main函数作为程序运行的入口,正常情况下,函数会执行毫秒级别的操作,然后返回一个0表示程序正常终止。为了避免应用启动即终止,苹果设计了runloop机制来维持线程的生命,runloop在每一次的循环当中不断的去处理事件,或控制线程的休眠和唤醒。runloop还结合了libdispatch的任务派发机制,可以循环地处理async到队列中的任务
启动runloop
从runloop对外暴露的接口来看,启动方式一共存在三种:
无条件的启动。这种启动方式缺少停止runloop的手段,唯一的结束方式是kill掉线程
- (void)run;
设定超时时间。直接run会调用这个接口并且传入distantFuture,表示无限长的超时时间。如果超时时间不是无限长,那么runloop会在处理完事件或者超时后终止,优于直接run
- (void)runUntilDate: (NSDate *)limitDate;
设置超时时间和运行模式。比起第二种,允许我们让runloop运行在某个模式下,灵活性更高
- (BOOL)runMode: (NSRunLoopMode)mode beforeDate: (NSDate *)limitDate;
如果runloop在启动之后没有任何sources、timers或者ports事件可以处理,那么会自动退出,否则会在处理完成后让线程陷入休眠,等待这些事件重新唤醒线程处理。下面是最常用来表示runloop处理逻辑的示意图:
除开图中列出的事件之外,main loop会处理timer之后检测队列中是否存在待执行的block然后开始执行
什么情况下需要启动runloop
主线程的runloop会在应用启动后被UIApplication启动,其他线程则需要我们主动去run。从接触iOS开发到现在,笔者了解的需要主动启动runloop只有这么两类:
子线程使用timer
由于NSTimer本身就不是一个能确保稳定回调的定时器机制,并且主线程会经常处在忙碌状态,这又进一步降低了NSTimer的准确性。为了提高定时的准确性,多数人会采用子线程启动runloop的方式来实现定时器功能
线程保活
线程保活是一种不太常见的需求,但是如果你曾经了解过AFNetworking的做法,会发现创建了子线程之后,采用一个空port的方式来启动runloop,避免线程被中止回收。但实际上这种做法很容易导致线程既无法被回收,也不能被使用的情况
上面两种不同的应用场景,实际上是使用timer和port维持runloop不会因为没有事件处理直接退出,而且在这些源事件来临之前,线程大多数情况下处在休眠状态不造成额外损耗
串行队列的runloop
假设现在需要使用一个子线程的runloop来实现定时器,由于runloop在停止之前,线程会一直存活,因此可能会想利用这个存活的线程处理其他的任务。因此除了NSTimer之外,我们添加一个GCD Timer定时的派发任务给这个启动runloop的队列:
dispatch_queue_t serialQueue = dispatch_queue_create("serial.queue", DISPATCH_QUEUE_SERIAL); dispatch_async(serialQueue, ^{ NSLog(@"the task run in the thread: %d", mach_thread_self()); [NSTimer scheduledTimerWithTimeInterval: 0.5 repeats: YES block: ^(NSTimer * _Nonnull timer) { NSLog(@"ns timer in the thread: %d", mach_thread_self()); }]; [[NSRunLoop currentRunLoop] runUntilDate: [NSDate dateWithTimeIntervalSinceNow: 600]]; }); dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, serialQueue); dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 0.5 * NSEC_PER_SEC, 0 * NSEC_PER_SEC); dispatch_source_set_event_handler(timer, ^{ NSLog(@"gcd timer in the thread: %d", mach_thread_self()); }); dispatch_resume(timer);
按照预期,这段代码充分利用了已经被保活的线程,除了已有的NSTimer之外,线程还能在空闲的时间去处理不断派发的任务,但实际上只有NSTimer的任务被执行:
2018-04-14 10:32:54.667718+0800 PThreads[7693:94702] the task run in the thread: 4355 2018-04-14 10:32:55.168871+0800 PThreads[7693:94702] ns timer in the thread: 4355 2018-04-14 10:32:55.672011+0800 PThreads[7693:94702] ns timer in the thread: 4355 2018-04-14 10:32:56.169150+0800 PThreads[7693:94702] ns timer in the thread: 4355 2018-04-14 10:32:56.669411+0800 PThreads[7693:94702] ns timer in the thread: 4355 2018-04-14 10:32:57.169665+0800 PThreads[7693:94702] ns timer in the thread: 4355 2018-04-14 10:32:57.669234+0800 PThreads[7693:94702] ns timer in the thread: 4355 2018-04-14 10:32:58.172068+0800 PThreads[7693:94702] ns timer in the thread: 4355 2018-04-14 10:32:58.669446+0800 PThreads[7693:94702] ns timer in the thread: 4355 2018-04-14 10:32:59.169223+0800 PThreads[7693:94702] ns timer in the thread: 4355 2018-04-14 10:32:59.671802+0800 PThreads[7693:94702] ns timer in the thread: 4355
