"__block" variable results in nil value when go out of block

Question

I wanna use __block variable to get value in block. But when out of block, the __block variable seems to be nil. Why this would happen?

    NSString *fileName = [Tools MD5Encode:url];
    __block NSString *filePath = nil;
    [fileList enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        NSString *aFileName = obj;
        if ([aFileName isEqualToString:fileName]) {
            NSString *path = [VERSIONS_INFO_DATA_DIRECTORY stringByAppendingPathComponent:aFileName];
            filePath = path;
            NSLog(@"filePath1 %@", filePath);
            *stop = YES;
        }
    }];
    //NSLog(@"filePath2 %@", filePath);
    //filePath seems to be nil
    return filePath;

When I change the code to [path copy], it works. But I have no idea whether this is a good idea. Any decision?

    NSString *fileName = [Tools MD5Encode:url];
    __block NSString *filePath = nil;
    [fileList enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        NSString *aFileName = obj;
        if ([aFileName isEqualToString:fileName]) {
            NSString *path = [VERSIONS_INFO_DATA_DIRECTORY stringByAppendingPathComponent:aFileName];
            filePath = [path copy];
            NSLog(@"filePath1 %@", filePath);
            *stop = YES;
        }
    }];
    //NSLog(@"filePath2 %@", filePath);
    return [filePath autorelease];

Answer 1

http://www.mikeash.com/pyblog/friday-qa-2011-09-30-automatic-reference-counting.html

Specifically:

Without ARC, __block also has the side effect of not retaining its contents when it's captured by a block. Blocks will automatically retain and release any object pointers they capture, but __block pointers are special-cased and act as a weak pointer. It's become a common pattern to rely on this behavior by using __block to avoid retain cycles.

Under ARC, __block now retains its contents just like other captured object pointers. Code that uses __block to avoid retain cycles won't work anymore. Instead, use __weak as described above.

So you need to copy.