On Dec 6, 2013, at 9:24 AM, Seth Willits <sli...@araelium.com> wrote:
> On Dec 6, 2013, at 8:05 AM, dangerwillrobinsondan...@gmail.com wrote:
>>>> On 6 Dec 2013, at 11:26 am, Graham Cox <graham....@bigpond.com> wrote:
>>>>
>>>> NSBlockOperation* op = [NSBlockOperation
>>>> blockOperationWithBlock:^
>>>> {
>>>> CGContextClipToRect( ncx, tileRect );
>>>>
>>>> [self drawTile:tileRect inContext:ncx];
>>>> }];
>>>
>>>
>>> A question for blocks experts:
>>>
>>> Is the value of <tileRect> here captured when the block is created,or when
>>> it is run?
>>>
>>> If the latter, it’s going to be probably wrong most of the time, so how can
>>> I make sure it is captured when the block is created?
>>>
>>
>>
>> It's the latter IIRC
>> You'll want to capture it outside and prefix it to be block scoped.
>
> No it’s not and no you don’t.
To expand on this, whenever you evaluate a block literal expression that uses
one or more normal local variables, those variables are immediately copied into
the block, essentially as if the block literal were a struct with a field that
looks exactly like the local variable. When you copy the block and it has to
“move" to the heap, the exact same structure gets allocated there and
member-by-member copied from the stack version. Any subsequent changes to the
variable aren’t tracked, because how could they be?
A __block variable is special; when you capture a __block variable, what’s
actually captured is a pointer to the __block variable’s byref struct. I’ve
seen a lot of confusion and over-use of __block variables, which can probably
be traced back to the name being somewhat misleading. You don’t need to make a
__block variable just to capture something in a block. You need to make a
__block variable only if it’s important to you that the block and the enclosing
function are working on the same variable, not just the same value; that is, if
you’re going to modify the variable in one place and need those modifications
to be seen by the other. The usual use case is a block that’s supposed to set
some variable in the enclosing scope, but you could also imagine a function
running a state machine and a long-lived block that’s supposed to react to
state changes.
John.
_______________________________________________
Cocoa-dev mailing list (Cocoa-dev@lists.apple.com)
Please do not post admin requests or moderator comments to the list.
Contact the moderators at cocoa-dev-admins(at)lists.apple.com
Help/Unsubscribe/Update your Subscription:
https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com
This email sent to arch...@mail-archive.com