导致保活线程无法处理async任务的原因有两个:
runloop和queue的区别
runloop和queue各自维护着自己的一个任务队列,在runloop的每个周期里面,会检测自身的任务队列里面是否存在待执行的task并且执行。但主线程的情况比较特殊,在main runloop的每个周期,会去检测main queue是否存在待执行任务,如果存在,那么copy到自身的任务队列中执行
async的实现不同
在非主线程之外,runloop和queue的任务队列是互不干扰的,因此两者处理任务的机制也是完全不同的。当async任务到队列时,GCD会尝试寻找一个线程来执行任务。由于串行队列同时只能与一个线程挂钩,因此GCD会让该线程执行完已有任务后,才执行async到队列中的任务。但由于线程被保活,任务是一个条件死循环condition-loop,因此async的任务始终无法被处理
为了证明这些原因,可以通过CFRunLoopPerformBlock将任务直接加入到runloop自身的任务队列中,检测这个任务是否被执行:
__block CFRunLoopRef serialRunLoop = NULL; dispatch_queue_t mainQueue = dispatch_get_main_queue(); dispatch_queue_t serialQueue = dispatch_queue_create("serial.queue", DISPATCH_QUEUE_SERIAL); dispatch_async(serialQueue, ^{ NSLog(@"the task run in the thread: %d", mach_thread_self()); [NSTimer scheduledTimerWithTimeInterval: 0.5 repeats: YES block: ^(NSTimer * _Nonnull timer) { NSLog(@"ns timer in the thread: %d", mach_thread_self()); }]; serialRunLoop = [NSRunLoop currentRunLoop].getCFRunLoop; [[NSRunLoop currentRunLoop] runUntilDate: [NSDate dateWithTimeIntervalSinceNow: 600]]; }); dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, mainQueue); dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 0.5 * NSEC_PER_SEC, 0 * NSEC_PER_SEC); dispatch_source_set_event_handler(timer, ^{ dispatch_async(serialQueue, ^{ NSLog(@"gcd timer in the thread: %d", mach_thread_self()); }); CFRunLoopPerformBlock(serialRunLoop, NSDefaultRunLoopMode, ^{ NSLog(@"perform block in thread: %d", mach_thread_self()); }); }); dispatch_resume(timer);
再次运行之后,async的任务依旧无法被处理,但是perform block的任务总是能在timer唤醒休眠的线程后被处理:
2018-04-14 11:01:29.925924+0800 PThreads[15619:198121] the task run in the thread: 4355 2018-04-14 11:01:30.428175+0800 PThreads[15619:198121] ns timer in the thread: 4355 2018-04-14 11:01:30.428982+0800 PThreads[15619:198121] perform block in thread: 4355 2018-04-14 11:01:30.429410+0800 PThreads[15619:198121] perform block in thread: 4355 2018-04-14 11:01:30.932411+0800 PThreads[15619:198121] ns timer in the thread: 4355 2018-04-14 11:01:30.932674+0800 PThreads[15619:198121] perform block in thread: 4355 2018-04-14 11:01:31.430281+0800 PThreads[15619:198121] ns timer in the thread: 4355 2018-04-14 11:01:31.430546+0800 PThreads[15619:198121] perform block in thread: 4355 2018-04-14 11:01:31.929485+0800 PThreads[15619:198121] ns timer in the thread: 4355 2018-04-14 11:01:31.929691+0800 PThreads[15619:198121] perform block in thread: 4355 2018-04-14 11:01:32.432369+0800 PThreads[15619:198121] ns timer in the thread: 4355 2018-04-14 11:01:32.432726+0800 PThreads[15619:198121] perform block in thread: 4355 2018-04-14 11:01:32.930981+0800 PThreads[15619:198121] ns timer in the thread: 4355 2018-04-14 11:01:32.931179+0800 PThreads[15619:198121] perform block in thread: 4355 2018-04-14 11:01:33.429207+0800 PThreads[15619:198121] ns timer in the thread: 4355 2018-04-14 11:01:33.429519+0800 PThreads[15619:198121] perform block in thread: 4355
