Learn about Objective C blocks. Its syntax, usage

1. Block
Khoa Pham - 2359Media

2. Block
Definition
Syntax
Capture
Usage

3. Definition

4. 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

5. 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.

6. Block
struct + captured state information

7. Syntax

8. 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/

9. Syntax
*
[]
()
^

10. 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/

11. Syntax
int a;

12. Syntax
int a;
a is an int

13. Syntax
int *a;

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

15. Syntax
int a[];

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

17. Syntax
int f();

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

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

20. Syntax
int *a[];

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

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

23. Syntax
int (*)a[];

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

25. Syntax
int *f();

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

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

28. Syntax
int (*f)();

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

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

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

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

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

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

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

36. __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,

37. __block (Kiwi example)
describe(@"it takes a while", ^{
__block NSDictionary *apiResponse = nil;
beforeAll(^{
__block BOOL requestCompleted = NO;
AFJSONRequestOperation *operation = [AFJSONRequestOperationJSONRequestOperationWithRequest: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", ^{
[[[apiResponseobjectForKey:@"children"] should] containObjects:@"foo", @"bar", @"baz", nil];
});
});

38. Capture

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

40. 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”

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

42. 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

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

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

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

46. Take care

47. 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!");
}];

48. 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 ?

49. 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);
}];

50. 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];
}];

51. Usage

52. DSL
Masonry
Kiwi
FTGValidator

53. 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);
}];

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

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

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

57. 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];

58. 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)];
};

59. 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/

60. Thanks