ios - NSOperationQueue : cancel an operation after a timeout given

Question

基本上，如果我添加到队列的操作在特定超时后没有响应，我想执行取消:

NSOperationQueue *   queue = ...

[self.queue addOperationWithBlock:^{
        // my block...
    } timeoutInSeconds:5.0 hasTimedOutWithBlock:^{
        // called after 5.0, operation should be canceled at the end
}];

谢谢大家!

Answer 1

你可以按照你的要求做一些事情，但我可能建议向第一个 block 添加一个参数，第一个 block 可以通过该参数检查操作是否被取消。

[queue addOperationWithBlock:^(NSOperation *operation) {

    // do something slow and synchronous here, 

    // if this consists of a loop, check (and act upon) `[operation isCancelled]` periodically 

} timeout:5.0 timeoutBlock:^{

    // what else to do when the timeout occurred

}];

也许您不需要检查 isCancelled，但在某些情况下您会(通常响应取消的负担取决于操作本身)，因此添加这可能是一个谨慎的参数.

无论如何，如果那是您想要的，您可以执行如下操作:

@implementation NSOperationQueue (Timeout)

- (NSOperation *)addOperationWithBlock:(void (^)(NSOperation *operation))block timeout:(CGFloat)timeout timeoutBlock:(void (^)(void))timeoutBlock
{
    NSBlockOperation *blockOperation = [[NSBlockOperation alloc] init];  // create operation
    NSBlockOperation __weak *weakOperation = blockOperation;             // prevent strong reference cycle

    // add call to caller's provided block, passing it a reference to this `operation`
    // so the caller can check to see if the operation was canceled (i.e. if it timed out)

    [blockOperation addExecutionBlock:^{
        block(weakOperation);
    }];

    // add the operation to this queue

    [self addOperation:blockOperation];

    // if unfinished after `timeout`, cancel it and call `timeoutBlock`

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(timeout * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        // if still in existence, and unfinished, then cancel it and call `timeoutBlock`

        if (weakOperation && ![weakOperation isFinished]) {
            [weakOperation cancel];
            if (timeoutBlock) {
                timeoutBlock();
            }
        }
    });

    return blockOperation;
}

@end

提供了该代码示例后，我必须承认，在极少数情况下，类似上述的内容可能会有用。一般来说，使用另一种模式会更好地解决这个问题。大多数时候，当你想要一个可取消的操作时，你会实现一个 NSOperation 子类(通常是一个并发的 NSOperation 子类)。请参阅 Operation Queues 的定义自定义操作对象部分Concurrency Programming Guide 章节以获取更多信息。