__block directive, a simple explanation

Nowadays, if you pass a variable inside a block and try to assign a new value to it, you will encounter the error Variable is not assignable (missing __block type specifier).

This is a definitive improvement from the previous behavior where your variables simply remained unmodified. Now consider the following example:

    NSMutableString *digits = [NSMutableString stringWithString:@"12345"];

    void (^someBlock) (void) = ^{
        [digits appendString:@"67890"];

    };

    someBlock();

    NSLog(@"%@", digits);

Since digits was modified inside a block without the __block directive, you could think it will print the sequence "12345".

Surprisingly, digits actually holds the value "1234567890".

Why?

The answer is simple. When you make use of a variable inside a block. It is imported into the block structure as a const. Hence, the behavior is consistent with the declaration:

NSMutableString * const importedDigits = digits;

You would not be able to point importedDigits to a different address, but nothing stops you from changing the content of the object digits being referenced.

By using the __block directive, the variable is no longer imported to the block. Instead, it's passed by reference and its behavior will be consistent with the declaration:

NSMutableString **digitsReference = &digits;

In this case, you can move where digitsReference is pointing at and you will actually be affecting the content of the variable digits.

For more information on the topic, check out the Block Implementation Specification in Clang's documentation.