ESRunLoopOperation.m

#import "ESRunLoopOperation.h"

@interface ESRunLoopOperation ()
// read/write versions of public properties
//@property ESOperationState state;
@property (copy) NSError *error;
@end

@implementation ESRunLoopOperation
{
@private
	NSRecursiveLock *_stateLock;
}
@synthesize state=_state;

- (id)init
{
    self = [super init];
    if (self != nil) 
	{
		NSAssert((_state == kESOperationStateInited), @"Operation initialized with invalid state: %d", _state);
		if (_state != kESOperationStateInited)
			return nil;
		_stateLock = [[NSRecursiveLock alloc] init];
	}
    return self;
}

- (void)dealloc
{
	NSAssert((_state != kESOperationStateExecuting), @"Run loop operation dealloced while still executing");
}

#pragma mark * Properties

- (NSThread *)actualRunLoopThread
// Returns the effective run loop thread, that is, the one set by the user 
// or, if that's not set, the main thread.
{
    NSThread *result;
    result = self.runLoopThread;
    if (result == nil)
        result = [NSThread mainThread];
    return result;
}

- (BOOL)isActualRunLoopThread
// Returns YES if the current thread is the actual run loop thread.
{
    return [[NSThread currentThread] isEqual:self.actualRunLoopThread];
}

- (NSSet *)actualRunLoopModes
{
    NSSet * result;
    result = self.runLoopModes;
    if ((result == nil) || 
		([result count] == 0))
        result = [NSSet setWithObject:NSDefaultRunLoopMode];
    return result;
}

#pragma mark * Core state transitions

- (ESOperationState)state
{
	ESOperationState state;
	[_stateLock lock];
	state = _state;
	[_stateLock unlock];
    return state;
}

- (void)setState:(ESOperationState)newState
// Change the state of the operation, sending the appropriate KVO notifications.
{
    // any thread
	
	[_stateLock lock];
	
	ESOperationState oldState;
	
	// The following check is really important.  The state can only go forward, and there 
	// should be no redundant changes to the state (that is, newState must never be 
	// equal to _state).
	
	NSAssert((newState > _state), @"Invalid state transition from %d to %d", _state, newState);
	
	// Transitions from executing to finished must be done on the run loop thread.
	
	NSAssert(((newState != kESOperationStateFinished) || self.isActualRunLoopThread), @"Attempted transition to finish on non run loop thread");
	
	// inited    + executing -> isExecuting
	// inited    + finished  -> isFinished
	// executing + finished  -> isExecuting + isFinished
	
	oldState = _state;
	if ((newState == kESOperationStateExecuting) || (oldState == kESOperationStateExecuting))
		[self willChangeValueForKey:@"isExecuting"];
	if (newState == kESOperationStateFinished)
		[self willChangeValueForKey:@"isFinished"];
	_state = newState;
	if (newState == kESOperationStateFinished)
		[self didChangeValueForKey:@"isFinished"];
	if ((newState == kESOperationStateExecuting) || (oldState == kESOperationStateExecuting))
		[self didChangeValueForKey:@"isExecuting"];
	
	[_stateLock unlock];
}

- (void)startOnRunLoopThread
// Starts the operation. The actual -start method is very simple, 
// deferring all of the work to be done on the run loop thread by this 
// method.
{
    NSParameterAssert(self.isActualRunLoopThread);
	// If we got canceled and finished waiting for this to get scheduled, bail
	if (self.state != kESOperationStateExecuting)
		return;
    if ([self isCancelled]) 
	{
        // We were cancelled before we even got running.  Flip the the finished 
        // state immediately.
        [self finishWithError:[NSError errorWithDomain:NSCocoaErrorDomain code:NSUserCancelledError userInfo:nil]];
    }
	else 
	{
        [self operationDidStart];
    }
}

- (void)cancelOnRunLoopThread
// Cancels the operation.
{
    NSParameterAssert(self.isActualRunLoopThread);
	
    // We know that a) state was kQRunLoopOperationStateExecuting when we were 
    // scheduled (that's enforced by -cancel), and b) the state can't go 
    // backwards (that's enforced by -setState), so we know the state must 
    // either be kQRunLoopOperationStateExecuting or kQRunLoopOperationStateFinished. 
    // We also know that the transition from executing to finished always 
    // happens on the run loop thread.  Thus, we don't need to lock here.  
    // We can look at state and, if we're executing, trigger a cancellation.
    
    if (self.state == kESOperationStateExecuting)
        [self finishWithError:[NSError errorWithDomain:NSCocoaErrorDomain code:NSUserCancelledError userInfo:nil]];
}

