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在非主线程里面使用NSTimer创建和取消定时任务

为什么要在非主线程创建NSTimer

  • 将 timer 添加到主线程的Runloop里面本身会增加线程负荷
  • 如果主线程因为某些原因阻塞卡顿了,timer 定时任务触发的时间精度肯定也会受到影响
  • 有些定时任务不是UI相关的,本来就没必要在主线程执行,给主线程增加不必要的负担。当然也可以在定时任务执行时,手动将任务指派到非主线程上,但这也是有额外开销的。

 

NSTimer的重要特性

  • NSTimer上的定时任务是在创建NSTimer的线程上执行的。NSTimer的销毁和创建必须在同一个线程上操作
  • NSTimer要被添加到当前线程的 Runloop 里面且 Runloop 被启动,定时任务(selector或者invocation)才会触发。

 

如何创建NSTimer对象

多数情况下,如此一行代码创建的NSTimer就能正常工作:

[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFire) userInfo:nil repeats:YES]

因为这段创建代码是在主线程里面执行的,主线程里面会有系统创建好了的且已经启动了的 Runloop :[NSRunLoop mainRunLoop]。通过[NSTimer scheduledTimerWithTimeInterval:target:selector:userInfo:repeats:]创建时,会自动将创建的NSTimer对象加到当前的 Runloop 里面,所以 timer 能够创建后立马就能工作。

 

根据以上,可以这么创建自定义线程和运行在上面的 timer :

创建线程对象和NSTimer对象,定义函数

  • 子类化NSThreadMythread,重载了deallocexit函数,在里面加了 log 输出,方便跟踪执行过程
    • Mythread.h文件为默认,略去
    • Mythread.m文件:
 1 #import "Mythread.h" 2 @implementation Mythread 3 - (void)dealloc 4 { 5     NSLog(@"Thread:%p dealloc",self); 6 } 7 + (void)exit 8 { 9     NSLog(@"Thread:%p exit",self);10     // 注意这是个类函数11     [super exit];12 }13 @end

 

  • 创建NSThreadNSTimer对象
1 @property (nonatomic , strong) NSThread *timerThread;2 3 @property (nonatomic , strong) NSTimer *timer;
  • 定义设置NSTimer的函数
 1 - (void)createTimer 2  3 { 4  5     self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFire) userInfo:nil repeats:YES]; 6  7     // 创建的线程中,runloop是不会自己启动的,需要手动启动 8  9     [[NSRunLoop currentRunLoop] run];10 11     NSLog(@"createTimer,currentThread:%@,isMainThread:%@",[NSThread currentThread],@([NSThread isMainThread]));12 13 }
  • 定义self.timer的定时任务函数
1 - (void)timerFire2 3 {4 5     static NSInteger counter = 0;6 7     NSLog(@"%@,main:%@,counter:%@",[NSThread currentThread],@([NSThread isMainThread]),@(counter++));8 9 }  
  • 定义销毁self.timer的函数
 1 - (void)destoryTimerAndThread 2  3 { 4  5     NSLog(@"destoryTimerAndThread,currentThread:%@,isMainThread:%@",[NSThread currentThread],@([NSThread isMainThread])); 6  7     [self.timer invalidate]; 8  9     @autoreleasepool {10 11         self.timerThread = nil;12 13         self.timer = nil;14 15     }16 17     // 释放打开的资源和清空申请的内存后,才可以退出,不然就会内存泄露18 19     [Mythread exit];20 21 }
  • 定义启动新线程的函数
 1 - (void)createAndStartThread 2  3 { 4  5     NSLog(@"createAndStartThread,currentThread:%@,isMainThread:%@",[NSThread currentThread],@([NSThread isMainThread])); 6  7     self.timerThread = [[NSThread alloc] initWithTarget:self selector:@selector(createTimer) object:nil]; 8  9     [self.timerThread start];10 11 }
  • 定义销毁新线程的函数
1 - (void)destoryThread2 3 {4 5     [self performSelector:@selector(destoryTimer) onThread:self.timerThread withObject:nil waitUntilDone:NO];6 7 }

调用过程

  1. 主线程中调用createAndStartThread启动线程和NSTimer
  2. 隔一小会,主线程中调用destoryThread销毁NSTimer和线程
  3. 隔一小会,主线程中调用createAndStartThread启动
  4. 隔一小会,主线程中调用destoryThread销毁NSTimer和线程

console输出结果

 1 16:16:07.166 : createAndStartThread,currentThread:<NSThread: 0x127d0b750>{number = 1, name = main},isMainThread:1 2  3 16:16:08.171 : timerFire,currentThread:<Mythread: 0x127d05810>{number = 2, name = (null)},isMainThread:0,counter:0 4  5 16:16:09.173 : timerFire,currentThread:<Mythread: 0x127d05810>{number = 2, name = (null)},isMainThread:0,counter:1 6  7 16:16:10.174 : timerFire,currentThread:<Mythread: 0x127d05810>{number = 2, name = (null)},isMainThread:0,counter:2 8  9 16:16:11.174 : timerFire,currentThread:<Mythread: 0x127d05810>{number = 2, name = (null)},isMainThread:0,counter:310 11 16:16:11.479 : destoryTimerAndThread,currentThread:<Mythread: 0x127d05810>{number = 2, name = (null)},isMainThread:012 13 16:16:11.481 : Thread:0x100011158 exit14 15 16:16:11.482 : Thread:0x127d05810 dealloc16 17 16:16:16.113 : createAndStartThread,currentThread:<NSThread: 0x127d0b750>{number = 1, name = main},isMainThread:118 19 16:16:17.124 : timerFire,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0,counter:420 21 16:16:18.124 : timerFire,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0,counter:522 23 16:16:19.124 : timerFire,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0,counter:624 25 16:16:20.126 : timerFire,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0,counter:726 27 16:16:21.126 : timerFire,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0,counter:828 29 16:16:21.382 : destoryTimerAndThread,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:030 31 16:16:21.383 : Thread:0x100011158 exit32 33 16:16:21.385 : Thread:0x127d21700 dealloc

示例说明

    • 为了节省显示空间,删除了部分 log 头信息
    • 创建和销毁时不一定要在主线程里面调用,只是为了方便比对输出结果
    • 在销毁 Timer 时,也不一定就要销毁线程,这里只是演示非主线程的创建和销毁

在非主线程里面使用NSTimer创建和取消定时任务