关于C#：“__ block”变量在走出块时导致nil值

“__block” variable results in nil value when go out of block

我想使用__block变量来获取块中的值。但是当阻塞时，__block变量似乎是零。为什么会这样？

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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;

当我将代码更改为[路径复制]时，它可以工作。但我不知道这是不是一个好主意。任何决定？

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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];

相关讨论

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

特别：

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.

所以你需要复制。