為什么要在非主線程創建NSTimer
- 將 timer 添加到主線程的Runloop里面本身會增加線程負荷
- 如果主線程因為某些原因阻塞卡頓了，timer 定時任務觸發的時間精度肯定也會受到影響
- 有些定時任務不是UI相關的，本來就沒必要在主線程執行，給主線程增加不必要的負擔。當然也可以在定時任務執行時，手動將任務指派到非主線程上，但這也是有額外開銷的。
NSTimer的重要特性
- NSTimer上的定時任務是在創建NSTimer的線程上執行的。NSTimer的銷毀和創建必須在同一個線程上操作
- NSTimer要被添加到當前線程的 Runloop 里面且 Runloop 被啟動，定時任務（selector或者invocation）才會觸發。
如何創建NSTimer對象
多數情況下，如此一行代碼創建的NSTimer就能正常工作：
[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFire) userInfo:nil repeats:YES]
因為這段創建代碼是在主線程里面執行的，主線程里面會有系統創建好了的且已經啟動了的 Runloop ：[NSRunLoop mainRunLoop]。通過[NSTimer scheduledTimerWithTimeInterval:target:selector:userInfo:repeats:]創建時，會自動將創建的NSTimer對象加到當前的 Runloop 里面，所以 timer 能夠創建后立馬就能工作。
根據以上，可以這么創建自定義線程和運行在上面的 timer ：
創建線程對象和NSTimer對象，定義函數
- 子類化NSThread為Mythread，重載了dealloc和exit函數，在里面加了 log 輸出，方便跟蹤執行過程
- Mythread.h文件為默認，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 // 注意這是個類函數 11 [super exit]; 12 } 13 @end
- 創建NSThread和NSTimer對象
1 @property (nonatomic , strong) NSThread *timerThread; 2 3 @property (nonatomic , strong) NSTimer *timer;
- 定義設置NSTimer的函數
1 - (void)createTimer 2 3 { 4 5 self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerFire) userInfo:nil repeats:YES]; 6 7 // 創建的線程中，runloop是不會自己啟動的，需要手動啟動 8 9 [[NSRunLoop currentRunLoop] run]; 10 11 NSLog(@"createTimer,currentThread:%@,isMainThread:%@",[NSThread currentThread],@([NSThread isMainThread])); 12 13 }
- 定義self.timer的定時任務函數
1 - (void)timerFire 2 3 { 4 5 static NSInteger counter = 0; 6 7 NSLog(@"%@,main:%@,counter:%@",[NSThread currentThread],@([NSThread isMainThread]),@(counter++)); 8 9 }
- 定義銷毀self.timer的函數
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 // 釋放打開的資源和清空申請的內存后，才可以退出，不然就會內存泄露 18 19 [Mythread exit]; 20 21 }
- 定義啟動新線程的函數
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 }
- 定義銷毀新線程的函數
1 - (void)destoryThread 2 3 { 4 5 [self performSelector:@selector(destoryTimer) onThread:self.timerThread withObject:nil waitUntilDone:NO]; 6 7 }
調用過程
- 主線程中調用createAndStartThread啟動線程和NSTimer
- 隔一小會，主線程中調用destoryThread銷毀NSTimer和線程
- 隔一小會，主線程中調用createAndStartThread啟動
- 隔一小會，主線程中調用destoryThread銷毀NSTimer和線程
console輸出結果
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
示例說明
- 為了節省顯示空間，刪除了部分 log 頭信息
- 創建和銷毀時不一定要在主線程里面調用，只是為了方便比對輸出結果
- 在銷毀 Timer 時，也不一定就要銷毀線程，這里只是演示非主線程的創建和銷毀