MQTT-Client-Framework GCD 使用

GCDTimer

@interface GCDTimer: NSObject

+ (GCDTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval
                                     repeats:(BOOL)repeats
                                       queue:(dispatch_queue_t)queue
                                       block:(void (^)(void))block;
- (void)invalidate;

@end

#import "GCDTimer.h"

@interface GCDTimer ()

@property (strong, nonatomic) dispatch_source_t timer;

@end

@implementation GCDTimer

+ (GCDTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval
                                     repeats:(BOOL)repeats
                                       queue:(dispatch_queue_t)queue
                                       block:(void (^)(void))block {
    GCDTimer *timer = [[GCDTimer alloc] initWithInterval:interval
                                                 repeats:repeats
                                                   queue:queue
                                                   block:block];
    return timer;
}

- (instancetype)initWithInterval:(NSTimeInterval)interval
                         repeats:(BOOL)repeats
                           queue:(dispatch_queue_t)queue
                           block:(void (^)(void))block {
    self = [super init];
    if (self) {
        self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
        dispatch_source_set_timer(self.timer, dispatch_time(DISPATCH_TIME_NOW, interval * NSEC_PER_SEC), interval * NSEC_PER_SEC, 0);
        dispatch_source_set_event_handler(self.timer, ^{
            if (!repeats) {
                dispatch_source_cancel(self.timer);
            }
            block();
        });
        dispatch_resume(self.timer);
    }
    return self;
}

- (void)dealloc {
    [self invalidate];
}

- (void)invalidate {
    if (self.timer) {
        dispatch_source_cancel(self.timer);
    }
}

@end

使用GCD 的 source timer 来完成 timer 做的事情.

MQTTCFSocketTransport.m

- (void)setQueue:(dispatch_queue_t)queue {
    _queue = queue;
    
    // We're going to use dispatch_queue_set_specific() to "mark" our queue.
    // The dispatch_queue_set_specific() and dispatch_get_specific() functions take a "void *key" parameter.
    // Later we can use dispatch_get_specific() to determine if we're executing on our queue.
    // From the documentation:
    //
    // > Keys are only compared as pointers and are never dereferenced.
    // > Thus, you can use a pointer to a static variable for a specific subsystem or
    // > any other value that allows you to identify the value uniquely.
    //
    // So we're just going to use the memory address of an ivar.
    
    dispatch_queue_set_specific(_queue, &QueueIdentityKey, (__bridge void *)_queue, NULL);
}

- (void)close {
    // https://github.com/novastone-media/MQTT-Client-Framework/issues/325
    // We need to make sure that we are closing streams on their queue
    // Otherwise, we end up with race condition where delegate is deallocated
    // but still used by run loop event
    if (self.queue != dispatch_get_specific(&QueueIdentityKey)) {
        dispatch_sync(self.queue, ^{
            [self internalClose];
        });
    } else {
        [self internalClose];
    }
}

dispatch_set_specific 来确认线程, 避免 race condition, 过度释放