- (BOOL)finishWithError:(NSError *)error
{
	NSAssert(self.isActualRunLoopThread, @"Entered finishWithError from non run loop thread");
	// If we got canceled and finished waiting for this to get scheduled, bail
	if (self.state != kESOperationStateExecuting)
	{
		return NO;
	}
    // error may be nil
    if (self.error == nil)
	{
		self.error = error;
	}
    [self operationWillFinish];
    self.state = kESOperationStateFinished;
	return YES;
}

#pragma mark * Subclass override points

- (void)operationDidStart
{
	NSAssert(self.isActualRunLoopThread, @"Entered operationDidStart from non run loop thread");
}

- (void)operationWillFinish
{
	NSAssert(self.isActualRunLoopThread, @"Entered operationWillFinish from non run loop thread");
}

#pragma mark * Overrides

- (BOOL)isConcurrent
{
    // any thread
    return YES;
}

- (BOOL)isExecuting
{
    // any thread
    return (self.state == kESOperationStateExecuting);
}

- (BOOL)isFinished
{
    // any thread
    return (self.state == kESOperationStateFinished);
}

- (void)start
{
    // any thread
	
	NSAssert((self.state == kESOperationStateInited), @"Operation started in invalid state %d", self.state);
    
    // We have to change the state here, otherwise isExecuting won't necessarily return 
    // true by the time we return from -start.  Also, we don't test for cancellation 
    // here because that would a) result in us sending isFinished notifications on a 
    // thread that isn't our run loop thread, and b) confuse the core cancellation code, 
    // which expects to run on our run loop thread.  Finally, we don't have to worry 
    // about races with other threads calling -start.  Only one thread is allowed to 
    // start us at a time.
    
    self.state = kESOperationStateExecuting;
    [self performSelector:@selector(startOnRunLoopThread) 
				 onThread:self.actualRunLoopThread 
			   withObject:nil 
			waitUntilDone:NO 
					modes:[self.actualRunLoopModes allObjects]];
}

- (void)cancel
{
    BOOL runCancelOnRunLoopThread;
    BOOL oldValue;
	
    // any thread
	
    // We need to synchronize here to avoid state changes to isCancelled and state
    // while we're running.
    
    @synchronized (self) 
	{
        oldValue = [self isCancelled];
        
        // Call our super class so that isCancelled starts returning true immediately.
        
        [super cancel];
        
        // If we were the one to set isCancelled (that is, we won the race with regards 
        // other threads calling -cancel) and we're actually running (that is, we lost 
        // the race with other threads calling -start and the run loop thread finishing), 
        // we schedule to run on the run loop thread.
		
        runCancelOnRunLoopThread = !(oldValue && (self.state == kESOperationStateExecuting));
    }
    if (runCancelOnRunLoopThread)
        [self performSelector:@selector(cancelOnRunLoopThread) 
					 onThread:self.actualRunLoopThread 
				   withObject:nil 
				waitUntilDone:YES 
						modes:[self.actualRunLoopModes allObjects]];
}

@end

/*
 Theory of Operation (http://developer.apple.com/library/ios/#samplecode/MVCNetworking/Listings/Networking_QRunLoopOperation_m.html#//apple_ref/doc/uid/DTS40010443-Networking_QRunLoopOperation_m-DontLinkElementID_31)
 -------------------
 Some critical points:
 
 1. By the time we're running on the run loop thread, we know that all further state 
 transitions happen on the run loop thread.  That's because there are only three 
 states (inited, executing, and finished) and run loop thread code can only run 
 in the last two states and the transition from executing to finished is 
 always done on the run loop thread.
 
 2. -start can only be called once.  So run loop thread code doesn't have to worry 
 about racing with -start because, by the time the run loop thread code runs, 
 -start has already been called.
 
 3. -cancel can be called multiple times from any thread.  Run loop thread code 
 must take a lot of care with do the right thing with cancellation.
 
