rest/json/coredata example code - a tour

github.com/carlbrown/SeismicJSON Carl BrownTwitter: @CarlBrwnEmail: [email protected]

REST/JSON/CoreData Example Code

1Turn on Camera and ScreenFlow!!

Asynchronous iOS Programming Rules

•Threads are bad

•Use Queues instead

•Apple's Concurrency Programming Guide*:

• Page 10: "The Move Away from Threads"

• Page 74: "Migrating Away from Threads"

• Page 74: "Replacing Threads with Dispatch Queues"* http://developer.apple.com/library/ios/#documentation/General/Conceptual/ConcurrencyProgrammingGuide/Introduction/Introduction.html

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Some things aren't thread-safe

•Any UIAnything in iOS

•CoreData

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UI tasks Must be on the Main Thread

Often called the "UI" thread for that reason4

CoreData Contexts and Objects are tied to a thread

• Never share a CoreData object between threads

• Never share a CoreData object between contexts

• Pass objects only by ID and fetch them again

• Always notify other contexts when you've made changes

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Don't cross the threads•Use the same serial queue to stay on the same thread

•Use dispatch_get_main_queue() or [NSOperationQueue mainQueue] to get to the Main Thread.

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SeismicJSONThe only project we'll be working with today

Feel free to run it and play with it for a couple of minutes

Also on github so you can see how it was written.

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First Exercise:Follow the Code

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This is a vital skill to have. We don't expect people to write in English without reading a lot of English first, but programmers consistently spend more time writing code than reading it while learning a new language.

Please Open MasterViewController.mThis is pretty much a tableViewController like you've seen before.

Ignore the #if conditionals this run-through

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Ignore the #if conditionals this run-through

MasterViewController• Cell labels filled in from Model Object

• custom XIB for TableViewCell

• dateFormatter in viewDidLoad

• segmentedController sets FRC sorting

• actionSheet for adding rows (you can figure out)

• some iPad stuff (not Rocket Science)

• #if conditional stuff (for later)

should be mostly familiar

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DetailViewController•Really simple

•simpler than the tableViewCell

•Just a bunch of labels

•nothing to discuss here

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ActivityIndicatingImageViewThis has a UIActivityIndicatorView, implements a new custom @protocol and refers to a NetworkManager.

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So openActivityIndicatingImageView.m

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Start Spinner upon waking

-(void) awakeFromNib { _activityIndicator = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray]; [_activityIndicator setFrame:self.frame]; [_activityIndicator setHidesWhenStopped:YES]; [self addSubview:_activityIndicator]; [_activityIndicator startAnimating];}

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Stop it when we get image

-(void) setImage:(UIImage *)image { [super setImage:image]; if (image) { [self.activityIndicator stopAnimating]; } else { [self.activityIndicator startAnimating]; }}

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setImageFileName 1/3

-(void) setImageFileName:(NSString *)imageFileName {

_imageFileName = imageFileName;

if (_imageFileName==nil) { [self setImage:nil]; return; }

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//If the file already exists, don't bother to fetch it again NSString *fullFilePath = [[[NetworkManager sharedManager]

cachedImageDirectory] stringByAppendingPathComponent:_imageFileName];

if ([[NSFileManager defaultManager] fileExistsAtPath:fullFilePath]) {

[self imageDidBecomeAvailableAtPath:fullFilePath]; return; }

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[[NetworkManager sharedManager] fetchImagewithFilename:

imageFileName andNotifyTarget:self]; }

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OK, now we're getting somewhere

-(void) fetchImagewithFilename:(NSString *) filename andNotifyTarget:(NSObject <ImageFetchDelegate> *) target { //Stuff we've seen before ImageFetchOperation *imageFetchOperation =

[[ImageFetchOperation alloc] init];

[imageFetchOperation setUrlToFetch:[self imageURLForImageFileName:filename]];

[imageFetchOperation setNotificationTarget:target];

[self.fetchQueue addOperation:imageFetchOperation];

}

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ImageFetchOperation.m

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connectionDidFinishLoading (abridged)

- (void)connectionDidFinishLoading:(NSURLConnection *)connection { if (self.response.statusCode==200) { NSError *error=nil; NSString *filename = [self.urlToFetch lastPathComponent]; NSString *fullFilePath = [

[[NetworkManager sharedManager] cachedImageDirectory] stringByAppendingPathComponent:filename];

