AFNetworking/QHTTPOperation/QRunLoopOperation.m

/*
    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.

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*/

#import "QRunLoopOperation.h"

/*
    Theory of Operation
    -------------------
    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.
*/

@interface QRunLoopOperation ()

// read/write versions of public properties

@property (assign, readwrite) QRunLoopOperationState    state;
@property (copy,   readwrite) NSError *                 error;          

@end

@implementation QRunLoopOperation

- (id)init
{
    self = [super init];
    if (self != nil) {
        assert(self->_state == kQRunLoopOperationStateInited);
    }
    return self;
}

- (void)dealloc
{
    assert(self->_state != kQRunLoopOperationStateExecuting);
    [self->_runLoopModes release];
    [self->_runLoopThread release];
    [self->_error release];
    [super dealloc];
}

#pragma mark * Properties

@synthesize runLoopThread = _runLoopThread;
@synthesize runLoopModes  = _runLoopModes;

- (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;
}

@synthesize error         = _error;

#pragma mark * Core state transitions

- (QRunLoopOperationState)state
{
    return self->_state;
}

- (void)setState:(QRunLoopOperationState)newState
    // Change the state of the operation, sending the appropriate KVO notifications.
{
    // any thread

    @synchronized (self) {
        QRunLoopOperationState  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 self->_state).
        
        assert(newState > self->_state);

        // Transitions from executing to finished must be done on the run loop thread.
        
        assert( (newState != kQRunLoopOperationStateFinished) || self.isActualRunLoopThread );

        // inited    + executing -> isExecuting
        // inited    + finished  -> isFinished
        // executing + finished  -> isExecuting + isFinished

        oldState = self->_state;
        if ( (newState == kQRunLoopOperationStateExecuting) || (oldState == kQRunLoopOperationStateExecuting) ) {
            [self willChangeValueForKey:@"isExecuting"];
        }
        if (newState == kQRunLoopOperationStateFinished) {
            [self willChangeValueForKey:@"isFinished"];
        }
        self->_state = newState;
        if (newState == kQRunLoopOperationStateFinished) {
            [self didChangeValueForKey:@"isFinished"];
        }
        if ( (newState == kQRunLoopOperationStateExecuting) || (oldState == kQRunLoopOperationStateExecuting) ) {
            [self didChangeValueForKey:@"isExecuting"];
        }
    }
}

- (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.
{
    assert(self.isActualRunLoopThread);
    assert(self.state == kQRunLoopOperationStateExecuting);

    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.
{
    assert(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 == kQRunLoopOperationStateExecuting) {
        [self finishWithError:[NSError errorWithDomain:NSCocoaErrorDomain code:NSUserCancelledError userInfo:nil]];
    }
}

- (void)finishWithError:(NSError *)error
{
    assert(self.isActualRunLoopThread);
    // error may be nil

    if (self.error == nil) {
        self.error = error;
    }
    [self operationWillFinish];
    self.state = kQRunLoopOperationStateFinished;
}

#pragma mark * Subclass override points

- (void)operationDidStart
{
    assert(self.isActualRunLoopThread);
}

- (void)operationWillFinish
{
    assert(self.isActualRunLoopThread);
}

#pragma mark * Overrides

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

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

- (void)start
{
    // any thread

    assert(self.state == kQRunLoopOperationStateInited);
    
    // 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 = kQRunLoopOperationStateExecuting;
    [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 synchronise 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 == kQRunLoopOperationStateExecuting;
    }
    if (runCancelOnRunLoopThread) {
        [self performSelector:@selector(cancelOnRunLoopThread) onThread:self.actualRunLoopThread withObject:nil waitUntilDone:YES modes:[self.actualRunLoopModes allObjects]];
    }
}

@end