CocoaShop - Intermediate Objective-C

1. INTERMEDIATE OBJECTIVE-C CocoaShops Curtis Herbert Carl Leiby Kotaro Fujita Mike Zornek @parrots @carlism @wild37 @zorn
2. http://bit.ly/CocoashopsAug2014
3. THE BASICS
4. HISTORY OF OBJ-C • 1983 - C + Smalltalk come together as a C pre-compiler • 1988 - NeXT licensed the language • 1996 - Apple buys NeXT as foundation for OS X (“Rhapsody”) • 2006 - Objective-C 2.0 • 2008 - iPhone SDK
5. THE IN THE ROOM Swift is the future, right?
6. THE BASICS CORE LANGUAGE FEATURES
7. C PRIMITIVES FLOAT DOUBLE VOID CHAR BOOL INT
8. int myAge = 6; myAge = myAge + 6; int alsoMyAge = myAge; C PRIMITIVES
9. WHERE THINGS GET FUN: OBJ-C IS A DYNAMIC RUNTIME A quick detour to the deep end
10. [tableView reloadData]; Send the message “reloadData” to the object “tableView” objc_msgsend(tableview, @selector(reloadData)); MESSAGE SENDING
11. “Objective-C decides which method implementation to call right before doing so (during runtime). The idea is that the connection between the name of a method and the implementation is dynamic. C++ for example does this during compile time.”
12. AN EXAMPLE OF THIS VOODOO UIView *myNewView = [[UIView alloc] init]; object_setClass(myNewView, [MyOtherClass class]); (this isn’t a common thing to do, don’t worry)
13. -[NSArray objectForKey:]: unrecognized selector sent to instance 0x102301c40 
 *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSArray objectForKey:]: unrecognized selector sent to instance 0x102301c40'
14. CLASS OBJ-C PRIMITIVES SEL ID NIL
15. OBJ-C PRIMITIVES SEL CLASS ID id name = @“Curtis”; id date = [NSDate new]; NIL
16. OBJ-C PRIMITIVES SEL CLASS ID - (id) someMethod; NIL
17. OBJ-C PRIMITIVES SEL CLASS ID SEL mySelector = @selector(update); [self performSelector:mySelector]; NIL
18. OBJ-C PRIMITIVES SEL CLASS ID SEL mySelector = @selector(update); if ([self respondsToSelector:mySelector]) { } NIL
19. OBJ-C PRIMITIVES SEL CLASS ID SEL mySelector = @selector(updateName:); [self performSelector:mySelector withObject:@“Joe”]; NIL
20. OBJ-C PRIMITIVES SEL CLASS ID SEL mySelector = NSSelectorFromString(@“myMethod”); [self performSelector:mySelector]; NIL
21. OBJ-C PRIMITIVES SEL CLASS ID Class targetClass = [NSDate class]; if ([myObject isKindOfClass:targetClass]) … NIL
22. OBJ-C PRIMITIVES SEL CLASS ID myObject = nil; NSString *name = [myObject showName]; NIL
23. OBJ-C DIRECTIVES • Instructions to the compiler • @IBOutlet, @property, @class, etc • @“string”, @[@“string1”], @{@“key” : @“value”}
24. HANDS-ON #1
25. CORE LANGUAGE FEATURES Deep breaths - back to more familiar territory
26. OBJ-C CLASSES UIVIEWCONTROLLER NSSTRING NSNUMBER UITABLEVIEW NSARRAY UIVIEW NSDATE NSDICTIONARY NSURL UILABEL UIIMAGEVIEW NSFORMATTER CLLOCTIONMANAGER UISWITCH UICOLLECTIONVIEW
27. STANDARD OO STUFF NSObject UIView : UIResponder UIResponder : NSObject Inherits from subclass Inherits from subclass
28. UIView : UIResponder <NSCoding, UIAppearance> Conforms to protocols STANDARD OO STUFF
29. COMPONENTS OF A CLASSES HEADER Properties, method signatures Exposed to the outside world IMPLEMENTATION The executable code
30. @import Foundation; @class PCHActivity; @interface PCHActivitySharingProvider : NSObject <NSCoder> - (void)shareActivity:(PCHActivity *)activity; @end
31. #import “PCHActivitySharingProvider.h” #import <AFNetworking/AFNetworking.h> @implementation PCHActivitySharingProvider - (void)shareActivity:(PCHActivity *)activity { //pre-process the activity, do weird stuff [self showShareUIWithActivity:activity]; //do something else } - (void)showShareUIWithActivity:(PCHActivity *)activity { //do awesome stuff with activity } @end