NSLog(@"About to write file: %@",fullFilePath); if (![self.fetchedData writeToFile:fullFilePath

options:NSDataWritingAtomic error:&error]) { NSLog(@"error occurred writing file: %@",

[error localizedDescription]); } if (self.notificationTarget) { [self.notificationTarget

imageDidBecomeAvailableAtPath:fullFilePath]; } }}

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Only save if no error







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Get filename/path







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Save File







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Let the View Know







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[control]-[⌘]-Jback to ActivityIndicatingImageView

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imageDidBecomeAvailableAtPath1/2

-(void) imageDidBecomeAvailableAtPath:(NSString *) path { if (![[path lastPathComponent]

isEqualToString:self.imageFileName]) { NSLog(@"Warning: notified of incorrect file:

%@, should have been %@",[path lastPathComponent],self.imageFileName);

//try again [self setImageFileName:self.imageFileName]; return; }

Only load the file we're expecting (race condition checking)

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imageDidBecomeAvailableAtPath2/2

//load image off the main queue UIImage *imageToLoad=[UIImage imageWithContentsOfFile:path]; dispatch_async(dispatch_get_main_queue(), ^{ [self setImage:imageToLoad]; [self setNeedsDisplay]; });}

Set our image with the file now on disk

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Summary of ActivityIndicatingImageView

• Start the view with a spinner telling the user we are working on something

• See if the file is already on disk, and use it if so.

• If not, we ask the Network Manager to get the file for us

• The Network Manager creates an operation to get our file (presumably from the network) and write it to disk

• The Network Manager tells us the file is ready

• We load the file into our image property

• Now that we have an image, the spinner hides

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Networking Strategy•Always* load the UI from local storage

•Core Data or local file or something

•Always* put network data in local storage

•Then tell the UI to refresh itself

•Put up a placeholder if no data

Carl's Recommended

*Except with live web pages or HTTP streaming30

Some people argue with me about this, but it's served me well for years

Why do it that way?•Separates network code from UI code

•Easier to test

•Much faster response if previous data

•Much better user experience offline

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Why wouldn't you?•Pointless if the network is infinitely fast and infinitely reliable*

•More effort than "Unbreakable Glass" loading screens

*c.f. http://en.wikipedia.org/wiki/Fallacies_of_Distributed_Computing32

NSOperations and GCD

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NSOperation•Been around since the first iPhone OS SDK

•Way to encapsulate the pieces of a task in one place

•Can be queried, suspended or canceled

•Simple selector call or block variants

•NSOperations are placed in NSOperationQueues

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NSOperationQueue• Long-lived (presumably) queue that can contain

numerous operations

• Can be serial or concurrent

• Can be suspended or canceled

• Nice (but verbose) Objective-C syntax

• Will stay on the same thread, if serial

• [NSOperationQueue mainQueue] is always on the Main Thread

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Dispatch Queues•C-style (concise) syntax

•quicker to use in-place

•much less typing than declaring an NSOperation and adding to Queue

•Harder to manage or cancel

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Which to use?• No hard-and-fast rules, but...

• I tend to use NSOperations for:

• things I'm going to do several times

• things that have non-trivial complexity

• I tend to use dispatch_async() for things:

• with less than 10 or so lines of code

• done only once in the App

• that won't need to change when spec changes

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Waiting in Cocoa•Don't Sleep

•Don't use locks

•Yield to the RunLoop

•See the FetchOperation for example

•Sleeping or Locking Freezes the Thread

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Be Nice to Threads•POSIX Threads are a finite resource

•The system will spin up more if tasks are waiting

•But when no more can start, things will hang

•See: WWDC2012 Session Session 712 - Asynchronous Design Patterns with Blocks, GCD, and XPC

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Back to our Application

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Please OpenNotificationOrParentContext.h

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Project Variations

//Make this a 1 to show notifications, and a 0 to show parent contexts#define kUSE_NSNOTIFICATIONS_FOR_CONTEXT_MERGE 0//if using notifications, set this to 1 to have them in the App Delegate#define kNSNOTIFICATIONS_HANDLED_IN_APPDELEGATE 0

Note: I'm not usually a fan of this kind of conditionalcompilation, but for this case, I thought it would

let you play with project in the debugger in acleaner way than with traditional if's.

