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NSThread为Mythread，重载了dealloc和exit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NSThread和NSTimer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)timerFire
2 
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)destoryThread
2 
3 {
4 
5     [self performSelector:@selector(destoryTimer) onThread:self.timerThread withObject:nil waitUntilDone:NO];
6 
7 }

调用过程

主线程中调用createAndStartThread启动线程和NSTimer
隔一小会，主线程中调用destoryThread销毁NSTimer和线程
隔一小会，主线程中调用createAndStartThread启动
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:3
10 
11 16:16:11.479 : destoryTimerAndThread,currentThread:<Mythread: 0x127d05810>{number = 2, name = (null)},isMainThread:0
12 
13 16:16:11.481 : Thread:0x100011158 exit
14 
15 16:16:11.482 : Thread:0x127d05810 dealloc
16 
17 16:16:16.113 : createAndStartThread,currentThread:<NSThread: 0x127d0b750>{number = 1, name = main},isMainThread:1
18 
19 16:16:17.124 : timerFire,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0,counter:4
20 
21 16:16:18.124 : timerFire,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0,counter:5
22 
23 16:16:19.124 : timerFire,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0,counter:6
24 
25 16:16:20.126 : timerFire,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0,counter:7
26 
27 16:16:21.126 : timerFire,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0,counter:8
28 
29 16:16:21.382 : destoryTimerAndThread,currentThread:<Mythread: 0x127d21700>{number = 3, name = (null)},isMainThread:0
30 
31 16:16:21.383 : Thread:0x100011158 exit
32 
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