Assign a variable inside a Block to a variable outside a Block

Question

I'm getting an error

Variable is not assignable (missing __block type specifier)

on the line aPerson = participant;. How can I make sure the block can access the aPerson variable and the aPerson variable can be returned?

Person *aPerson = nil;

[participants enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {   
    Person *participant = (Person*)obj;

    if ([participant.gender isEqualToString:@"M"]) {
        aPerson = participant;
        *stop = YES;
    }
}];

return aPerson;

score 823 · Accepted Answer · edited Apr 07 '19 at 05:58

823

You need to use this line of code to resolve your problem:

__block Person *aPerson = nil;

For more details, please refer to this tutorial: Blocks and Variables

edited Apr 07 '19 at 05:58

Cœur

37,241
25
195
267

answered Nov 01 '11 at 05:31

Devarshi

16,440
13
72
125

105

Please note that `__block` is prefixed with **two** underscores. – Eric Brotto Aug 20 '12 at 09:03
14

using `__block` my error gone, but value is not retaining out of block. Any help here – User16119012 Feb 24 '14 at 14:00
7

The block is called well after where it looks like it should be called, in other words, blocks don't run 'right away'. – Tom Andersen Dec 08 '14 at 20:46
1

If blocks don't run RIGHT AWAY then this should not be accepted answer as that makes it a huge deal breaker. – durazno May 16 '16 at 04:57
3

This answer is correct with respect to posted question, feel free to post another question if you have a different scenario in mind. – Devarshi May 16 '16 at 12:29

score 43 · Answer 2 · answered Mar 02 '14 at 23:57

Just a reminder of a mistake I made myself too, the

 __block

declaration must be done when first declaring the variable, that is, OUTSIDE of the block, not inside of it. This should resolve problems mentioned in the comments about the variable not retaining its value outside of the block.

score 19 · Answer 3 · edited Jan 19 '16 at 10:55

19

Just use the __block prefix to declare and assign any type of variable inside a block.

For example:

__block Person *aPerson = nil;

__block NSString *name = nil;

edited Jan 19 '16 at 10:55

Luke

11,426
43
60
69

answered Jan 10 '14 at 07:15

Umesh Sawant

329
2
6

score 11 · Answer 4 · edited Oct 09 '13 at 12:21

11

To assign a variable inside block which outside of block always use __block specifier before that variable your code should be like this:-

__block Person *aPerson = nil;

edited Oct 09 '13 at 12:21

Luke

11,426
43
60
69

answered Oct 09 '13 at 12:19

Gaurav

553
5
9

score 10 · Answer 5 · edited Jan 19 '16 at 10:55

10

__block Person *aPerson = nil;

edited Jan 19 '16 at 10:55

Luke

11,426
43
60
69

answered Apr 29 '15 at 13:44

Ketan Patel

550
4
11

score 3 · Answer 6 · answered Jul 02 '15 at 08:39

3

Try __weak if you get any warning regarding retain cycle else use __block

Person *strongPerson = [Person new];
__weak Person *weakPerson = person;

Now you can refer weakPerson object inside block.

answered Jul 02 '15 at 08:39

PradeepKN

617
7
10

score 3 · Answer 7 · answered Jun 12 '16 at 06:37

3

yes block are the most used functionality , so in order to avoid the retain cycle we should avoid using the strong variable,including self inside the block, inspite use the _weak or weakself.

answered Jun 12 '16 at 06:37

user6311313

31
1

score 3 · Answer 8 · answered Oct 18 '19 at 07:22

When I saw the same error, I tried to resolve it like:

   __block CGFloat docHeight = 0.0;


    [self evaluateJavaScript:@"document.height" completionHandler:^(id height, NSError *error) {
        //height
        NSLog(@"=========>document.height:@%@",height);
        docHeight = [height floatValue];
    }];

and its working fine

Just add "__block" before Variable.

Assign a variable inside a Block to a variable outside a Block

8 Answers8

Linked

Related