当 NSOperationQueue 完成所有任务时获取通知

Question

NSOperationQueue 有waitUntilAllOperationsAreFinished，但我不想同步等待它。我只想在队列完成时在 UI 中隐藏进度指示器。

最好的方法是什么？

我无法从我的NSOperations 发送通知，因为我不知道哪一个会是最后一个，并且[queue operations] 在收到通知时可能还不是空的（或者更糟糕的是 - 重新填充）。

Answer 1

使用 KVO 观察队列的 operations 属性，然后您可以通过检查 [queue.operations count] == 0 来判断队列是否已完成。

在您正在执行 KVO 的文件的某处，像这样声明 KVO 的上下文 (more info)：

static NSString *kQueueOperationsChanged = @"kQueueOperationsChanged";

设置队列时，请执行以下操作：

[self.queue addObserver:self forKeyPath:@"operations" options:0 context:&kQueueOperationsChanged];

然后在您的observeValueForKeyPath 中执行此操作：

- (void) observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object 
                         change:(NSDictionary *)change context:(void *)context
{
    if (object == self.queue && [keyPath isEqualToString:@"operations"] && context == &kQueueOperationsChanged) {
        if ([self.queue.operations count] == 0) {
            // Do something here when your queue has completed
            NSLog(@"queue has completed");
        }
    }
    else {
        [super observeValueForKeyPath:keyPath ofObject:object 
                               change:change context:context];
    }
}

（这是假设您的 NSOperationQueue 位于名为 queue 的属性中）

在您的对象完全解除分配之前的某个时间点（或者当它停止关心队列状态时），您需要像这样从 KVO 注销：

[self.queue removeObserver:self forKeyPath:@"operations" context:&kQueueOperationsChanged];

附录：iOS 4.0 有一个 NSOperationQueue.operationCount 属性，根据文档，它是 KVO 兼容的。不过，这个答案在 iOS 4.0 中仍然有效，因此对于向后兼容仍然有用。

Answer 2

如果您期望（或渴望）符合这种行为的东西：

t=0 add an operation to the queue.  queueucount increments to 1
t=1 add an operation to the queue.  queueucount increments to 2
t=2 add an operation to the queue.  queueucount increments to 3
t=3 operation completes, queuecount decrements to 2
t=4 operation completes, queuecount decrements to 1
t=5 operation completes, queuecount decrements to 0
<your program gets notified that all operations are completed>

您应该知道，如果将一些“短”操作添加到队列中，您可能会看到这种行为（因为操作是作为添加到队列的一部分开始的）：

t=0  add an operation to the queue.  queuecount == 1
t=1  operation completes, queuecount decrements to 0
<your program gets notified that all operations are completed>
t=2  add an operation to the queue.  queuecount == 1
t=3  operation completes, queuecount decrements to 0
<your program gets notified that all operations are completed>
t=4  add an operation to the queue.  queuecount == 1
t=5  operation completes, queuecount decrements to 0
<your program gets notified that all operations are completed>

在我的项目中，我需要知道最后一个操作何时完成，在大量操作被添加到串行 NSOperationQueue（即 maxConcurrentOperationCount=1）之后，并且只有在它们全部完成之后。

谷歌搜索我从一位 Apple 开发人员那里找到了这个声明，以回答“是串行 NSoperationQueue FIFO 吗？”这个问题。 --

如果所有操作具有相同的优先级（在 操作被添加到队列中）并且所有操作总是 - 当它们被放入操作队列时，isReady==YES，然后是串行 NSOperationQueue 是先进先出的。

克里斯·凯恩 可可框架，苹果

在我的情况下，可以知道最后一个操作何时添加到队列中。因此，在添加最后一个操作之后，我向队列中添加了另一个优先级较低的操作，它只发送队列已清空的通知。鉴于 Apple 的声明，这确保了仅在所有操作完成后才发送单个通知。

如果以不允许检测最后一个操作的方式添加操作（即非确定性），那么我认为您必须使用上面提到的 KVO 方法，并添加额外的保护逻辑来尝试检测是否可以添加更多操作。

:)

Answer 3

添加一个依赖于所有其他的 NSOperation 以使其最后运行怎么样？

Answer 4

另一种方法是使用 GCD。请参阅this 作为参考。

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

dispatch_group_async(group,queue,^{
 NSLog(@"Block 1");
 //run first NSOperation here
});

dispatch_group_async(group,queue,^{
 NSLog(@"Block 2");
 //run second NSOperation here
});

