Created
December 18, 2013 17:17
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Python-style generators in Objective-C
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| @interface CDYieldEnumerator : NSEnumerator | |
| @end | |
| typedef BOOL(^YieldBlock)(id); | |
| #define YIELD_BLOCK ^(YieldBlock yield) | |
| #define YIELD(OBJECT) do { if (!yield(OBJECT)) return; } while (0) | |
| @implementation CDYieldEnumerator { | |
| dispatch_queue_t _queue; | |
| dispatch_semaphore_t _queueLock; | |
| dispatch_semaphore_t _yieldLock; | |
| BOOL _started; | |
| id _nextObject; | |
| void (^_block)(YieldBlock); | |
| } | |
| - (instancetype)initWithBlock:(void(^)(YieldBlock yield))block | |
| { | |
| if ((self = [super init])) { | |
| _queue = dispatch_queue_create([[NSString stringWithFormat:@"CDYieldEnumerator@%p", (__bridge void *)self] UTF8String], NULL); | |
| _block = [block copy]; | |
| _queueLock = dispatch_semaphore_create(0); | |
| _yieldLock = dispatch_semaphore_create(0); | |
| } | |
| return self; | |
| } | |
| - (void)dealloc | |
| { | |
| [self cleanUp]; | |
| } | |
| - (void)cleanUp | |
| { | |
| _queue = nil; | |
| _block = nil; | |
| dispatch_semaphore_signal(_queueLock); | |
| } | |
| - (void)start | |
| { | |
| __weak __typeof(self) weakSelf = self; | |
| void (^block)(YieldBlock) = _block; | |
| dispatch_semaphore_t yieldLock = _yieldLock; | |
| dispatch_semaphore_t queueLock = _queueLock; | |
| dispatch_async(_queue, ^{ | |
| block(^BOOL(id object) { | |
| { | |
| __strong __typeof (weakSelf) strongSelf = weakSelf; | |
| strongSelf->_nextObject = object; | |
| } | |
| dispatch_semaphore_signal(yieldLock); | |
| dispatch_semaphore_wait(queueLock, DISPATCH_TIME_FOREVER); | |
| { | |
| __strong __typeof(weakSelf) strongSelf = weakSelf; | |
| return strongSelf && strongSelf->_queue; | |
| } | |
| // return NO if work needs to be cancelled | |
| // another approach would be to throw an exception and catch it immediately in this method | |
| }); | |
| dispatch_semaphore_signal(yieldLock); | |
| }); | |
| } | |
| - (id)nextObject | |
| { | |
| if (!_queue) return nil; | |
| _nextObject = nil; | |
| if (!_started) { | |
| _started = YES; | |
| [self start]; | |
| } else { | |
| // Let the queue know we need the next element | |
| dispatch_semaphore_signal(_queueLock); | |
| } | |
| dispatch_semaphore_wait(_yieldLock, DISPATCH_TIME_FOREVER); | |
| if (!_nextObject) { | |
| [self cleanUp]; | |
| } | |
| return _nextObject; | |
| } | |
| @end |
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| + (void)test | |
| { | |
| NSEnumerator *enumerator = [[CDYieldEnumerator alloc] initWithBlock:YIELD_BLOCK { | |
| YIELD(@1); | |
| YIELD(@2); | |
| for (NSNumber *number in @[@3, @4, @5]) { | |
| YIELD(number); | |
| } | |
| YIELD(@6); | |
| [self expensiveComputation]; // Will never be executed | |
| YIELD(@7); | |
| [@[@8] enumerateObjectsUsingBlock:^(NSNumber *number, NSUInteger idx, BOOL *stop) { | |
| // This would break the generator since YIELD can call return | |
| // A possible workaround is exceptions, but they would be slower and create other problems (auto breakpoints, mem management) | |
| YIELD(number); | |
| }]; | |
| }]; | |
| for (int i = 0; i < 6; i++) NSLog([enumerator nextObject]); // Logs 1 2 3 4 5 6 | |
| } |
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