Optimizing Your App for Multitasking on iPad in iOS 9

In Multitasking and PiP for iOS 9, the apps are suggested to be good multitasking citizen in order to keep the best user experience even in a resource constrainted environment. This session aims to provide useful information for adopting new multitasking features.

Sample Code : iconReel

The Easy stuff

• use instructment to find and fix memory leaks
• fix retain cycles and unbound memory growth
• fix inefficient algorithms like high space and time complexities

Great Performance Involves Tradeoffs

Great Performance is a trade-off between memory, CPU, Disk space, I/O and GPU.

Working Set is a set of objects and resources is required right now

• Keep it small
• may change based on context
• Keep it bounded in terms of memory

CPU Management

• Main thread's top priority is responding to user events
• Don't do unnecessary work
• use QoS for setting higher priority of task
• use Qos overriding to boost when the lower priority task is being waited by a higher priority
• providing a hint for QoS overriding
• dispatch_sync() and dispatch_block_wait()

Memory Warnings

Occured when

• system is under memory pressure
• The app process is approaching its memory limit
• Clear cache data
• release images
• release view controllers

API for indicating memory warning

-[UIApplicationDelegate applicationDidReceiveMemoryWarning:]

-[UiViewController didReceiveMemoryWarning]

UIApplicationDidReceiveMemoryWarningNotification

DISPATCH_SOURCE_TYPE_MEMORYPRESSURE

NSCache

• NSDictionary-like
• Used for objects that can be regenerated on demand
• Trims under memory pressure
• Trims for application lifecycle change
• NOT ON DISK

Advanced Adaptive Memory

• Minimize dirty memory usage
  • use less of it
  • convert to purgeable as if:
    • automatically reclaimed when not in use
    • "nice to have" data can be recomputed
• Maximize purgeable memory usage
• NSPurgeableData
  • beginContentAccess considered as dirty
  • endContentAccess considered as purgeable
  • isContentDiscarded has memory been reclaimed

File data characteristics

• Absolutely essential
• Expensive to generate
• Can be pre-computed
• Static once generated

Maximize clean memory

* Data in a file can be "memory mapped" (not caching the whole file in the memory but a pointer to a location of the file location)
* memory and file content must be match exactly so it is useful for read-only data
* Data can be removed and reload when needed

API for memory mapped

typedef NS_OPTIONS(NSUInteger, NSDataReadingOptions) {
    NSDataReadingMappedIfSafe,
    NSDataReadingMappedAlways,
};
@interface NSData (NSDataCreation)
- (nullable instancetype)initWithContentsOfFile:(NSString *)path
                                         options:(NSDataReadingOptions)readOptionsMask
                                         error:(NSError **)errorPtr;
@end

• not useful for small chunks of data cause
  • fragmentation
  • Exhaustion