ITP 342 Mobile App Dev

Transcription

1 ITP 342 Mobile App Dev Grand Central Dispatch Background Processing

2 Grand Central Dispatch (GCD) New API for splitting up the work your app needs to do into smaller chunks that can be spread across multiple thread and, with the right hardware, multiple CPUs Much of the new API is accessed using blocks In-line function capability to C & Objective-C 2

3 Threading Basics Most modern operating systems support the notion of threads of execution Each process can contain multiple threads which all run concurrently If there's just one processor core, the operating system will switch between all executing threads If more than one core is available, the threads will be distributed among them All threads in a process share the same executable program code and the same global data Each thread can also have some data that is exclusive to the thread 3

4 Threading Basics Mutex (mutual exclusion) or lock Special structure that threads can make use of Ensures that a particular chunk of code can't be run by multiple threads at once Use in a critical section of your code This makes them thread-safe 4

5 Thread-Safe In Cocoa Touch, the Foundation framework (NSString, NSArray, etc.) is thread-safe The UIKit framework is for the most part not threadsafe Main thread is where all the action of your ios app occurs All method calls that deal with any UIKit object should be executed in the main thread documentation/cocoa/conceptual/ Multithreading/ThreadSafetySummary/ ThreadSafetySummary.html 5

6 Units of Work It is possible to implement some concurrency without too much low-level thread-twisting Split up long-running tasks into units of work and put those units into queues for execution The system manages the queues for us, executing units of work on multiple threads 6

7 GCD: Low-Level Queueing GCD made its debut in Mac OS X 10.6 Available since ios 4.0 Works with Objective-C, C, and C++ Key concepts is the queue The system provides a number of predefined queues including one that does work on the main thread You can also create your own queues FIFO queues = first-in, first-out 7

8 Queueing Units of work added to a GCD queue will always be started in the order they were placed in the queue They may not always finish in the same order due to distributing its work among multiple threads Each queue has access to a pool of threads that are reused throughout the lifetime of the app GCD will try to maintain a pool of threads dependent on the machine's architecture Dual-core processors starting with ipad2 and iphone 4S Earlier devices are single-core 8

9 Blocks Blocks: known as closures or lambdas in other languages Let a particular chunk of code be treated like any other C-language type A block can be assigned to a variable, passed as an argument to a function or method, and executed A block can take one or more parameters and specify a return value To declare a block variable, use the caret (^) symbol along with some additional parenthesized bits to declare parameters and return types To define the block itself, you do roughly the same, but follow it up with the actual code defining the block wrapped in curly braces 9

10 Blocks - Example // Declare a block variable "loggerblock" with no parameters // and no return value. void (^loggerblock)(void); // Assign a block to the variable declared above. // A block without parameters and with no return value needs no // "decorations" like the use of void in the variable declaration. loggerblock = ^{ NSLog(@"I'm just glad they didn't call it a lambda"); }; // Execute the block, just like calling a function. loggerblock(); // this produces some output in the console 10

11 Defining Blocks Similar to the concept of a function pointer in C, but a few critical differences Blocks can be defined in-line in your code Right at the point where it's going to be passed to another method or function A block can access variables available in the scope where it's created By default, the block makes a copy of any variable you access this way You can make an outside variable "read/write" by prepending the storage qualifier block before its declaration (that's 2 underscores) 11

12 Blocks - Example // define a variable that cannot be changed by a block int a = 0; // define a block that tries to modify a variable in its scope void (^sillyblock)(void) = ^{ a = 47; }; // check the value of our variable before calling the block NSLog(@"a == %d", a); // outputs "a == 0" // execute the block sillyblock(); // check the values of our variable again, after calling the block NSLog(@"a == %d", a); // outputs "a == 0" 12

13 Blocks - Example // define a variable that can be changed by a block block int a = 0; // define a block that tries to modify a variable in its scope void (^sillyblock)(void) = ^{ a = 47; }; // check the value of our variable before calling the block NSLog(@"a == %d", a); // outputs "a == 0" // execute the block sillyblock(); // check the values of our variable again, after calling the block NSLog(@"a == %d", a); // outputs "a == 47" 13

14 dispatch_async Make code run entirely in the background by wrapping all the code in a block and passing it to a GCD function called dispatch_async First parameter is a GCD queue Using dispatch_get_global_queue() function DISPATCH_QUEUE_PRIORITY_HIGH DISPATCH_QUEUE_PRIORITY_LOW Second parameter is a block to assign to the queue dispatch_async (dispatch_get_global_queue (DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ // code }); 14

15 The Main Thread One problem is UIKit thread-safety Can't message any GUI object from a background thread Solution is to pass the work to the main thread Call dispatch_async() with the first parameter of dispatch_get_main_queue() dispatch_async (dispatch_get_main_queue(), ^{ // code }); 15

16 Concurrent Blocks Let's do things concurrently Use a dispatch group All blocks that are dispatched asynchronously within the context of a group, via the dispatch_group_async() function, are set loose to execute as fast as they can Including being distributed to multiple threads for concurrent execution, if possible Use dispatch_group_notify() to specify an additional block that will be executed when all the blocks in the group have been run to completion 16

17 Worker Create a single-view app To the storyboard, add Button (round rect button) Text View Activity Indicator View In the Attributes Inspector, make sure Hides When Stopped is checked Create IBOutlets Button à startbutton TextView à resultstextview Create IBAction Button à dowork 17

