On 21 Nov 2012, at 10:56, Andreas Grosam <agro...@onlinehome.de> wrote:
> I've defined a class Foo that defines a block via a property:
>
> @property (copy) void (^progressHandler)(RXProgressState progressState,
> NSInteger totalBytesWritten, NSInteger totalBytesExpectedToWrite);
>
> The property is synthesized by the compiler, ARC enabled.
>
> The block is usually invoked on a private thread for several times until an
> action finishes (an asynchronous NSURLConnection).
> A client of class Foo should be able to set the block property at any time
> from any thread - that is it should also be able to set the block to NULL.
> Now, I'm worried about thread safety.
>
> First, is it a good measurement to use a property with the "atomic" attribute?
>
> Secondly, I'm invoking the block as follows (from within a
> NSURLConnectionDelegate method):
>
> progress_handler_block_t block;
> if ( (block=self.progressHandler) != NULL) {
> block(RXProgressStateStart, 0, self.source.size);
> }
>
>
> since it appears to me, that
>
> if (_progressHandler) {
> _progressHandler(RXProgressStateStart, 0, self.source.size);
> }
>
> or
>
> if (self.progressHandler) {
> self.progressHandler(RXProgressStateStart, 0, self.source.size);
> }
>
> isn't thread safe.
People are making this seem very complicated!
You say you're running with ARC, so the way you are doing things is fine if you
mark the property as atomic to make sure the contents are not inadvertently
released 'during' your call to the accessor. The local reference to the block
will retain it after it's returned from the accessor.
The only caveat (and I think this is already obvious to you given what you've
said) is that the 'old' block might be run instead of the 'new' block if
another thread assigns to the property between the "block=self.progressHandler"
and your invocation of the block.
Jamie.
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