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OK, to theAppDelegate.m

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Open Source Control View

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Click the TimeMachine45

Pick the Initial Commit Scroll in the center of the screen

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Got rid of some Xcode 4.4-isms

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Removed Observer48

applicationWillResignActive:!(UIApplication *) application

•Happens when user gets Texts, notifications, Alerts, phone calls or hits the home button

•Here I'm removing the notification observer so we won't try to get notifications while not Active

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Added Observer50

Kicked off Network Fetch

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applicationDidBecomeActive:(UIApplication *)application

•Happens when App becomes full-focus

•After launch

•Or after returning from dealing with alert

•Or after dealing with "most recently used apps" along bottom of screen

•Here I'm adding a notification observer

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This Runs "changesSaved:"

[[NSNotificationCenter defaultCenter] addObserver:selfselector:@selector(changesSaved:) name:NSManagedObjectContextDidSaveNotification

object:nil];

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Handler Code

- (void)changesSaved:(NSNotification *)notification { if (![NSThread isMainThread]) { dispatch_async(dispatch_get_main_queue(), ^{ [self changesSaved:notification]; }); return; } if ([notification object] != self.managedObjectContext) { [self.managedObjectContext

mergeChangesFromContextDidSaveNotification:notification]; }}

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If not on Main, go there



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Merge changes



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Queue Concurrency Type

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Reset DB each run58

Back to normal view now

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NetworkManager.m

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Singleton Pattern

+ (NetworkManager *)sharedManager { static dispatch_once_t pred; dispatch_once(&pred, ^{ sharedManager = [[self alloc] init]; //Initialization Stuff }); return sharedManager;}

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Kicked off from AppDelegate

-(void) startMainPageFetch { [self setHostReach:[Reachability

reachabilityWithHostName:[self.baseURL host]]]; [self.hostReach startNotifier];

[self queuePageFetchForRelativePath:@"/earthquakes/feed/geojson/significant/month"];

}

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Inform users of network status or be Rejected




}

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Just do it. If you want to understand it, read Apple's writeup

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Start fetch of first batch




}

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Make NSOp & Queue it

-(void) queuePageFetchForRelativePath:(NSString *) relativePath { EarthquakeFetchOperation *earthquakeFetchOperation =

[[EarthquakeFetchOperation alloc] init]; [earthquakeFetchOperation setUrlToFetch:

[self urlForRelativePath:relativePath]]; [earthquakeFetchOperation setMainContext:self.mainContext]; [earthquakeFetchOperation setDelegate:self]; [self.fetchQueue addOperation:earthquakeFetchOperation];}

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EarthquakeFetchOperation.h

#import <Foundation/Foundation.h>#import "BaseFetchOperation.h"

@interface EarthquakeFetchOperation : BaseFetchOperation@property (nonatomic, weak) NSManagedObjectContext *mainContext;

@end

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BaseFetchOperation.h

@interface BaseFetchOperation : NSOperation <NSURLConnectionDataDelegate>

@property (nonatomic, strong) NSURL *urlToFetch;@property (nonatomic, strong) NSMutableData *fetchedData;@property (nonatomic, assign, getter=isDone) BOOL done;@property (nonatomic, assign) NSURLConnection *connection;@property (nonatomic, retain) NSHTTPURLResponse *response;

@property (nonatomic, weak) NSObject<FetchNotifierDelegate> *delegate;

-(void) finish;

@end

@protocol FetchNotifierDelegate <NSObject>-(void) fetchDidFailWithError:(NSError *) error;-(void) incrementActiveFetches;-(void) decrementActiveFetches;@end

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Methods needed for URL fetching

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BaseFetchOperation.m

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Entry Point

- (void)main { if ([self isCancelled]) { return; } if (!_urlForJSONData) { NSLog(@"Cannot start without a URL"); return; } [self setFetchedData:[NSMutableData data]]; //Initialize NSURLRequest *request =

[NSURLRequest requestWithURL:[self urlToFetch]]; if (self.delegate) { [self.delegate incrementActiveFetches]; } [self setConnection:[NSURLConnection

connectionWithRequest:request delegate:self]]; CFRunLoopRun();}

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Sanity Check




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Make request




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Inform user we're active




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Start Connection




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Give Up Control of thread




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Finish

-(void) finish { [self setDone:YES]; if (self.delegate) { [self.delegate decrementActiveFetches]; } CFRunLoopStop(CFRunLoopGetCurrent());}

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Inform user we're done


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Stop the runloop & get off


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Other methods there

• didReceiveResponse

• remember response

• truncate data

• (can get more than one response)

• didReceiveData

• append data

• didFailWithError

• report error to our delegate80

EarthquakeFetchOperation

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connectionDidFinishLoading 1/n