32. - (void)share; - (void)shareActivity:(PCHActivity *)activity; - (void)shareActivity:(PCHActivity *)activity withName:(NSString *)name forUser:(PCHUser *)user; + (instancetype)sharedInstance; METHODS
33. share shareActivity: shareActivity:withName:forUser: sharedInstance METHODS
34. @implementation PCHActivitySharingProvider PCHActivity *_activity; - (void)shareActivity:(PCHActivity *)activity { _activity = activity; [self showShareUIWithActivity:activity]; } ... INTERNAL VARIABLES (IVARS)
35. @implementation PCHActivitySharingProvider PCHActivity *_activity; - (void)setActivity:(PCHActivity *)activity { _activity = activity; } - (PCHActivity *)activity { return _activity; } ...
36. @import Foundation; @class CBCActivity; @interface CBCActivitySharingProvider : NSObject <NSCoder> @property CBCActivity *activity; - (void)shareActivity:(CBCActivity *)activity; @end PROPERTIES
37. myObject.activity myObject.activity = blah; self.activity = blah; return self.activity; “DOT SYNTAX” VS “OLD SCHOOL” [myObject activity]; [myObject setActivity:blah]; [self setActivity:blah]; return [self activity];
38. CONVENTIONS - ACCESSORS setFont: and font not setFont: and getFont
39. CONVENTIONS - CLARITY Code Commentary insertObject:atIndex: Good. insert:at: Not clear; what is being inserted? what does “at” signify? removeObjectAtIndex: Good. removeObject: Good, because it removes object referred to in argument. remove: Not clear; what is being removed? https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CodingGuidelines/Articles/ NamingBasics.html#//apple_ref/doc/uid/20001281-BBCHBFAH
40. CONVENTIONS - DESCRIPTIVE Code Commentary destinationSelection Good. destSel Not clear. setBackgroundColor: Good. setBkgdColor: Not clear. https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CodingGuidelines/ Articles/NamingBasics.html#//apple_ref/doc/uid/20001281-BBCHBFAH
41. CONVENTIONS - PREFIXING • Class Prefix — UIColor, NSString, CLLocation, etc • Apple uses 2-letter prefix • Prefix your classes with your initials Prefix Cocoa Framework NS Foundation NS Application Kit AB Address Book IB Interface Builder UI UIKit
42. HANDS-ON #2
43. POINTERS Not the tips and tricks kind
44. - (void)myMethod { int a = 10; } C primitives only exist in memory as long as they’re in scope. The memory for a is gone after the method returns.
45. int a = 10; int b = a; b = 20; C primitives are passed by making copies. 10 ➡️
46. int a = 10; int b = a; b = 20; C primitives are passed by making copies. 10 10 ➡️
47. int a = 10; int b = a; b = 20; C primitives are passed by making copies. 10 20 ➡️
48. int a = 10; [self myAdder:a]; a = 20; C primitives are passed by making copies. 10 - (void)myAddder:(int)c { adder += 40; } ➡️
49. int a = 10; [self myAdder:a]; a = 20; C primitives are passed by making copies. 10 10 - (void)myAddder:(int)c { adder += 40; } ➡️
50. int a = 10; [self myAdder:a]; a = 20; C primitives are passed by making copies. 10 50 - (void)myAddder:(int)c { adder += 40; } ➡️
51. int a = 10; [self myAdder:a]; a = 20; C primitives are passed by making copies. 20 - (void)myAddder:(int)c { adder += 40; } ➡️
52. Objects don’t work this way in Objective-C. They use pointers.
53. NSNumber *myNumber = [[NSNumber alloc] init]; NSNumber A pointer is a variable whose value is the address of another variable.
