REST/JSON/CoreData Example Code - A Tour

REST/JSON/CoreData
Example Code
github.com/carlbrown/
SeismicJSON
Carl Brown
Twitter: @CarlBrwn
Email: CarlB@PDAgent.com

1
Turn 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

2

Some things aren't thread-safe
•Any UIAnything in iOS
•CoreData

3

UI tasks Must be on the Main
Thread

Often called the "UI" thread for that reason
4

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

5

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.

6

SeismicJSON
The 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.

7

First Exercise:
Follow the Code

8

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
MasterView
Controller.m
This is pretty much a
tableViewController like
you've seen before.

Ignore the #if conditionals
this run-through

9

Ignore the #if conditionals this run-through

MasterViewController
should be mostly familiar
• 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)

10

DetailViewController
•Really simple

•simpler than the tableViewCell

•Just a bunch of labels

•nothing to discuss here

11

Activity
Indicating
ImageView
This has a
UIActivityIndicatorView,
implements a new
custom @protocol and
refers to a
NetworkManager.

12

So open
ActivityIndicating
ImageView.m

13

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

Start Spinner upon
waking
14

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

Stop it when we get
image
15

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

_imageFileName = imageFileName;

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

setImageFileName 1/3
16

//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;
}

17

[[NetworkManager sharedManager]
fetchImagewithFilename:
imageFileName andNotifyTarget:self];

}

18

-(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];

}

OK, now we're getting
somewhere
19

ImageFetch
Operation.m

20

- (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];
}
}
}

connectionDidFinish
Loading (abridged)
21

NSError *error=nil;
}
}
}
}

Only save if no error
22

NSError *error=nil;
}
}
}
}

Get ﬁlename/path
23

NSError *error=nil;
}
}
}
}

Save File
24

NSError *error=nil;
}
}
}
}

Let the View Know
25

[control]-[⌘]-J
back to ActivityIndicatingImageView
26

-(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)

imageDidBecomeAvailableAtPath
1/2
27

//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

imageDidBecomeAvailableAtPath
2/2
28

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

29

Recommended
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
*Except with live web pages or HTTP streaming
30

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

31

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_Computing
32

NSOperations and GCD

33

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

34

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

35

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

36

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

37

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

38

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

39

Back to our Application

40

Please Open
NotiﬁcationOr
ParentContext.h

41

//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 conditional
compilation, but for this case, I thought it would
let you play with project in the debugger in a
cleaner way than with traditional if's.

Project Variations
42

OK, to the
AppDelegate.
m

43

Open Source
Control View

44

Click the TimeMachine
45

Scroll in the center of
Pick the Initial Commit the screen

46

Got rid of some Xcode
4.4-isms
47

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

49

Kicked off Network
Fetch
51

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

52

[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(changesSaved:)
name:NSManagedObjectContextDidSaveNotification
object:nil];

This Runs
"changesSaved:"
53

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

Handler Code
54

});
return;
}
}
}

If not on Main, go there
55

});
return;
}
}
}

Merge changes
56

Queue Concurrency
Type
57

Reset DB each run
58

Back to
normal view
now

59

Network
Manager.m

60

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

//Initialization Stuff

});
return sharedManager;
}

Singleton Pattern
61

-(void) startMainPageFetch {
[self setHostReach:[Reachability
reachabilityWithHostName:[self.baseURL host]]];
[self.hostReach startNotifier];

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

Kicked off from
AppDelegate
62


}

Inform users of network status
or be Rejected
63

Just do it. If you want to
understand it, read Apple's writeup
64


}

Start fetch of ﬁrst batch
65

-(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];
}

Make NSOp & Queue it
66

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

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

@end

EarthquakeFetchOperation.h
67

@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

BaseFetchOperation.h
68

Methods needed for
URL fetching
69

BaseFetchOp
eration.m

70

- (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();
}

Entry Point
71

- (void)main {
return;
}
return;
}



CFRunLoopRun();
}

Sanity Check
72

- (void)main {
return;
}
return;
}



CFRunLoopRun();
}

Make request
73

- (void)main {
return;
}
return;
}



CFRunLoopRun();
}

Inform user we're
active
74

- (void)main {
return;
}
return;
}



CFRunLoopRun();
}

Start Connection
75

- (void)main {
return;
}
return;
}



CFRunLoopRun();
}

Give Up Control of
thread
76

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

Finish
77

-(void) finish {
[self setDone:YES];
}
}

Inform user we're done
78

-(void) finish {
[self setDone:YES];
}
}

Stop the runloop & get
off
79

Other methods there
• didReceiveResponse

• remember response

• truncate data

• (can get more than one response)

• didReceiveData

• append data

• didFailWithError

• report error to our delegate
80

Earthquake
FetchOperation
81

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
[self finish];
return;
}


connectionDidFinishLoading
1/n
82

{
[self finish];
return;
}


Sanity Check/
Housekeeping
83

{
[self finish];
return;
}


Don't parse bad
response
84

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

connectionDidFinish
Loading 2/n
85


#else

#endif

Parse JSON
86


#else

#endif

If the JSON was good
87


#else

#endif

Make new
ManagedObjectContext
88

NSDictionary *jsonDict = (NSDictionary *) objectFromJSON;

if (jsonDict) {

NSArray *events = [jsonDict objectForKey:@"features"];

if (events) {

for (NSDictionary *eventDict in events) {

connectionDidFinish
Loading 3/n
89


if (jsonDict) {


if (events) {


If we got a dictionary
90


if (jsonDict) {


if (events) {


Get Array of
Earthquakes
91


if (jsonDict) {


if (events) {


If Array/JSON is valid
92


if (jsonDict) {


if (events) {


Iterate over it
93

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
doubleValue]];
NSNumber *eventMagnitude = [NSNumber numberWithFloat:[
[eventDict valueForKeyPath:@"properties.mag"] floatValue]];
NSString *eventWebPath = [@"http://earthquake.usgs.gov"
stringByAppendingPathComponent:[eventDict
valueForKeyPath:@"properties.url"]];

connectionDidFinish
Loading 4/n
94

doubleValue]];
doubleValue]];

Extract values from
eventDict
95

doubleValue]];
doubleValue]];

Using keyPaths
96

doubleValue]];
doubleValue]];

and/or Array elements
97

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];

connectionDidFinish
Loading 5/n
98

eventLocation,
eventDate];

Make a fetch request
99

eventLocation,
eventDate];

matching our event
100

eventLocation,
eventDate];

And run it
101

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"];
}

connectionDidFinish
Loading 6/n
102



}

If there isn't already
one
103



}

Make a new Object
104



}

Set all its attributes
105

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

connectionDidFinish
Loading 7/n
106

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

Save and
check for errors
107

#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

connectionDidFinish
Loading 8/n
108

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

If we're merging via
Parent
109

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

On the Main Thread
110

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

Tell the main context to
save
111

Review
• Asynchronous programming

• NSOperations

• Grand Central Dispatch (GCD)

• More Blocks

• Notiﬁcations

• App Lifecycle

• Network I/O (HTTP/REST)

• JSON parsing
112

Questions?
Now, Or Later:
CarlB@PDAgent.com
@CarlBrwn (Twitter/App.net)

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

113

REST/JSON/CoreData Example Code - A Tour

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