Block Khoa Pham - 2359Media

Block Definition Syntax Capture Usage

Definition

Block The closure that Apple adds to C Is an object (NSBlock) The compiler translate block literals into struct and functions. So we don’t see the alloc call

Block It is said to be the only object that can be allocated on the stack, by default. It is moved to heap when copied.

Block struct + captured state information

Syntax

Syntax The syntax of Objective C block http://arigrant. com/blog/2014/1/18/the-syntax-of-objective-c-blocks From C declarators to Objective C block syntax http://nilsou.com/blog/2013/08/21/objective-c-blocks-syntax/ Cheatsheet http://fuckingblocksyntax.com/

Syntax * [] () ^

Syntax () > [] > *, ^ Start from the variable name to right Then to the left Operator precedence http://unixwiz.net/techtips/reading-cdecl.html CDECL http://cdecl.org/

Syntax int a;

Syntax int a; a is an int

Syntax int *a;

Syntax int *a; a is a pointer to an int

Syntax int a[];

Syntax int a[]; a is an array of int

Syntax int f();

Syntax int f(); f is a function that returns an int

Syntax int f(long); f is a function that returns an int, and accepts a long

Syntax int *a[];

Syntax int *a[]; a is an array of pointers to int

Syntax int *(a[]); a is an array of pointers to int

Syntax int (*)a[];

Syntax int (*)a[]; a is a pointer to an array of ints

Syntax int *f();

Syntax int *f(); f is function that returns a pointer to an int

Syntax int *(f()); f is function that returns a pointer to an int

Syntax int (*f)();

Syntax int (*f)(); f is a pointer to a function which accepts nothing and returns an int

Syntax void (^successBlock)(NSDictionary *response); successBlock is a block pointer to a function which takes a dictionary and returns nothing

Syntax ^ is the block pointer, which can only be applied to function int ^f(); // Error

^ unary operator int (^doubleMe)(int) = ^(int a){ return a * 2; } int b = doubleMe(2);

^ unary operator Transform function implementation into a block Infer the return type

^ unary operator BOOL (^customBlock)(NSArray *) = ^(NSArray *array) { // return array.count == 3; // Please don’t if (array.count == 3) { return YES; } return NO; };

__block What does the block keyword mean http://stackoverflow.com/questions/7080927/what-does-the-block-keyword- mean

__block Access to __block variable http://clang.llvm.org/docs/Block-ABI-Apple.html#layout-of-block-marked- variables By default, variables used withtin the block are copied Rewrite access,

__block (Kiwi example) describe(@"it takes a while" , ^{ __block NSDictionary *apiResponse = nil; beforeAll(^{ __block BOOL requestCompleted = NO; AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest: request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) { requestCompleted = YES; apiResponse = JSON; } failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error, id JSON) { requestCompleted = YES; } ]; [operation start]; [KWSpec waitWithTimeout:3.0 forCondition: ^BOOL() { return requestCompleted; }]; }); it(@"includes the related objects in the response" , ^{ [[[apiResponse objectForKey: @"children" ] should] containObjects: @"foo", @"bar", @"baz", nil]; }); });

Capture

Capture The ability to capture values from the enclosing scope, making them similar to closures or lambdas in other programming languages.

weakSelf vs strongSelf __weak __typeof__(self) weakSelf = self; self.block = ^{ __typeof__(self) strongSelf = weakSelf; [strongSelf doSomething]; [strongSelf doSomethingElse]; }; The block property is declared as “copy”

weakSelf vs strongSelf Understand weakSelf and strongSelf http://www.fantageek. com/1090/understanding-weak-self-and- strong-self/

weakSelf vs strongSelf Block is allocated on the stack. It has no effect on the storage of lifetime of anything it accesses. When they are copied, they take their captured scope with them, retaining any objects they refer

weakSelf vs strongSelf Block captures the variable along with its decorators (i.e. weak qualifier),

weakSelf vs strongSelf Block captures the variable along with its decorators (i.e. weak qualifier),

weakSelf vs strongSelf You should only use a weak reference to self, if self will hold on to a reference of the block.

Take care

UIView animation block [UIView animateWithDuration:0.5 delay:1.0 options: UIViewAnimationCurveEaseOut animations:^{ self.basketTop.frame = basketTopFrame; self.basketBottom.frame = basketBottomFrame; } completion:^(BOOL finished){ NSLog(@"Done!"); }];

UIView animation block - (void)loopThisBlock { [UIView animateWithDuration:0.2 animations:^{ someView.alpha = (someView.alpha + 1.0) % 2; } completion:^(BOOL finished) { [self loopThisBlock]; }]; } What if the block is executed infinitely ?

AFNetworking callback block AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager]; [manager GET:@"http://example.com/resources.json" parameters:nil success:^ (AFHTTPRequestOperation *operation, id responseObject) { NSLog(@"JSON: %@", responseObject); } failure:^(AFHTTPRequestOperation *operation, NSError *error) { NSLog(@"Error: %@", error); }];

Notification handler block [[NSNotificationCenter defaultCenter] addObserverForNotificationName:@"NotificationName" object:nil queue:[NSOperationQueue mainQueue] block:^(NSNotification *notification) { //reload the table to show the new whiz bangs NSAssert(notification, @"Notification must not be nil"); [self.tableView reloadData]; }];

Usage

DSL Masonry Kiwi FTGValidator

DSL REQUIRE_STRING(@"90001").to.matchRegExWithPattern(@"^[0-9][0-9][0-9][0-9][0- 9]$").with.message(@"Value must be US zip code format"), [view1 mas_makeConstraints:^(MASConstraintMaker *make) { make.edges.equalTo(superview).with.insets(padding); }];

DSL - (FTGStringRule * (^)(NSString *anotherValue))equalTo { return ^(NSString *anotherValue){ [self setValidation:^BOOL(NSString *value) { return [value isEqualToString:anotherValue]; }]; return self; }; }

Syntactic sugar https://github.com/supermarin/ObjectiveSugar [@3 times:^{ NSLog(@"Hello!"); }]; [cars each:^(id object) { NSLog(@"Car: %@", object); }];

Syntactic sugar https://github.com/supermarin/ObjectiveSugar [@3 times:^{ NSLog(@"Hello!"); }]; [cars each:^(id object) { NSLog(@"Car: %@", object); }];

Condition http://blog.vikingosegundo.de/2012/10/05/pattern-switch-value-object/ NSArray *filter = @[caseYES, caseNO]; id obj1 = @"YES"; id obj2 = @"NO"; [obj1 processByPerformingFilterBlocks:filter]; [obj2 processByPerformingFilterBlocks:filter];

Mapping with block http://www.merowing.info/2014/03/refactoring-tricks/#.VNw2IlOUc8Y NSString *(^const format)(NSUInteger, NSString *, NSString *) = ^(NSUInteger value, NSString *singular, NSString *plural) { return [NSString stringWithFormat:@"%d %@", value, (value == 1 ? singular : plural)]; };

Reference 1. http://www.galloway.me.uk/2012/10/a-look-inside-blocks-episode-1/ 2. https://blackpixel.com/writing/2014/03/capturing-myself.html 3. http://albertodebortoli.github.io/blog/2013/04/21/objective-c-blocks- under-the-hood/ 4. http://stackoverflow.com/questions/20134616/how-are-nsblock-objects- created 5. http://nilsou.com/blog/2013/08/21/objective-c-blocks-syntax/ 6. http://clang.llvm.org/docs/Block-ABI-Apple.html#blocks-as-objects 7. http://albertodebortoli.github.io/blog/2013/08/03/objective-c-blocks- caveat/

Thanks