/*

    File:       QWatchedOperationQueue.m

    Contains:   An NSOperationQueue subclass that calls you back when operations finish.

    Written by: DTS

    Copyright:  Copyright (c) 2011-2013 Apple Inc. All Rights Reserved.

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*/

#import "QWatchedOperationQueue.h"

@interface QWatchedOperationQueue ()

@property (atomic, assign, readwrite) id            target;

@end

@implementation QWatchedOperationQueue

@synthesize target       = _target;

@synthesize targetThread = _targetThread;

- (id)initWithTarget:(id)target

    // See comment in header.

{

    assert(target != nil);

    self = [super init];

    if (self != nil) {

        self->_target = target;

        self->_targetThread = [[NSThread currentThread] retain];

        self->_operationToAction = CFDictionaryCreateMutable(NULL, 0, &kCFTypeDictionaryKeyCallBacks, NULL);

        assert(self->_operationToAction != NULL);

    }

    return self;

}

- (void)dealloc

{

    // can be called on any thread

    if (self->_operationToAction != NULL) {

        assert(CFDictionaryGetCount(self->_operationToAction) == 0);

        CFRelease(self->_operationToAction);

    }

    [self->_targetThread release];

    [super dealloc];

}

- (void)addOperation:(NSOperation *)op finishedAction:(SEL)action

    // See comment in header.

{

    // can be called on any thread

    assert(op != nil);

    assert(action != nil);

    // Add the operation-to-action map entry.  We do this synchronised 

    // because we can be running on any thread.

    @synchronized (self) {

        assert( ! CFDictionaryContainsKey(self->_operationToAction, (const void *) op) );

        CFDictionarySetValue(self->_operationToAction, (const void *) op, (const void *) action);

    }

    // Retain ourselves so that we can't go away while the operation is running, 

    // and then observe the finished property of the operation.

    [self retain];

    [op addObserver:self forKeyPath:@"isFinished" options:0 context:&self->_target];

    // Call into the real NSOperationQueue.

    [self addOperation:op];

}

- (void)invalidate

    // See comment in header.

{

    assert([NSThread currentThread] == self.targetThread);

    // Because self.target is only referenced by this and -didFinishOperation:, 

    // and both of these can only be called on the target thread, this doesn't 

    // require synchronisation and is guaranteed to be effective against any 

    // 'in flight' operations.

    self.target = nil;

}

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

    // Called when the finished property of the operation changes.  We do very 

    // little here, but rather push the work off to the target thread.

{

    // can be called on any thread

    if (context == &self->_target) {

        assert([keyPath isEqual:@"isFinished"]);

        @synchronized (self) {

            assert( CFDictionaryContainsKey(self->_operationToAction, (const void *) object) );

        }

        assert([object isKindOfClass:[NSOperation class]]);

        // We ignore the change if isFinished is not set.  Various operations 

        // end up triggering KVO notification on isFinished when they're not 

        // actually finished so, just to be sure, we perform a definitive check 

        // here.

        if ( [((NSOperation *) object) isFinished] ) {

            [self performSelector:@selector(didFinishOperation:) onThread:self.targetThread withObject:object waitUntilDone:NO];

        }

    } else {

        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];

    }

}

- (void)didFinishOperation:(NSOperation *)op

    // Called on the target thread when an operation finishes. 

{

    SEL     action;

    assert([NSThread currentThread] == self.targetThread);

    assert([op isKindOfClass:[NSOperation class]]);

    assert([op isFinished]);

    // Pull the action out of the operation-to-action map, remembering the 

    // action that was required.

    @synchronized (self) {

        assert( CFDictionaryContainsKey(self->_operationToAction, (const void *) op) );

        action = (SEL) CFDictionaryGetValue(self->_operationToAction, (const void *) op);

        CFDictionaryRemoveValue(self->_operationToAction, (const void *) op);

        assert(action != nil);

    }

    // Remove ourselves as an observer for this operation. We can now 

    // safely release the retain on ourselves that we took in 

    // -addOperation:finishedAction:.

    [op removeObserver:self forKeyPath:@"isFinished"];

    [self autorelease];

    // If we haven't been invalidated, and the operation hasn't been 

    // cancelled, call the action method.

    // IMPORTANT: You have to be very careful here.  It's quite possible that the action 

    // we sent to target can call us recursively, perhaps to add more operations but, 

    // more interestingly, to call -invalidate.  This will, in turn, set target to nil. 

    // So it's important we not reference target after doing the -performSelector:withObject:.

    //

    // If you crash inside the -performSelector:withObject: code but target is nil, it's most 

    // likely the completion of this operation has caused the client to shut down, and 

    // the client is crashing, not because of problems with this class, but because it's failing 

    // to ensure that it's still alive during the shutdown process.

    if ( (self.target != nil) && ! [op isCancelled] ) {

        [self.target performSelector:action withObject:op];

    }

}

@end