并行队列的runloop
队列的串并行属性决定了队列能不能被多个线程处理任务,因此同样的代码在并行队列执行,产生的结果必然是有所区别的:
__block CFRunLoopRef serialRunLoop = NULL; dispatch_queue_t mainQueue = dispatch_get_main_queue(); dispatch_queue_t serialQueue = dispatch_queue_create("concurrent.queue", DISPATCH_QUEUE_CONCURRENT); dispatch_async(serialQueue, ^{ NSLog(@"the task run in the thread: %d", mach_thread_self()); [NSTimer scheduledTimerWithTimeInterval: 0.5 repeats: YES block: ^(NSTimer * _Nonnull timer) { NSLog(@"ns timer in the thread: %d", mach_thread_self()); }]; serialRunLoop = [NSRunLoop currentRunLoop].getCFRunLoop; [[NSRunLoop currentRunLoop] runUntilDate: [NSDate dateWithTimeIntervalSinceNow: 600]]; }); dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, mainQueue); dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 0.5 * NSEC_PER_SEC, 0 * NSEC_PER_SEC); dispatch_source_set_event_handler(timer, ^{ dispatch_async(serialQueue, ^{ NSLog(@"gcd timer in the thread: %d", mach_thread_self()); }); CFRunLoopPerformBlock(serialRunLoop, NSDefaultRunLoopMode, ^{ NSLog(@"perform block in thread: %d", mach_thread_self()); }); }); dispatch_resume(timer);
输出结果如下:
2018-04-14 11:02:47.348083+0800 PThreads[15988:203200] the task run in the thread: 4867 2018-04-14 11:02:47.397174+0800 PThreads[15988:203203] gcd timer in the thread: 3843 2018-04-14 11:02:47.840678+0800 PThreads[15988:203206] gcd timer in the thread: 9219 2018-04-14 11:02:47.852870+0800 PThreads[15988:203200] ns timer in the thread: 4867 2018-04-14 11:02:47.853122+0800 PThreads[15988:203200] perform block in thread: 4867 2018-04-14 11:02:47.853552+0800 PThreads[15988:203200] perform block in thread: 4867 2018-04-14 11:02:48.340602+0800 PThreads[15988:203206] gcd timer in the thread: 9219 2018-04-14 11:02:48.352863+0800 PThreads[15988:203200] ns timer in the thread: 4867 2018-04-14 11:02:48.353149+0800 PThreads[15988:203200] perform block in thread: 4867 2018-04-14 11:02:48.840085+0800 PThreads[15988:203206] gcd timer in the thread: 9219 2018-04-14 11:02:48.853918+0800 PThreads[15988:203200] ns timer in the thread: 4867 2018-04-14 11:02:48.854120+0800 PThreads[15988:203200] perform block in thread: 4867 2018-04-14 11:02:49.340729+0800 PThreads[15988:203206] gcd timer in the thread: 9219 2018-04-14 11:02:49.354172+0800 PThreads[15988:203200] ns timer in the thread: 4867 2018-04-14 11:02:49.354470+0800 PThreads[15988:203200] perform block in thread: 4867 2018-04-14 11:02:49.840661+0800 PThreads[15988:203206] gcd timer in the thread: 9219 2018-04-14 11:02:49.853115+0800 PThreads[15988:203200] ns timer in the thread: 4867 2018-04-14 11:02:49.853288+0800 PThreads[15988:203200] perform block in thread: 4867 2018-04-14 11:02:50.340078+0800 PThreads[15988:203206] gcd timer in the thread: 9219 2018-04-14 11:02:50.354262+0800 