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：

 #import "Mythread.h"
 @implementation Mythread
 - (void)dealloc
 {
     NSLog(@"Thread:%p dealloc",self);
 }
 + (void)exit
 {
     NSLog(@"Thread:%p exit",self);
     // 注意这是个类函数
     [super exit];
 }
 @end

• 创建NSThread和NSTimer对象

 @property (nonatomic , strong) NSThread *timerThread;

 @property (nonatomic , strong) NSTimer *timer;

• 定义设置NSTimer的函数

 - (void)createTimer

 {

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

     // 创建的线程中，runloop是不会自己启动的，需要手动启动

     [[NSRunLoop currentRunLoop] run];

     NSLog(@"createTimer,currentThread:%@,isMainThread:%@",[NSThread currentThread],@([NSThread isMainThread]));

 }

• 定义self.timer的定时任务函数

 - (void)timerFire

 {

     static NSInteger counter = ;

     NSLog(@"%@,main:%@,counter:%@",[NSThread currentThread],@([NSThread isMainThread]),@(counter++));

 }　　

• 定义销毁self.timer的函数

 - (void)destoryTimerAndThread

 {

     NSLog(@"destoryTimerAndThread,currentThread:%@,isMainThread:%@",[NSThread currentThread],@([NSThread isMainThread]));

     [self.timer invalidate];

     @autoreleasepool {

         self.timerThread = nil;

         self.timer = nil;

     }

     // 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

     [Mythread exit];

 }

• 定义启动新线程的函数

 - (void)createAndStartThread

 {

     NSLog(@"createAndStartThread,currentThread:%@,isMainThread:%@",[NSThread currentThread],@([NSThread isMainThread]));

     self.timerThread = [[NSThread alloc] initWithTarget:self selector:@selector(createTimer) object:nil];

     [self.timerThread start];

 }

• 定义销毁新线程的函数

 - (void)destoryThread

 {

     [self performSelector:@selector(destoryTimer) onThread:self.timerThread withObject:nil waitUntilDone:NO];

 }

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

 ::07.166 : createAndStartThread,currentThread:<NSThread: 0x127d0b750>{number = , name = main},isMainThread:

 ::08.171 : timerFire,currentThread:<Mythread: 0x127d05810>{number = , name = (null)},isMainThread:,counter:

 ::09.173 : timerFire,currentThread:<Mythread: 0x127d05810>{number = , name = (null)},isMainThread:,counter:

 ::10.174 : timerFire,currentThread:<Mythread: 0x127d05810>{number = , name = (null)},isMainThread:,counter:

 ::11.174 : timerFire,currentThread:<Mythread: 0x127d05810>{number = , name = (null)},isMainThread:,counter:

 ::11.479 : destoryTimerAndThread,currentThread:<Mythread: 0x127d05810>{number = , name = (null)},isMainThread:

 ::11.481 : Thread:0x100011158 exit

 ::11.482 : Thread:0x127d05810 dealloc

 ::16.113 : createAndStartThread,currentThread:<NSThread: 0x127d0b750>{number = , name = main},isMainThread:

 ::17.124 : timerFire,currentThread:<Mythread: 0x127d21700>{number = , name = (null)},isMainThread:,counter:

 ::18.124 : timerFire,currentThread:<Mythread: 0x127d21700>{number = , name = (null)},isMainThread:,counter:

 ::19.124 : timerFire,currentThread:<Mythread: 0x127d21700>{number = , name = (null)},isMainThread:,counter:

 ::20.126 : timerFire,currentThread:<Mythread: 0x127d21700>{number = , name = (null)},isMainThread:,counter:

 ::21.126 : timerFire,currentThread:<Mythread: 0x127d21700>{number = , name = (null)},isMainThread:,counter:

 ::21.382 : destoryTimerAndThread,currentThread:<Mythread: 0x127d21700>{number = , name = (null)},isMainThread:

 ::21.383 : Thread:0x100011158 exit

 ::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