如何使用带有调度gcd的NSOperationQueue和NSBlockOperation？ [英] How to work with NSOperationQueue and NSBlockOperation with dispatch gcd?
问题描述
这里是代码
@interface ViewController ()
@property (nonatomic, strong) NSOperationQueue *queue;
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
_queue = [[NSOperationQueue alloc] init];
NSBlockOperation *aBlockOperation = [[NSBlockOperation alloc] init];
__weak NSBlockOperation* aWeakBlockOperation = aBlockOperation;
[aBlockOperation addExecutionBlock:^{
NSLog(@"queue should still have the operation. And it does. yay!: %@", [_queue operations]); // This should print correctly. It will show the NSBlock operation correctly residing inside the NSOperationQueue
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
NSLog(@"Now queue is empty??: %@", [_queue operations]); // This should print as being empty
NSLog(@"And a weak block is nil???: %@", aWeakBlockOperation); // This should print out **nil**
if (![aWeakBlockOperation isCancelled]) {
// Now i have no way to check the operation
}
});
}];
[_queue addOperation:aBlockOperation];
@end
[编辑]
目标是要有这样的用户交互：
屏幕上有一个tableView包含5个或更多单元格。用户单击单元格时，后台进程将执行需要一段时间的后台进程。该应用程序将每隔3秒检查一次用户是否单击了另一个单元格。如果用户单击了另一个单元格，则应从队列中取消当前操作，并开始处理用户单击的新单元格。
[Edit] The goal is to have a user interaction like this: There is a tableView on screen with 5 or more cells. When ever a user click a cell, background process will perform background process that will take a while. The App will, at 3 second intervals, check to see if the user clicked on another cell. If the user clicked on another cell, I should cancel the current operation from queue, and begin processing the new one the user clicked on.
从上面的代码中我有2个我无法解决的问题。
From the code above i have 2 problems i cant solve.
-
我如何做到这一点，以便弱引用不会在dispatch_after块中释放？将其放置在此处的目的是将应用暂停3秒钟。如果dispatch_after不正确，那么我在那使用什么代码防止它变为nil？
How do i make it so that my weak reference isnt deallocated in the dispatch_after block? The goal of putting it there is to pause the app for exactly 3 seconds. If dispatch_after is incorrect, then what code do i use there to prevent it becoming nil?
为什么在我调用dispatch_after之后我的NSOperationQueue变成空？有没有办法使它不为空？
Why is it that my NSOperationQueue become empty after I call dispatch_after? Is there a way to make it not become empty?
推荐答案
dispatch_after 计划该块并立即返回。因此，您的 NSBlockOperation 的executionBlock几乎无事可做-它立即完成并从队列中删除。那时，该操作被释放，因此弱引用在以后调用 dispatch_after 块之前变为零。
dispatch_after schedules the block and returns immediately. So, your NSBlockOperation's executionBlock has almost no work to do — it immediately finishes and is removed from the queue. At that time, the operation is released and so the weak reference becomes nil before the dispatch_after block is called later.
如果首先执行 dispatch_after 并从该块内部计划操作，则可能会满足您的需求。您可以只使用 sleep ，但我不建议您这样做，因为您将不必要地阻塞线程。有关NSO操作和延迟的更多讨论，请参见此问题。
If you do the dispatch_after first and schedule the operation from inside that block, it might suit your needs. You could just use sleep, but I wouldn't recommend that since you will be unnecessarily blocking a thread. See this question for more discussion on NSOperation and delays.
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