54. NSNumber *myNumber = [[NSNumber alloc] init]; A pointer is a variable whose value is the address of another variable. NSNumber *
55. NSNumber *myNumber = [[NSNumber alloc] init]; NSNumber *5 A pointer is a variable whose value is the address of another variable.
56. *0 NSNumber *myNumber; The nil value means “this instance reference isn’t pointing to any instance.”
57. *0 NSNumber *myNumber = nil; if (myNumber) { //do something if not nil } nil is equivalent to 0
58. *5 *12 NSNumber NSNumber NSNumber *myNumber = [[NSNumber alloc] init]; NSNumber *myNumber2 = [[NSNumber alloc] init];
59. NSNumber *myNumber = [[NSNumber alloc] init]; NSNumber *myNumber2 = [[NSNumber alloc] init]; myNumber = myNumber2; *5 *5 NSNumber NSNumber
60. Objects in Objective-C are pass-by-reference.
61. *5 NSMutableString NSMutableString *myString = [[NSMutableString alloc] init]; - (void)appendCompany:(NSMutableString *)string { NSLog(string); }
62. *5 NSMutableString *5 NSMutableString *myString = [[NSMutableString alloc] init]; [self appendCompany:myString]; - (void)appendCompany:(NSMutableString *)string { NSLog(string); }
63. *5 NSMutableString *5 NSMutableString *myString = [[NSMutableString alloc] init]; [self appendCompany:myString]; - (void)appendCompany:(NSMutableString *)string { [string appendString:@“Test”]; }
64. HANDS-ON #3 NSLOG(@“%p“, &object) will show an object’s address
65. MEMORY MANAGEMENT Reference Counting: A trip back to the dark ages (pre 2011)
66. C primitives only exist in memory as long as they’re in scope. How do we clean up after objects which work differently?
67. *5 *12 NSNumber NSNumber NSNumber *myNumber = [[NSNumber alloc] init]; NSNumber *myNumber2 = [[NSNumber alloc] init]; myNumber = myNumber2;
68. Reference counting is a way to express ownership of an object. NSString 1 image: http://cocoadevcentral.com alloc
69. Reference counting is a way to express ownership of an object. NSString 2
70. Reference counting is a way to express ownership of an object. NSString 1
71. Reference counting is a way to express ownership of an object. NSString 0
72. Reference counting is a way to express ownership of an object. NSString dealloc
73. A BALANCING ACT • Alloc or copy variants: starts with retain count of 1 • Retain: +1 • Release: -1 • Autorelease: -1 (later)
74. https://developer.apple.com/library/ios/documentation/general/conceptual/devpedia-cocoacore/MemoryManagement.html
75. IF YOU CREATE, YOU RELEASE - (void)shareActivity { NSDate *today = [[NSDate alloc] init]; //do some stuff with the date [today release]; }
76. AUTORELEASE TO TRANSFER OWNERSHIP - (NSDate *)todaysDate { NSDate *today = [[NSDate alloc] init]; //do some stuff with the date return [today autorelease]; }
77. TAKING OWNERSHIP PCHActivity *_activity; - (void)shareActivity:(PCHActivity *)activity { [_activity release]; _activity = [activity retain]; } - (void)dealloc { [_activity release]; [super dealloc]; }
78. TAKING OWNERSHIP (COPY) PCHActivity *_activity; - (void)shareActivity:(PCHActivity *)activity { [_activity release]; _activity = [activity copy]; } - (void)dealloc { [_activity release]; [super dealloc]; }
79. 2014-03-30 02:30:36.172 tradeRumors[3997:20b] *** -[GameLayer update]: message sent to deallocated instance 0x59bf670 EXEC_BAD_ACCESS
80. PROFILING FOR LEAKS ⌘I
82. HANDS-ON #4 Use NSLog(@"%lu", (unsigned long)[self retainCount]) to print an object’s current reference count
83. AUTOMATIC REFERENCE COUNTING • Added in 2011 • Compiler automatically adds these retain/release calls • Different than garbage collection (which is at runtime) • Didn’t solve all the retain-related issues…
84. RETAIN CYCLES CBCActivityMonitor CBCActivity
85. RETAIN CYCLES CBCActivityMonitor CBCActivity “Weak reference”