 Some state transitions:
 
 1. init -> dealloc
 2. init -> cancel -> dealloc
 XXX  3. init -> cancel -> start -> finish -> dealloc
 4. init -> cancel -> start -> startOnRunLoopThreadThread -> finish dealloc
 !!!  5. init -> start -> cancel -> startOnRunLoopThreadThread -> finish -> cancelOnRunLoopThreadThread -> dealloc
 XXX  6. init -> start -> cancel -> cancelOnRunLoopThreadThread -> startOnRunLoopThreadThread -> finish -> dealloc
 XXX  7. init -> start -> cancel -> startOnRunLoopThreadThread -> cancelOnRunLoopThreadThread -> finish -> dealloc
 8. init -> start -> startOnRunLoopThreadThread -> finish -> dealloc
 9. init -> start -> startOnRunLoopThreadThread -> cancel -> cancelOnRunLoopThreadThread -> finish -> dealloc
 !!! 10. init -> start -> startOnRunLoopThreadThread -> cancel -> finish -> cancelOnRunLoopThreadThread -> dealloc
 11. init -> start -> startOnRunLoopThreadThread -> finish -> cancel -> dealloc
 
 Markup:
 XXX means that the case doesn't happen.
 !!! means that the case is interesting.
 
 Described:
 
 1. It's valid to allocate an operation and never run it.
 2. It's also valid to allocate an operation, cancel it, and yet never run it.
 3. While it's valid to cancel an operation before it starting it, this case doesn't 
 happen because -start always bounces to the run loop thread to maintain the invariant 
 that the executing to finished transition always happens on the run loop thread.
 4. In this -startOnRunLoopThread detects the cancellation and finishes immediately.
 5. Because the -cancel can happen on any thread, it's possible for the -cancel 
 to come in between the -start and the -startOnRunLoop thread.  In this case 
 -startOnRunLoopThread notices isCancelled and finishes straightaway.  And 
 -cancelOnRunLoopThread detects that the operation is finished and does nothing.
 6. This case can never happen because -performSelecton:onThread:xxx 
 callbacks happen in order, -start is synchronised with -cancel, and -cancel 
 only schedules if -start has run.
 7. This case can never happen because -startOnRunLoopThread will finish immediately 
 if it detects isCancelled (see case 5).
 8. This is the standard run-to-completion case. 
 9. This is the standard cancellation case.  -cancelOnRunLoopThread wins the race 
 with finish, and it detects that the operation is executing and actually cancels. 
 10. In this case the -cancelOnRunLoopThread loses the race with finish, but that's OK 
 because -cancelOnRunLoopThread already does nothing if the operation is already 
 finished.
 11. Cancellating after finishing still sets isCancelled but has no impact 
 on the RunLoop thread code.
 */

/*
 File:       QRunLoopOperation.m
 
 Contains:   An abstract subclass of NSOperation for async run loop based operations.
 
 Written by: DTS
 
 Copyright:  Copyright (c) 2010 Apple Inc. All Rights Reserved.
 
 Disclaimer: IMPORTANT: This Apple software is supplied to you by Apple Inc.
 ("Apple") in consideration of your agreement to the following
 terms, and your use, installation, modification or
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 not use, install, modify or redistribute this Apple software.
 
 In consideration of your agreement to abide by the following
 terms, and subject to these terms, Apple grants you a personal,
 non-exclusive license, under Apple's copyrights in this
 original Apple software (the "Apple Software"), to use,
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 without modifications, in source and/or binary forms; provided
 that if you redistribute the Apple Software in its entirety and
 without modifications, you must retain this notice and the
 following text and disclaimers in all such redistributions of
 the Apple Software. Neither the name, trademarks, service marks
 or logos of Apple Inc. may be used to endorse or promote
 products derived from the Apple Software without specific prior
 written permission from Apple.  Except as expressly stated in
 this notice, no other rights or licenses, express or implied,
 are granted by Apple herein, including but not limited to any
 patent rights that may be infringed by your derivative works or
 by other works in which the Apple Software may be incorporated.
 
 The Apple Software is provided by Apple on an "AS IS" basis. 
 APPLE MAKES NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 WITHOUT LIMITATION THE IMPLIED WARRANTIES OF NON-INFRINGEMENT,
 MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, REGARDING
 THE APPLE SOFTWARE OR ITS USE AND OPERATION ALONE OR IN
 COMBINATION WITH YOUR PRODUCTS.
 
 IN NO EVENT SHALL APPLE BE LIABLE FOR ANY SPECIAL, INDIRECT,
 INCIDENTAL OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
 TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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 */

//
//  ESRunLoopOperation.m
//
//  Created by Doug Russell
//  Copyright (c) 2011 Doug Russell. All rights reserved.
//  
//  Licensed under the Apache License, Version 2.0 (the "License");
//  you may not use this file except in compliance with the License.
//  You may obtain a copy of the License at
//  
//  http://www.apache.org/licenses/LICENSE-2.0
//  
//  Unless required by applicable law or agreed to in writing, software
//  distributed under the License is distributed on an "AS IS" BASIS,
//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//  See the License for the specific language governing permissions and
//  limitations under the License.
//