Modify captured variable

Block will capture variables that appeared in the same lexical scope. Normally these variables are captured as “const” value:

int val = 10;
void (^blk)(void) = ^{
    val = 20; // Error! val is a constant value and cannot be modified!
};

In order to modify the variable, you need to use the __block storage type modifier.

__block int val = 10;
void (^blk)(void) = ^{
    val = 20; // Correct! val now can be modified as an ordinary variable.
};

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