18 Worker Add methods in implementation - (NSString *) fetchsomethingfromserver { [NSThread sleepfortimeinterval:1]; from Server"; } - (NSString *) processdata: (NSString *)data { [NSThread sleepfortimeinterval:2]; return [data uppercasestring]; } - (NSString *) calculatefirstresult: (NSString *)data { [NSThread sleepfortimeinterval:3]; return [NSString stringwithformat:@"number of chars: %d", [data length]]; } - (NSString *) calculatesecondresult: (NSString *)data { [NSThread sleepfortimeinterval:4]; return [data stringbyreplacingoccurrencesofstring:@"e" withstring:@"e"]; } 18

19 Slow Worker Implement dowork: method - (IBAction) dowork: (id)sender { NSDate *starttime = [NSDate date]; NSString *fetcheddata = [self fetchsomethingfromserver]; NSString *processeddata = [self processdata:fetcheddata]; NSString *firstresult = [self calculatefirstresult:processeddata]; NSString *secondresult = [self calculatesecondresult:processeddata]; NSString *resultssummary = [NSString [%@]\nsecond: [%@]", firstresult, secondresult]; self.resultstextview.text = resultssummary; NSDate *endtime = [NSDate date]; NSLog(@"Completed in %f seconds", [endtime timeintervalsincedate:starttime]); } 19

20 Faster Worker - (IBAction)doWork:(id)sender { NSDate *starttime = [NSDate date]; // Disable the startbutton self.startbutton.enabled = NO; self.startbutton.alpha = 0.5f; // Start animating the spinner [self.spinner startanimating]; // Grand Central Dispatch dispatch_queue_t queue = dispatch_get_global_queue(dispatch_queue_priority_default, 0); dispatch_async(queue, ^{ NSString *fetcheddata = [self fetchsomethingfromserver]; NSString *processeddata = [self processdata:fetcheddata]; 20

21 Faster Worker } }); block NSString *firstresult; block NSString *secondresult; dispatch_group_t group = dispatch_group_create(); dispatch_group_async(group, queue, ^{ firstresult = [self calculatefirstresult:processeddata]; }); dispatch_group_async(group, queue, ^{ secondresult = [self calculatesecondresult:processeddata]; }); dispatch_group_notify(group, queue, ^{ NSString *resultssummary = [NSString [%@]\nsecond: [%@]", firstresult, secondresult]; // Have to be on the main thread for UIKit objects dispatch_async(dispatch_get_main_queue(), ^{ self.resultstextview.text = resultssummary; self.startbutton.enabled = YES; self.startbutton.alpha = 1; [self.spinner stopanimating]; }); NSDate *endtime = [NSDate date]; NSLog(@"Completed in %f seconds", [endtime timeintervalsincedate:starttime]); }); 21

22 Background Processing Allows your apps to run in the background Allows apps that require specific kinds of system functionality to continue to run in a constrained manner Since ios 4, iphone 3GS & on, all ipads For example, an app that plays an audio stream from an Internet radio station even if the user switches to another app 22

23 Not Running Application Life Cycle This is the state that all apps are in on a freshly rebooted device An app that has launched will return to this state only: In Info.plist, UIApplicatonExitsOnSuspend is set to YES It was previously Suspended and the system needs to clear out some memory It crashes while running 23

24 Application Life Cycle Active The normal running state of an app when it's displayed on the screen Can receive user input & update the display Background An app is given some time to execute some code but can't directly access the screen or get any user input All apps enter this state when the user presses the home button Most of them quickly move on to Suspended Apps that want to run in the background stay here until they're made Active again 24

25 Suspended Application Life Cycle A Suspended app is frozen All the memory that app was using while it was active is held just as it was If the user brings the app back to the Active state, it will pick up right where it left off If the system needs more memory for an Active app, any Suspended apps may be terminated (Not Running state) and their memory freed 25

26 Inactive Application Life Cycle Temporary rest stop between 2 states The only ways an app can stay Inactive for any length of time 1. If the user is dealing with a system prompt such as those shown for an incoming call or SMS message 2. If the user has locked the screen This state is basically a sort of limbo 26

27 State-Change Notifications To manage changes between these states, UIApplication defines a number of methods that its delegate can implement UIApplication also defines a matching set of notification names This allows other objects besides the app delegate to register for notifications when the app's state changes 27

28 Delegate Methods Delegate methods for tracking your application's execution state and their corresponding notification names Delegate Method" application: didfinishlaunchingwithoptions: applicationwillresignactive: applicationdidbecomeactive: applicationdidenterbackground: applicationwillenterforeground: applicationwillterminate: Notification Name" UIApplicationDidFinishLaunchingNotification" UIApplicationWillResignActiveNotification" UIApplicationDidBecomeActiveNotification" UIApplicationDidEnterBackgroundNotification" UIApplicationWillEnterForegroundNofication" UIApplicationWillTerminateNotification" 28

29 Executing State Changes Active à Inactive Use applicationwillresignactive: and/or UIApplicationWillResignActiveNotification "Pause" your app's display Inactive à Background Use applicationdidenterbackground:/ UIApplicationDidEnterBackgroundNotification Release any resources Have about 5 seconds Background à Inactive Use applicationwillenterforeground:/ UIApplicationWillEnterForeground Inactive à Active Use applicationdidbecomeactive:/uiapplicationdidbecomeactive 29

30 Project State Lab Create a single view app project called StateLab which will log messages to the console A selector is simply Objective-C's way of referring to a method Objective-C provides a handy built-in variable called _cmd that always contains the selector of the current method The NSStringFromSelector function returns an NSString representation of a given selector Shortcut for outputting the current method name NSLog(@"%@", NSStringFromSelector(_cmd)); 30