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;

Cœur · Accepted Answer · 2019-04-07 05:58:26Z

828

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 7, 2019 at 5:58

Cœur

39k25 gold badges207 silver badges282 bronze badges

answered Nov 1, 2011 at 5:31

Devarshi

16.8k14 gold badges76 silver badges126 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Eric Brotto Over a year ago

Please note that __block is prefixed with two underscores.

User16119012 Over a year ago

using __block my error gone, but value is not retaining out of block. Any help here

Tom Andersen Over a year ago

The block is called well after where it looks like it should be called, in other words, blocks don't run 'right away'.

durazno Over a year ago

If blocks don't run RIGHT AWAY then this should not be accepted answer as that makes it a huge deal breaker.

Devarshi Over a year ago

This answer is correct with respect to posted question, feel free to post another question if you have a different scenario in mind.

Denis Balko · Accepted Answer · 2014-03-02 23:57:14Z

43

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.

answered Mar 2, 2014 at 23:57

Denis Balko

1,6062 gold badges16 silver badges31 bronze badges

Comments

Luke · Accepted Answer · 2016-01-19 10:55:46Z

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, 2016 at 10:55

Luke

11.5k43 gold badges63 silver badges70 bronze badges

answered Jan 10, 2014 at 7:15

Umesh Sawant

3272 silver badges6 bronze badges

Comments

Luke · Accepted Answer · 2013-10-09 12:21:46Z

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 9, 2013 at 12:21

Luke

11.5k43 gold badges63 silver badges70 bronze badges

answered Oct 9, 2013 at 12:19

Gaurav

5616 silver badges9 bronze badges

Comments

Luke · Accepted Answer · 2016-01-19 10:55:54Z

10

__block Person *aPerson = nil;

edited Jan 19, 2016 at 10:55

Luke

11.5k43 gold badges63 silver badges70 bronze badges

answered Apr 29, 2015 at 13:44

Ketan Patel

5604 silver badges11 bronze badges

Comments

Mr.Javed Multani · Accepted Answer · 2019-10-18 07:22:35Z

4

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.

answered Oct 18, 2019 at 7:22

Mr.Javed Multani

13.5k5 gold badges56 silver badges55 bronze badges

Comments

PradeepKN · Accepted Answer · 2015-07-02 08:39:20Z

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 2, 2015 at 8:39

PradeepKN

6877 silver badges10 bronze badges

Comments

user6311313 · Accepted Answer · 2016-06-12 06:37:32Z

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, 2016 at 6:37

user6311313

311 bronze badge

Collectives™ on Stack Overflow

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

8 Answers 8

5 Comments

Comments

Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

8 Answers 8

5 Comments

Comments

Comments

Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related