86. @implementation PCHActivitySharingProvider __weak PCHActivity *_activity; - (void)shareActivity:(PCHActivity *)activity { _activity = activity; } ... WEAK VARIABLES
87. @import Foundation; @class PCHActivityMonitor; @interface PCHActivity : NSObject @property (weak) PCHActivityMonitor *monitor; @end PROPERTY ATTRIBUTES
88. weak - does not increase the reference count atomic - creates a lock in setters/getters readonly - only creates a getter for the property copy - creates a copy instead of retaining original object strong - increases the reference count nonatomic - does not lock, better performance readwrite - creates setter and getter assign - similar to weak, used for primitives
89. @import Foundation; @class PCHActivityMonitor; @interface PCHActivity : NSObject @property (strong, atomic, readwrite) PCHActivityMonitor *monitor; @property (assign, atomic, readwrite) BOOL isValid; @end PROPERTY DEFAULT ATTRIBUTES
90. @property PCHActivityMonitor *monitor; _monitor @property PCHActivity *activity; _activity SYNTHESIZED IVARS Property definition ivar name In general though, avoid accessing ivars, always use self.property. Use ivars in custom setters and gettings.
91. HANDS-ON #5 You can use NSLOG(@“%@“, object) again to verify unique copies
92. BLOCKS
93. ^() { //do some awesome stuff };
94. void (^myBlock)() = ^() { NSLog(@“Inside!”); }; NSLog(@“Outside”); myBlock(); THEY’RE OBJECTS
95. [UIView animateWithDuration:1 animations:^{ self.view.alpha = 0.0; }]; [myDictionary enumerateKeysAndObjectsUsingBlock:^(id k, id v, BOOL *stop) { NSLog(@"%@ => %@", k, v); }]; PASS THEM AROUND
96. int (^myBlock)(int) = ^(int a) { return a * 15; }; int result = myBlock(10); COMPONENTS OF A BLOCK
97. int (^myBlock)(int) = ^(int a) { return a * 15; }; int result = myBlock(10); DEFINITION Return type
98. int (^myBlock)(int) = ^(int a) { return a * 15; }; int result = myBlock(10); DEFINITION Name
99. int (^myBlock)(int) = ^(int a) { return a * 15; }; int result = myBlock(10); DEFINITION Parameter types
100. int (^myBlock)(int) = ^(int a) { return a * 15; }; int result = myBlock(10); IMPLEMENTATION Parameters
101. int (^myBlock)(int) = ^(int a) { return a * 15; }; int result = myBlock(10); IMPLEMENTATION Body
102. http://f*ckingblocksyntax.com (or http://goshdarnblocksyntax.com)
103. - (void)upload:(NSData *)data failure:(void (^)(NSError *))errorCallback; YOUR OWN BLOCKS AS PARAMS [server upload:myData failure:^(NSError *error) { NSLog(@“Error!?! %@“, error); }];
104. - (void)upload:(NSData *)data failure:(void (^)(NSError *))errorCallback { //perform upload NSError *error = //get the upload error if (errorCallback) { errorCallback(error); } } YOUR OWN BLOCKS AS PARAMS
105. typedef void (^CBCErrorHandler)(NSError *); @interface PCHServerAPI - (void)upload:(NSData *)data failure:(PCHErrorHandler)callback; - (void)synchronizeWithFailure:(PCHErrorHandler)callback; @end REUSING BLOCK DEFINITIONS
106. GOTCHA #1: __BLOCK NSString *test = @“Hello!; void (^myBlock)() = ^(){ test = @“Hello 2!”; }; myBlock(); NSLog(test);
107. GOTCHA #1: __BLOCK __block NSString *test = @“Hello!; void (^myBlock)() = ^(){ test = @“Hello 2!”; }; myBlock(); NSLog(test);
108. GOTCHA #2: RETAIN CYCLES @property (nonatomic, copy) void (^errorHandler)(NSError *); ... self.errorHandler = ^(){ [self displayAlert:@“Oh No!”]; };
109. _weak typeof(self) weakSelf = self; self.errorHandler = ^(){ __strong typeof(weakSelf) strongSelf = weakSelf; if (strongSelf) { [strongSelf displayAlert:@“Oh No!”]; } }; GOTCHA #2: RETAIN CYCLES
110. HANDS-ON #6 You’ll need to make sure reloadData happens on the main thread dispatch_async(dispatch_get_main_queue(), ^{ //main thread code });