PThreads[15988:203200] ns timer in the thread: 4867 2018-04-14 11:02:50.354537+0800 PThreads[15988:203200] perform block in thread: 4867 2018-04-14 11:02:50.840653+0800 PThreads[15988:203206] gcd timer in the thread: 9219 2018-04-14 11:02:50.854117+0800 PThreads[15988:203200] ns timer in the thread: 4867 2018-04-14 11:02:50.854406+0800 PThreads[15988:203200] perform block in thread: 4867 2018-04-14 11:02:51.339972+0800 PThreads[15988:203206] gcd timer in the thread: 9219 2018-04-14 11:02:51.353246+0800 PThreads[15988:203200] ns timer in the thread: 4867 2018-04-14 11:02:51.353472+0800 PThreads[15988:203200] perform block in thread: 4867 2018-04-14 11:02:51.839917+0800 PThreads[15988:203206] gcd timer in the thread: 9219
虽然并行队列的async功能并不会因为启动了runloop受到影响,但是可以发现如果不去保存runloop,这个保活的线程除了定时器能正常处理之外,其他时候不会再被GCD复用
使用port保活
如果不使用NSTimer这种稳定的唤醒机制来保活线程,而是采用port的方式,线程的表现是否依旧符合预期?
__block CFRunLoopRef serialRunLoop = NULL; dispatch_queue_t mainQueue = dispatch_get_main_queue(); dispatch_queue_t serialQueue = dispatch_queue_create("concurrent.queue", DISPATCH_QUEUE_CONCURRENT); dispatch_async(serialQueue, ^{ serialRunLoop = [NSRunLoop currentRunLoop].getCFRunLoop; [[NSRunLoop currentRunLoop] addPort: [NSPort new] forMode: NSDefaultRunLoopMode]; [[NSRunLoop currentRunLoop] runUntilDate: [NSDate dateWithTimeIntervalSinceNow: 600]]; }); dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, mainQueue); dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 0.5 * NSEC_PER_SEC, 0 * NSEC_PER_SEC); dispatch_source_set_event_handler(timer, ^{ CFRunLoopPerformBlock(serialRunLoop, NSDefaultRunLoopMode, ^{ NSLog(@"perform block in thread: %d", mach_thread_self()); }); }); dispatch_resume(timer);
此时任务总是不会被处理。由于runloop需要被唤醒才能处理队列任务,而perform block只是单纯的添加任务,没有唤醒功能。为了线程能够继续执行任务,这时候还需要不断的wake up线程:
dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, mainQueue); dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 0.5 * NSEC_PER_SEC, 0 * NSEC_PER_SEC); dispatch_source_set_event_handler(timer, ^{ CFRunLoopPerformBlock(serialRunLoop, NSDefaultRunLoopMode, ^{ NSLog(@"perform block in thread: %d", mach_thread_self()); }); CFRunLoopWakeUp(serialRunLoop); }); dispatch_resume(timer); /// 日志输出 2018-04-14 11:15:10.314064+0800 PThreads[19459:248764] perform block in thread: 2563 2018-04-14 11:15:10.763480+0800 PThreads[19459:248764] perform block in thread: 2563 2018-04-14 11:15:11.263651+0800 PThreads[19459:248764] perform block in thread: 2563 2018-04-14 11:15:11.763957+0800 PThreads[19459:248764] perform block in thread: 2563 2018-04-14 11:15:12.264006+0800 PThreads[19459:248764] perform block in thread: 2563 2018-04-14 11:15:12.763384+0800 PThreads[19459:248764] perform block in thread: 2563
结论
从测试来看,在子线程启动runloop并不是一个很明智的选择:这会导致线程保活期间被遗弃,失去了处理消息派发的能力,且无法响应其他线程的通信。其次,即便可以通过perform block来继续为保活线程添加任务处理,但在保活线程的runloop缺乏稳定的唤醒机制的情况下,还需要其他线程来提供唤醒能力,这增加了代码设计的成本,并且不会有额外的好处