NSOperation 依赖和 completionBlock

Question

我们遇到了一个关于NSOperationQueue的简单问题，简单的操作逻辑如下：

self.queue = [[NSOperationQueue alloc] init];

NSOperation *operationA = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"- Running operation A");
    [NSThread sleepForTimeInterval:1.2];
    NSLog(@"- Done operation A");
}];

NSOperation *operationB = [NSBlockOperation blockOperationWithBlock:^{
    NSLog(@"- Running operation B");
    [NSThread sleepForTimeInterval:2];
    NSLog(@"- Done operation B");
}];

[operationA setCompletionBlock:^{
    NSLog(@"-- Completion Block A");
}];

[operationB setCompletionBlock:^{
    NSLog(@"-- Completion Block B");
}];

[operationB addDependency:operationA];
[self.queue addOperations:@[operationA, operationB] waitUntilFinished:NO];

这是最终输出

2015-12-21 14:59:57.463 SampleProject[18046:310901] - Running operation A
2015-12-21 14:59:58.664 SampleProject[18046:310901] - Done operation A
2015-12-21 14:59:58.664 SampleProject[18046:310900] - Running operation B
2015-12-21 14:59:58.664 SampleProject[18046:310904] -- Completion Block A
2015-12-21 15:00:00.736 SampleProject[18046:310900] - Done operation B
2015-12-21 15:00:00.736 SampleProject[18046:310904] -- Completion Block B

正如我们所见，操作B在操作A的completionBlock之前执行。在我们的实际应用中，我们有很多 operation A 而只有一个 operation B 依赖于所有 operation A.但是我们遇到的问题是 operationB 在最后一个 operation A 的完成块被调用之前启动，这通常会给信息操作 B.

如何让 操作 B 在所有 操作 A 的完成块之后执行？

Answer 1

为什么不能调用完成块a里面的操作，这就是完成块的作用。

[operationA setCompletionBlock:^{
    NSLog(@"-- Completion Block A");
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    NSOperation *operationB = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"- Running operation B");
        [NSThread sleepForTimeInterval:2];
        NSLog(@"- Done operation B");
    }];
    [queue addOperations:@[operationB] waitUntilFinished:NO];
}];


[operationA setCompletionBlock:^{
    NSLog(@"-- Completion Block A when we dont need B");
}];

有一些更好的方法来执行此操作，而不是使用

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
     [self operationB];
}

Answer 2

正如您在测试完成块中发现的那样，它们不是 'part of the queue'，而是运行在操作队列之外（在另一个线程上）。因此，操作 A 的 completionBlock 将运行与操作 B 同时（大概）

我建议您重构代码以删除所有完成块。

你说你正在使用 completionBlocks 将信息从操作 A 传递到 B，对此有两种选择：为 B 提供所有 A 的引用（不是弱的）所以当 B 运行s 它可以从所有 A 中选择结果。或者，如果由于某种原因无法在 B 运行s 之前保留所有 A，则将您的 completionBlock 重新创建为另一个 NSOperation:

NSOperation *operationA = [NSBlockOperation blockOperationWithBlock:^{
    // do stuff
}];

NSOperation *operationATail = [NSBlockOperation blockOperationWithBlock:^{
    // do completionBlock stuff
}];

[operationATail addDependency:operationA];
[operationB addDependency:operationATail];
[self.queue addOperations:@[operationA, operationATail, operationB] waitUntilFinished:NO];

Answer 3

避免完成块 - 它们 out-of-queue 机制不合适用于同步任何操作或其通信。
引入依赖关系（B 依赖于 A）意味着 B 将仅在 A 成功完成后运行。

出于这个原因 - 任何简单的数据对象都可以用来在这两个操作之间安全地“传递信息” - 可以简单地共享它（只要你在定义这两个操作的块之外创建它就足够了）操作。当“B”运行s时，它可以假设“A”已经将需要的信息放入数据对象中，并简单地访问它。

self.queue = [[NSOperationQueue alloc] init];

NSMutableDictionary *info = [NSMutableDictionary new]; // data object for communication

NSOperation *operationA = [NSBlockOperation blockOperationWithBlock:^{
  NSLog(@"- Running operation A");
 [NSThread sleepForTimeInterval:1.2];

  info[@"DoThis"] = @YES;
  info[@"DoThat"] = @NO;
  NSLog(@"- Done operation A");
}];

NSOperation *operationB = [NSBlockOperation blockOperationWithBlock:^{
   NSLog(@"- Running operation B");

  if ([info[@"DoThis"] boolValue] NSLog(@"Op A said to do this.");
  if ([info[@"DoThat"] boolValue] NSLog(@"Op A said to do that.");

  [NSThread sleepForTimeInterval:2];
  NSLog(@"- Done operation B");
}];

[operationB addDependency:operationA];
[self.queue addOperations:@[operationA, operationB] waitUntilFinished:NO];

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