111. ARCHITECTURAL PATTERNS
112. These can all be abused.
113. SINGLETON PCHServerAPI UserViewController TimelineViewController AppDelegate [PCHServerAPI sharedInstance];
114. @implementation PCHServerAPI static CBCServerAPI *PCHServerAPISharedInstance = nil; + (instancetype)sharedServer { static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ PCHServerAPISharedInstance = [[PCHServerAPI alloc] init]; }); return PCHServerAPISharedInstance; }
115. You know how often you use global variables? Use singletons even less.
116. WITHOUT A SINGLETON? (static) shared downloader queue ServerAPI ServerAPI ServerAPI
117. @implementation PCHServerAPI static dispatch_queue_t PCHServerAPISharedQueue = nil; - (instancetype)init { if (self = [super init]) { static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ PCHServerAPISharedQueue = dispatch_queue_create(...); }); } return self; }
118. HANDS-ON #7
119. DATA SOURCE / DELEGATE (PROTOCOLS) UITableView • What data should be shown? • What happens during key events (like tapping a row)?
120. @interface CBCViewController : UIViewController <UITableViewDataSource> CBCViewController.h
121. - (void)awakeFromNib { [super awakeFromNib]; self.tableView.dataSource = self; } ... - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section { return 0; } ... CBCViewController.m
122. @protocol UITableViewDataSource <NSObject> @required - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section; - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath; @optional - (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView; ... UITableViewController.h
123. @interface UITableView : UIScrollView <NSCoding> ... @property (nonatomic, weak) id <UITableViewDataSource> dataSource; ... UITableViewController.h
124. NSUInteger rows = 0; if ([self.delegate respondsToSelector:@selector(tableView:numberOfRowsInSection:)]) { rows = [self.delegate tableView:self numberOfRowsInSection:section]; } UITableViewController.m
125. @interface UITableView : UIScrollView <NSCoding> ... @property (nonatomic, weak) id <UITableViewDataSource> dataSource; @property (nonatomic, weak) id <UITableViewDelegate> delegate; ...
126. Use this when you want two-way communications between classes, but you don’t want to have to hard-code references. Great for reusability / being able to swap out implementations.
127. HANDS-ON #8
128. @protocol PCHServerAPIDelegate <NSObject> @required - (void)downloadCountChangedTo:(NSUInteger)count; - (void)isDownloading:(BOOL)downloading; @end @interface PCHServerAPI : NSObject @property (nonatomic, weak) id <PCHServerAPIDelegate> delegate; @end
129. @interface PCHServerAPI : NSObject @property (nonatomic) NSUInteger downloadCount; @property (nonatomic) BOOL downloading; @end WHAT IF? “Hey, tell me if these change.”
