ios - 获取当前后台操作并处理它

Question

我想在后台操作完成时做一些事情，我正在使用以下代码来执行后台操作:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0) { () -> Void in
    //perform the background operation
})

但是换个角度看，如何获取当前的后台操作并进行处理呢？
非常感谢:)

Answer 1

一般不会“获取当前后台操作”。如果你想在后台操作完成时做一些事情，你通常只是把它放在 block 本身中。例如，如果你想在后台任务完成时更新 UI(在主队列上)，你可以这样做:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // perform the background operation

    // now do whatever you want when the above is done

    dispatch_async(dispatch_get_main_queue(), ^{
        // do whatever you want here
    });
});

如果您有一系列已分派(dispatch)到后台的 block ，并且您想等待所有 block 完成，您通常会使用分派(dispatch)组:

dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_queue_t notificationQueue = dispatch_get_main_queue();

dispatch_group_async(group, queue, ^{
    // do something
});

dispatch_group_async(group, queue, ^{
    // do something else
});

dispatch_group_notify(group, notificationQueue, ^{
    // whatever you want when the above dispatched blocks finish
});

或者您可以使用操作队列，并为给定操作设置一个 completionBlock，或者创建一个依赖于其他操作的新操作(仅当其他操作完成时才会触发)。

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
NSOperationQueue *completionQueue = [NSOperationQueue mainQueue];

NSOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{
    // do something
}];
[queue addOperation:operation1];

NSOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{
    // do something else
}];
[queue addOperation:operation2];

NSOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{
    // whatever you want when the above operations finish
}];
[completionOperation addDependency:operation1];
[completionOperation addDependency:operation2];
[completionQueue addOperation:completionOperation];

显然，上述所有示例都是在 Objective-C 中，但通用模式在 Swift 中也同样适用。

有关异步编程技术的更多信息，请参阅 Concurrency Programming Guide .或引用WWDC 2012 video - Asynchronous Design Patterns with Blocks, GCD, and XPC .