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我正在使用 Core Data 开发 iOS 应用程序，我从 Core Data 异步获取结果。我需要找到一种方法来实现此提取的取消功能，以防用户决定他们已经等待太久，并希望在中途取消提取。我知道使用 NSProgress 在 iOS 8 中使用 Core Data 是可能的，但我找不到有关如何执行此操作的任何示例或示例代码。

这是我的示例获取方法：

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"MyObject"];
fetchRequest.predicate = [NSPredicate predicateWithFormat:@"read == %@", @(NO)];

NSPersistentStoreAsynchronousFetchResultCompletionBlock resultBlock = ^(NSAsynchronousFetchResult *result) {
   NSLog(@"Number of Unread Items: %ld", (long)result.finalResult.count);   

   [result.progress removeObserver:self
                        forKeyPath:@"completedUnitCount"
                           context:ProgressObserverContext];

   [result.progress removeObserver:self
                        forKeyPath:@"totalUnitCount"
                           context:ProgressObserverContext];
};

NSAsynchronousFetchRequest *asyncFetch = [[NSAsynchronousFetchRequest alloc]
                                          initWithFetchRequest:fetchRequest
                                          completionBlock:resultBlock];

[context performBlock:^{
    //Assumption here is that we know the total in advance and supply it to the NSProgress instance
    NSProgress *progress = [NSProgress progressWithTotalUnitCount:preComputedCount];
    [progress becomeCurrentWithPendingUnitCount:1];

    NSAsynchronousFetchResult *result = (NSAsynchronousFetchResult *)[context 
                                                               executeRequest:asyncFetch 
                                                                        error:nil];

    [result.progress addObserver:self
                 forKeyPath:@"completedUnitCount"
                    options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew
                    context:ProgressObserverContext];

    [result.progress addObserver:self
                 forKeyPath:@"totalUnitCount"
                    options:NSKeyValueObservingOptionOld|NSKeyValueObservingOptionNew
                    context:ProgressObserverContext];

    [progress resignCurrent];
}];

我意识到通过使用 NSProgress 取消我的获取，我通过了 NSUserCancelled 错误，但是我想再看一个如何完成此操作的示例。

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