130. KEY-VALUE OBSERVING (KVO) • Listen for changes for a given key-path • One-way: doesn’t need to know about subscribers • Ad-hoc: no formal contract (protocol) • Automatic • Limitless subscribers
131. KEY-VALUE OBSERVING (KVO) UserViewController TimelineViewController AppDelegate PCHServerAPI .downloadCount .downloading
132. Step 1 - Register - (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:(NSKeyValueObservingOptions)options context:(void *)context Step 2 - Listen for Callbacks - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
133. [self.serverAPI addObserver:self forKeyPath:@“downloadCount” options:0 context:nil]; ... - (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context { if ([keyPath isEqualToString:@"downloadCount"]) { //do something with the change } }
134. NSStringFromSelector(@selector(downloadCount)); A FEW IMPROVEMENTS
135. A FEW IMPROVEMENTS static void * MyContext = &MyContext; ... - (void)observeValueForKeyPath:(NSString *)keyPath... { if (context == MyContext) { } else { [super observeValueForKeyPath:keyPath ofObject:object change:change context:context]; } }
136. UNSUBSCRIBING - A PAIN @try { [self.server removeObserver:self forKeyPath:NSStringFromSelector(@selector(downloadCount)) context:MyContext]; } @catch (NSException * __unused exception) {} • Can’t check if you’re a subscriber • Will crash if you try to unsubscribe when not already subscribed
137. @interface PCHServerAPI : NSObject @property (nonatomic) NSUInteger downloadCount; @property (nonatomic, readonly) BOOL downloading; @end - (BOOL)downloading { return self.downloadCount > 0; } LETS GET TRICKY
138. + (NSSet *)keyPathsForValuesAffectingDownloading { return [NSSet setWithObjects: NSStringFromSelector(@selector(downloadCount)), nil]; } DERIVED PROPERTIES KEYPATH RELATIONSHIPS
139. - (void)setDownloadCount:(NSUInteger)downloadCount { BOOL shouldUpdateDownloading = //custom logic if (shouldUpdateDownloading) { [self willChangeValueForKey:@"downloading"]; } _downloadCount = downloadCount; if (shouldUpdateDownloading) { [self didChangeValueForKey:@"downloading"]; } } DERIVED PROPERTIES MANUAL NOTIFICATIONS
140. Use this when you want to allow multiple objects to listen to changes without the baggage of a protocol.
141. HANDS-ON #9 • viewWillAppear is a great spot to hook up KVO in this example • it’s opposite, viewDidDisappear, is a great place to unsubscribe
142. KVO OVERLOAD PCHServerAPI .downloading
143. KVO OVERLOAD PCHServerAPI .downloading
144. KVO OVERLOAD ServerAPI .downloading Nothing technically wrong with this. Just gets kinda messy.
145. NSNOTIFICATIONCENTER PCHServerAPI Started downloading Started downloading
146. NSNOTIFICATIONCENTER PCHServerAPI @“PCHDownloadStart” @“PCHDownloadStart”
147. REGISTERING id subscriptionToken; subscriptionToken = [[NSNotificationCenter defaultCenter] removeObserverForName:@“PCHDownloadStart” object:nil queue:nil usingBlock:^(NSNotification *notification) { //do something with the notification }];
148. POSTING [[NSNotificationCenter defaultCenter] postNotificationName:@“PCHDownloadStart” object:nil userInfo:nil];
149. POSTING DATA [[NSNotificationCenter defaultCenter] postNotificationName:@“PCHDownloadStart” object:nil userInfo:@{@“DownloadCount”: @(4)}];
150. DEREGISTERING [[NSNotificationCenter defaultCenter] removeObserver:subscriptionToken];
151. extern NSString * const PCHServerDownloadStartNotification; extern NSString * const PCHServerDownloadCountKey; NSString * const PCHServerDownloadStartNotification = @“org.phillycocoa.downloadStartNotification”; NSString * const PCHServerDownloadCountKey = @"PCHServerDownloadCountKey"; CLEANUP - USE CONSTANTS
152. CLEANUP - USE CONSTANTS [[NSNotificationCenter defaultCenter] postNotificationName:PCHServerDownloadStartNotification object:nil userInfo:@{PCHServerDownloadCountKey: @(4)}];
153. Use this when you want to allow multiple objects to be notified when something happens without needing any link between the objects.
154. HANDS-ON #10 • setDownloads is a great place to send a notification (if conditions are met) • you’ll need to use [NSNumber numberForBOOL:] to send bool values in the user info dictionary, and [number boolValue] to get it on the other side
155. ERROR HANDLING
156. NSError *error = nil; BOOL success = [myContext save:&error]; WHAT’S THAT & FOR?
157. • Exceptions exist in Objective-C, but are expensive • Instead return a successful BOOL if no return type needed, or nil if an object is expected. • Use an NSError parameter with information about the error (same as you would use an exception to bubble up information).
