen content
- handle user interaction, key SDK concept
- two major types:
- yours -> present per-screen content
- from apple -> manage multiple screens (i.e. switched by tab-bar controller)
- rarely subclassed (customization throught delegation)
- Key Methods: loadView, viewDidLoad, viewWillAppear, viewDidAppear
- write in Xcode: view controllers
- layout in interf. builder: views

- Apple view controllers:
- navigation controller: provides navigation bar on top of window
- stack-based naviagation: manages stack of view controllers, easy to customize...

- Tab bar controllers:
- "flat" navigation, no hierarchy
- provides "tab bar" on the bottom of the device
- different views on the same data or show different parts of the app (i.e. clock app)
- only five icons shown at a time, but more can be added on a separate screen (accessed via the "more" button)
- tab icons: developer provided only monochrome image with apha -> appropriate colors are provided by framework

- Other special views:
- tool bars:
- icons don't retain a title
- icons don't remain highlighted
- used for one-shot actions, often to present follow-up modal or alert views (by default, they slide up from the bottom, to indicate that they are temporary, vs. "navigation" affordance by left-right movement)
- table views:
- tableview delegate + data source
- table view cells are re-usable: already instantiated cells get re-used once the scroll by. They shouldn't be deleted and re-instantiated.
- Autorotation + Autoresizing maks can be specified with interface builder
- Size of the screen can change when incoming call arrives, or when keyboard appears -> well-behaved apps handle these conditions
- memory warning: -didReceiveMemoryWarning
- release non-critical objects ASAP
- initialize application lazily

- Animations:

- implement UIView change properties by wrapping with UIAnimation! --> very nice feature. setup animation ... commit animation.
- animation finished callback called when animation is done

- Text Display:
- UILabel
- UIWebView: can display HTML, PDF, Word files
- NSStringDrawing

Touch Screen:

- UITouch: information per touch

- UIEvent: collects all touch events
- allTouches, touchesForWindow, etc..
- myview.multipleTouchEnabled = YES;
- "Touches" sample code on developer connection

- Accelerometer:
- x axis is inverted (runs from right to left)
- interface orientation
- device orientation
- analog user input

- raw accelerometer data:
UIAccelerometer accel [uiaccelerometer sharedAccelerometer];
accel.updateInterval = 1 / f; (f = 1 - 100hz)
accel.delegate = self; -> delivery begins immediately

- filtering data:
- low-pass
- isolates current acceleration, discrete inputs --> bubbleLevel
- high-pass filter: isolates instantaneous movement
- used to find shakes and oscillations
-> accelerometerGraph

Session: Optimizing & Profiling

- Tools: Instruments (memory footprint, leak detection, cpu usage), Shark (profiling of method calls -> doesn't work that well yet)
- static memory: -mthumb compiler flag reduces to 16-bit ARM instruction subset (35% savings in size)
- PNGs are optimized
- plists are converte to compact binary format by Xcode -> use plists for structured data files
- Dynamic memory:
- autoreleasing is is expensive: try to do it manually (avoid conveniece Methods)
- use local autorelease pools to prevent object accumulation
- drain local autorelease pools periodically
- declare properties as nonatomic -> access is 10 times faster, but not thread safe
- avoid leaks, use Instruments
- Drawing Envirnment:
- use UIKit view animations when possible (very optimized)
- mark views as opaque to improve performa