//or from for loop
for (NSOperation *operation in operations)
{
   dispatch_group_async(group,queue,^{
      [operation start];
   });
}

dispatch_group_notify(group,queue,^{
 NSLog(@"Final block");
 //hide progress indicator here
});

Answer 5

从 iOS 13.0 开始，operationCount 和 operation 属性已被弃用。自己跟踪队列中的操作数量并在它们全部完成后触发通知同样简单。此示例也适用于 Operation 的异步子类化。

class MyOperationQueue: OperationQueue {
            
    public var numberOfOperations: Int = 0 {
        didSet {
            if numberOfOperations == 0 {
                print("All operations completed.")
                
                NotificationCenter.default.post(name: .init("OperationsCompleted"), object: nil)
            }
        }
    }
    
    public var isEmpty: Bool {
        return numberOfOperations == 0
    }
    
    override func addOperation(_ op: Operation) {
        super.addOperation(op)
        
        numberOfOperations += 1
    }
    
    override func addOperations(_ ops: [Operation], waitUntilFinished wait: Bool) {
        super.addOperations(ops, waitUntilFinished: wait)
        
        numberOfOperations += ops.count
    }
    
    public func decrementOperationCount() {
        numberOfOperations -= 1
    }
}

下面是 Operation 的子类，便于异步操作

class AsyncOperation: Operation {
    
    let queue: MyOperationQueue

enum State: String {
    case Ready, Executing, Finished
    
    fileprivate var keyPath: String {
        return "is" + rawValue
    }
}

var state = State.Ready {
    willSet {
        willChangeValue(forKey: newValue.keyPath)
        willChangeValue(forKey: state.keyPath)
    }
    
    didSet {
        didChangeValue(forKey: oldValue.keyPath)
        didChangeValue(forKey: state.keyPath)
        
        if state == .Finished {
            queue.decrementOperationCount()
        }
    }
}

override var isReady: Bool {
    return super.isReady && state == .Ready
}

override var isExecuting: Bool {
    return state == .Executing
}

override var isFinished: Bool {
    return state == .Finished
}

override var isAsynchronous: Bool {
    return true
}

public init(queue: MyOperationQueue) {
    self.queue = queue
    super.init()
}

override func start() {
    if isCancelled {
        state = .Finished
        return
    }
    
    main()
    state = .Executing
}

override func cancel() {
    state = .Finished
}

override func main() {
    fatalError("Subclasses must override main without calling super.")
}

}

Answer 6

我就是这样做的。

设置队列，并注册操作属性中的更改：

myQueue = [[NSOperationQueue alloc] init];
[myQueue addObserver: self forKeyPath: @"operations" options: NSKeyValueObservingOptionNew context: NULL];

...和观察者（在本例中为self）实现：

- (void) observeValueForKeyPath:(NSString *) keyPath ofObject:(id) object change:(NSDictionary *) change context:(void *) context {

    if (
        object == myQueue
        &&
        [@"operations" isEqual: keyPath]
    ) {

        NSArray *operations = [change objectForKey:NSKeyValueChangeNewKey];

        if ( [self hasActiveOperations: operations] ) {
            [spinner startAnimating];
        } else {
            [spinner stopAnimating];
        }
    }
}

- (BOOL) hasActiveOperations:(NSArray *) operations {
    for ( id operation in operations ) {
        if ( [operation isExecuting] && ! [operation isCancelled] ) {
            return YES;
        }
    }

    return NO;
}

在这个例子中，“spinner”是一个UIActivityIndicatorView，表示正在发生一些事情。显然你可以改变以适应...

