"Missing __block type specifier" compilation error after refactoring code but ok before refactor

ios, objective-c, objective-c-blocks

Solution

You seem to be confused over how parameters work in Objective-C.

In your original code you have:

MyObject* object = [[MyObject alloc] init];

which declares `object` as a local variable of the method. Then within the block you write:

object.image = [UIImage imageWithData:data];

So your block references the local variable `object` and (as you have no `__block` attribute on the declaration) the block gets a constant copy of the contents of variable. This is fine because you are not changing what is in `object`, which is a reference to your `MyObject` instance, but are calling a method on that instance which changes the internal state of that instance.[*]

Now let's look at your refactoring. You remove a chunk of your `someMethod`'s code and place it in a new method `fetchFbImage`. In `someMethod` you call `fetchFbImage`:

[self fetchFbImage:object.image forFbid:fbid];

This passes the current value of `object.image` to the method it does not pass the property in such a way that it can be assigned to in `fetchFbImage`. The type of the argument `targetImage` is `UIImage *` - a reference to an `UIImage` - it is not "a property of type `UIImage *`" - you cannot have arguments of such a type, properties cannot be passed around only their values or reference to objects which have properties.

When a method is called each parameter is effectively a local variable which is initialised to the argument value passed, so the above method call effectively performs:

UIImage *targetImage = object.image;

where `targetImage` is a local variable in the scope of `fetchFbImage` and `object` is the local variable in the scope of `someMethod`.

Now within the block inside of `fetchFbImage` you write:

targetImage = [UIImage imageWithData:data];

This is wrong for two reasons:

- You cannot assign to `targetImage` within the block. This is a local variable belonging to `fetchFbImage`, that local variable is not attributed with `__block`, and so the block has a constant copy of it - and you cannot assign to constants. This is why the compiler issues the error message.

- However your issue is bigger than this, you are assuming that an assignment to `fetchFbImage`'s local variable `targetImage` will some how invoke the property `object.image` - and there is no way it can do that. `targetImage` is just a local variable which was initialised with the value of `object.image` by the method call.

The only way to fix this is to pass `fetchFbImage` a reference to your `MyObject` instance and then within the block inside of `fetchFbImage` to assign to the `image` property of that object just as your pre-refectored code did.

So your code will look something like:

- (void) fetchFbImage:(MyObject *)targetObject forFbid:(NSString *)fbid
{
   ...
   targetObject.image = [UIImage imageWithData:data];
   ...
}

...

[self fetchFbImage:object forFbid:fbid];

HTH

Addendum

Seeing your comment on another answer it appears you would like `fetchFbImage` to have no knowledge of `MyObject` and be able to fetch images regardless of where they will be referenced from.

A simple way to do this is to follow the same design you have for `FBRequest` - use a completion handler. For convenience define a type for the completion block:

typedef void (^ImageConsumer)(NSImage *image);

Now define your method to take one of these:

- (void) fetchFbImageForFbid:(NSString *)fbid completionHandler:(ImageConsumer)handler

In your block within `fetchFbImageForFbid` pass the image to the handler:

handler([UIImage imageWithData:data]);

And in the call in `someMethod` pass a block which assigns the value to your property:

[self fetchFbImageForFbid:fbid
        completionHandler:^(NSImage *image) { object.image = image; }
];

[*] If this is confusing think of the reference as the address of a house. The address is constant, how many people are in the house is not. You can tell someone "go to this address, there is a great party on" - the contents ("state") of the house changes, its address does not.

Problem

Original source