Question

I’ve started using Objective-C blocks today. I wrote the following code:

NSArray *array = @[@25, @"abc", @7.2];

void (^print)(NSUInteger index) = ^(NSUInteger index)
{
    NSLog(@"%@", array[index]);
};

for (int n = 0; n < 3; n++)
    print(n);

Which works properly. I needed to change the array variable after its declaration, though, so I tried using the following code:

NSArray *array;

void (^print)(NSUInteger index) = ^(NSUInteger index)
{
    NSLog(@"%@", array[index]);
};

array = @[@25, @"abc", @7.2];

for (int n = 0; n < 3; n++)
    print(n);

However, that doesn’t work. The console just prints (null) three times. Why is it that this doesn’t work, while it did work with my first piece of code?

Answer 1

It’s because the block captures variables by value and when the block is created (unless you use __block).

What you probably want is:

NSArray *array = @[@25, @"abc", @7.2];

void (^print)(NSUInteger index) = ^(NSUInteger index)
{
    NSLog(@"%@", array[index]);
};

for (int n = 0; n < 3; n++)
    print(n);

Example with __block:

__block NSArray *array;

void (^print)(NSUInteger index) = ^(NSUInteger index)
{
    NSLog(@"%@", array[index]);
};

array = @[@25, @"abc", @7.2];

for (int n = 0; n < 3; n++)
    print(n);

Note that it’s a little less efficient to use __block if you don’t actually need to modify the variable inside the block and have it reflected outside.