- (void)connectionDidFinishLoading:(NSURLConnection *)connection { if ([self isCancelled]) { [self finish]; return; } if (self.response.statusCode==200) {

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Sanity Check/Housekeeping


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Don't parse bad response


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id objectFromJSON = [NSJSONSerialization JSONObjectWithData:self.fetchedData options:0 error:&error];

if (objectFromJSON) {#if kUSE_NSNOTIFICATIONS_FOR_CONTEXT_MERGE NSManagedObjectContext *context = [[NSManagedObjectContext alloc] init]; [context setPersistentStoreCoordinator:

self.mainContext.persistentStoreCoordinator];#else

NSManagedObjectContext *context = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];

[context setParentContext:[self mainContext]];#endif

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Parse JSON






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If the JSON was good






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Make new ManagedObjectContext






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NSDictionary *jsonDict = (NSDictionary *) objectFromJSON;

if (jsonDict) { NSArray *events = [jsonDict objectForKey:@"features"]; if (events) { for (NSDictionary *eventDict in events) {

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If we got a dictionary



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Get Array of Earthquakes



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If Array/JSON is valid



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Iterate over it



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NSString *eventLocation = [eventDict valueForKeyPath:@"properties.place"];

NSDate *eventDate = [NSDate dateWithTimeIntervalSince1970:[[eventDict valueForKeyPath:@"properties.time"] doubleValue]];

NSNumber *eventLong = [NSNumber numberWithDouble:[[[eventDict valueForKeyPath:@"geometry.coordinates"] objectAtIndex:0]

doubleValue]];NSNumber *eventLat =[NSNumber numberWithDouble:[[[eventDict

valueForKeyPath:@"geometry.coordinates"] objectAtIndex:1] doubleValue]];

NSNumber *eventMagnitude = [NSNumber numberWithFloat:[[eventDict valueForKeyPath:@"properties.mag"] floatValue]];

NSString *eventWebPath = [@"http://earthquake.usgs.gov" stringByAppendingPathComponent:[eventDict valueForKeyPath:@"properties.url"]];

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Extract values from eventDict








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Using keyPaths








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and/or Array elements








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NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:

NSStringFromClass([Earthquake class])];[fetchRequest setFetchLimit:1];NSPredicate *eventInfo =

[NSPredicate predicateWithFormat:@"location = %@ AND date = %@",

eventLocation, eventDate];[fetchRequest setPredicate:eventInfo];NSError *fetchError=nil;NSArray *existingEventsMatchingThisOne =

[context executeFetchRequest:fetchRequest error:&fetchError];

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Make a fetch request






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matching our event






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And run it






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if ([existingEventsMatchingThisOne count]==0) { //Didn't find one already, make a new one NSManagedObject *newManagedObject =

[NSEntityDescription insertNewObjectForEntityForName:NSStringFromClass([Earthquake class])

inManagedObjectContext:context]; [newManagedObject setValue:eventLocation forKey:@"location"]; [newManagedObject setValue:eventDate forKey:@"date"]; [newManagedObject setValue:eventLat forKey:@"latitude"]; [newManagedObject setValue:eventLong forKey:@"longitude"]; [newManagedObject setValue:eventMagnitude forKey:@"magnitude"]; [newManagedObject setValue:eventWebPath forKey:@"webLinkToUSGS"];}

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If there isn't already one




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Make a new Object




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Set all its attributes




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// Save the context.error = nil;if (![context save:&error]) { // stuff NSLog(@"Unresolved error %@, %@", error, [error userInfo]); abort();}

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Save and check for errors

// Save the context.error = nil;if (![context save:&error]) { // stuff NSLog(@"Unresolved error %@, %@", error, [error userInfo]); abort();}

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#if kUSE_PARENT_CONTEXTS_FOR_CONTEXT_MERGE dispatch_sync(dispatch_get_main_queue(), ^{ NSError *error = nil; if (![self.mainContext save:&error]) { // Stuff NSLog(@"Unresolved error %@, %@", error,

[error userInfo]); abort(); } });#endif

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If we're merging via Parent



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On the Main Thread



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Tell the main context to save



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Review

• Asynchronous programming

• NSOperations

• Grand Central Dispatch (GCD)

• More Blocks

• Notifications

• App Lifecycle

• Network I/O (HTTP/REST)

• JSON parsing112

Questions?Now, Or Later:[email protected]@CarlBrwn (Twitter/App.net)

Today's App was: https://github.com/carlbrown/SeismicJSON

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rest/json/coredata example code - a tour

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