How can I modify an ObjC method's parameters within a block?

Question

If I have an Objective-C method that takes an object parameter, and said method uses a block to do its work internally, is there a way to modify that object from within the block?

It is my understanding that blocks capture variables from their parent scope by referring to them within the block, and that they are copied by default. And if I want to be able to modify rather than work with a copy of a surrounding object, I can prefix its declaration with __block, but I'm unable to do so with method parameters since I didn't declare it myself, right?

For example:

- (void)doWorkWithString:(NSString *)someString
{
    [NSFoo doAwesomeClassMethodWithBlock:^{
        // How can I modify someString here directly?
        // By just changing someString, I'm changing the captured copy
    }];
}

Answer 1

"that takes an object parameter"

First of all, you are almost certainly confused by the fact that you cannot have a parameter or variable of object type. You can only have pointers to objects. So yes, non-__block variables that are captured by a block are copied by the block. But the variables here are either primitives or object pointers, not "objects".

If you only need to mutate the object that an object pointer points to, and that does not involve changing the pointer to point to another object, then you are not changing the variable. And since you are not changing the variable, all this "copying the variable" and __block stuff is completely irrelevant.

Answer 2

What you say about capture is correct; what you probably want to do is supply the objects that should be the subject of the block as arguments to it — much like if you were calling a C function. So e.g.

void (^ someBlock)(NSString *) =
    ^(NSString *someString)
    {
        NSLog(@"length is %d", [someString length]);
    };

...

someBlock(@"String 1");
someBlock(@"A second string");