Answer 7

我正在使用一个类别来执行此操作。

NSOperationQueue+Completion.h

//
//  NSOperationQueue+Completion.h
//  QueueTest
//
//  Created by Artem Stepanenko on 23.11.13.
//  Copyright (c) 2013 Artem Stepanenko. All rights reserved.
//

typedef void (^NSOperationQueueCompletion) (void);

@interface NSOperationQueue (Completion)

/**
 * Remarks:
 *
 * 1. Invokes completion handler just a single time when previously added operations are finished.
 * 2. Completion handler is called in a main thread.
 */

- (void)setCompletion:(NSOperationQueueCompletion)completion;

@end

NSOperationQueue+Completion.m

//
//  NSOperationQueue+Completion.m
//  QueueTest
//
//  Created by Artem Stepanenko on 23.11.13.
//  Copyright (c) 2013 Artem Stepanenko. All rights reserved.
//

#import "NSOperationQueue+Completion.h"

@implementation NSOperationQueue (Completion)

- (void)setCompletion:(NSOperationQueueCompletion)completion
{
    NSOperationQueueCompletion copiedCompletion = [completion copy];

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

        dispatch_async(dispatch_get_main_queue(), ^{
            copiedCompletion();
        });
    });
}

@end

用法：

NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{
    // ...
}];

NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{
    // ...
}];

[operation2 addDependency:operation1];

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[queue addOperations:@[operation1, operation2] waitUntilFinished:YES];

[queue setCompletion:^{
    // handle operation queue's completion here (launched in main thread!)
}];

来源：https://gist.github.com/artemstepanenko/7620471

Answer 8

使用KVO观察队列的operationCount属性怎么样？然后，当队列变空以及停止为空时，您会听到它。处理进度指示器可能就像执行以下操作一样简单：

[indicator setHidden:([queue operationCount]==0)]

Answer 9

添加最后一个操作，如：

NSInvocationOperation *callbackOperation = [[NSInvocationOperation alloc] initWithTarget:object selector:selector object:nil];

所以：

- (void)method:(id)object withSelector:(SEL)selector{
     NSInvocationOperation *callbackOperation = [[NSInvocationOperation alloc] initWithTarget:object selector:selector object:nil];
     [callbackOperation addDependency: ...];
     [operationQueue addOperation:callbackOperation]; 

}

Answer 10

ReactiveObjC 我觉得这很好用：

// skip 1 time here to ignore the very first call which occurs upon initialization of the RAC block
[[RACObserve(self.operationQueue, operationCount) skip:1] subscribeNext:^(NSNumber *operationCount) {
    if ([operationCount integerValue] == 0) {
         // operations are done processing
         NSLog(@"Finished!");
    }
}];

Answer 11

仅供参考，您可以使用 swift 3 中的 GCD dispatch_group 实现此目的。您可以在所有任务完成后收到通知。

let group = DispatchGroup()

    group.enter()
    run(after: 6) {
      print(" 6 seconds")
      group.leave()
    }

    group.enter()
    run(after: 4) {
      print(" 4 seconds")
      group.leave()
    }

    group.enter()
    run(after: 2) {
      print(" 2 seconds")
      group.leave()
    }

    group.enter()
    run(after: 1) {
      print(" 1 second")
      group.leave()
    }


    group.notify(queue: DispatchQueue.global(qos: .background)) {
      print("All async calls completed")
}

Answer 12

你可以创建一个新的NSThread，或者在后台执行一个选择器，然后在那里等待。当NSOperationQueue 结束时，您可以发送自己的通知。

我正在考虑类似的事情：

- (void)someMethod {
    // Queue everything in your operationQueue (instance variable)
    [self performSelectorInBackground:@selector(waitForQueue)];
    // Continue as usual
}

...

- (void)waitForQueue {
    [operationQueue waitUntilAllOperationsAreFinished];
    [[NSNotificationCenter defaultCenter] postNotification:@"queueFinished"];
}

Answer 13

如果你使用这个Operation 作为你的基类，你可以将whenEmpty {} 块传递给OperationQueue：

let queue = OOperationQueue()
queue.addOperation(op)
queue.addOperation(delayOp)

queue.addExecution { finished in
    delay(0.5) { finished() }
}

queue.whenEmpty = {
    print("all operations finished")
}

Answer 14

没有 KVO

private let queue = OperationQueue()

private func addOperations(_ operations: [Operation], completionHandler: @escaping () -> ()) {
    DispatchQueue.global().async { [unowned self] in
        self.queue.addOperations(operations, waitUntilFinished: true)
        DispatchQueue.main.async(execute: completionHandler)
    }
}

Answer 15

如果您来这里是在寻找使用 combine 的解决方案 - 我最终只是在听我自己的状态对象。

@Published var state: OperationState = .ready
var sub: Any?

sub = self.$state.sink(receiveValue: { (state) in
 print("state updated: \(state)")
})