158. NSError *error = nil; BOOL success = [myContext save:error]; *0
159. *0 *0 NSError *error = nil; BOOL success = [myContext save:error]; -(BOOL)save:(NSError*)error;
160. *0 *5 NSError NSError *error = nil; BOOL success = [myContext save:error]; -(BOOL)save:(NSError*)error { error = [[NSError alloc] init]; }
161. *0 NSError *error = nil; BOOL success = [myContext save:error];
162. ** pointer to a pointer & location in memory A POINTER TO A POINTER? * pointer to an object
163. NSError *error = nil; A POINTER TO A POINTER? *5 NSError if (error) { *error = [NSError errorWithDomain:@“...” code:1 userInfo:nil]; } } -(BOOL)save:(NSError**)error { BOOL success = [myContext save:&error];
164. A LITTLE MORE ON NSERROR [NSError errorWithDomain:domain code:code userInfo:userInfo]; Domain - reverse DNS, most likely the same as your app ID (“com.consumedbycode.slopes”, for example) Code - you pick these and define their meanings. Internal to app logic. UserInfo - can use to pass back a dictionary of values to the handler. Usually a description using NSLocalizedDescriptionKey.
165. extern NSString * const PCHSlopesErrorDomain; extern NSString * const PCHSlopesErrorUnknownTypeDescription; extern int const PCHSlopesErrorUnknownTypeCode; NSString * const PCHSlopesErrorDomain = @“org.phillycocoa.slopes”; NSString * const PCHSlopesErrorUnknownTypeDescription = @"File type for export unknown.”; int const PCHSlopesErrorUnknownTypeCode = 100; CLEANUP - USE CONSTANTS
166. HANDS-ON #11
167. A FEW MORE LANGUAGE FEATURES
168. CATEGORIES • A category allows you to add methods to an existing class without subclassing it or needing to know any of the details of how it's implemented. • Effects all instances of that class within the application.
169. @interface NSArray (Utils) - (id)randomObject; @end @implementation NSArray (Utils) -(id)randomObject { if ([self count]) { return [self objectAtIndex:arc4random_uniform([self count])]; } else { return nil; } } @end CATEGORIES NSArray+utils.h NSArray+utils.m
170. @interface NSArray (CBCUtils) - (id)pch_randomObject; @end @implementation NSArray (CBCUtils) -(id)pch_randomObject { if ([self count]) { return [self objectAtIndex:arc4random_uniform([self count])]; } else { return nil; } } @end BEST PRACTICE: NAMESPACE NSArray+utils.h NSArray+utils.m
171. - (void)build; - (void)destroy; CATEGORIES FOR EXPOSING “PROTECTED” METHODS PCHActivity.h @interface CBCActivity - (void)build; @end PCHActivity+Protected.h @interface CBCActivity(Protected) -(void)destroy; @end
172. @interface PCHServerAPI: NSObject @property (nonatomic) BOOL downloading; @end EXTENSIONS
173. @interface PCHServerAPI: NSObject @property (nonatomic, readonly) BOOL downloading; @end EXTENSIONS
174. #import “PCHServerAPI.h” @interface PCHServerAPI() @property (nonatomic, readwrite) BOOL downloading; @end @implementaion ... EXTENSIONS
175. INSTANCETYPE Remember: id means “this could be any type” instancetype lets the compiler infer the type and can only be used for return types
176. INSTANCETYPE - (id)init { self = [super init]; return self; } - (bool)testMethod { ... } bool test = [[AClass alloc] init] testMethod]; //error!
177. INSTANCETYPE - (id)init { self = [super init]; return self; } - (bool)testMethod { ... } bool test = [((AClass *)[AClass alloc] init]) testMethod]; //yay!
178. INSTANCETYPE - (instancetype)init { self = [super init]; return self; } - (bool)testMethod { ... } bool test = [[AClass alloc] init] testMethod]; //yay!
179. HANDS-ON #12
180. RESOURCES http://www.objc.io http://nshipster.com http://iosdevweekly.com