前阵子在整理公司项目的时候，发现老代码在使用NSTimer时出现了内存泄露。然后整理了一些NSTimer的相关内容。比较简单，各位见笑啦。NSTimerfire我们先用NSTimer来做个简单的计时器，每隔5秒钟在控制台输出 Fire 。比较想当然的做法是这样的：@interfaceDetailViewController()@property(nonatomic,weak) NSTimer *timer;@end@implementationDetailViewController- (IBAction)fireButtonPressed:(id)sender {
_timer = [NSTimer scheduledTimerWithTimeInterval:3.0f
target:selfselector:@selector(timerFire:)
userInfo:nilrepeats:YES];
[_timer fire];
}
-(void)timerFire:(id)userinfo {NSLog(@"Fire");
}@end运行之后确实在控制台每隔3秒钟输出一次 Fire ，然而当我们从这个界面跳转到其他界面的时候却发现：控制台还在源源不断的输出着 Fire 。看来Timer并没有停止。invalidate既然没有停止，那我们在DemoViewController的dealloc里加上invalidate的方法：-(void)dealloc {[_timer invalidate];NSLog(@"%@ dealloc",NSStringFromClass([selfclass]));}再次运行，还是没有停止。原因是Timer添加到Runloop的时候，会被Runloop强引用：Note in particular that run loops maintain strong references to their timers, so you don’t have to maintain your own strong reference to a timer after you have added it to a run loop.然后Timer又会有一个对Target的强引用（也就是 self ）：Target is the object to which to send the message specified by aSelector when the timer fires. The timer maintains a strong reference to target until it (the timer) is invalidated.也就是说NSTimer强引用了self，导致self一直不能被释放掉，所以也就走不到self的dealloc里。既然如此，那我们可以再加个invalidate按钮：-(IBAction)invalidateButtonPressed:(id)sender{[_timer invalidate];
}嗯这样就可以了。（在SOF上有人说该在invalidate之后执行_timer = nil，未能理解为什么，如果你知道原因可以告诉我：）在invalidate方法的文档里还有这这样一段话：You must send this message from the thread on which the timer was installed. If you send this message from another thread, the input source associated with the timer may not be removed from its run loop, which could prevent the thread from exiting properly.NSTimer在哪个线程创建就要在哪个线程停止，否则会导致资源不能被正确的释放。看起来各种坑还不少。dealloc那么问题来了：如果我就是想让这个NSTimer一直输出，直到DemoViewController销毁了才停止，我该如何让它停止呢？NSTimer 被 Runloop 强引用了，如果要释放就要调用invalidate方法。但是我想在DemoViewController的dealloc里调用invalidate方法，但是self被NSTimer强引用了。所以我还是要释放NSTimer先，然而不调用invalidate方法就不能释放它。然而你不进入到dealloc方法里我又不能调用invalidate方法。嗯…HWWeakTimerweakSelf问题的关键就在于self被NSTimer强引用了，如果我们能打破这个强引用问题自然而然就解决了。所以一个很简单的想法就是：weakSelf：__weaktypeof(self) weakSelf =self;
_timer = [NSTimer scheduledTimerWithTimeInterval:3.0f
target:weakSelf
selector:@selector(timerFire:)
userInfo:nilrepeats:YES];然而这并没有什么卵用，这里的__weak和__strong唯一的区别就是：如果在这两行代码执行的期间self被释放了，NSTimer的target会变成nil。target既然没办法通过__weak把self抽离出来，我们可以造个假的target给NSTimer。这个假的target类似于一个中间的代理人，它做的唯一的工作就是挺身而出接下了NSTimer的强引用。类声明如下：@interfaceHWWeakTimerTarget: NSObject@property(nonatomic, weak) id target;@property(nonatomic, assign) SEL selector;@property(nonatomic, weak) NSTimer* timer;@end@implementationHWWeakTimerTarget
- (void)fire:(NSTimer *)timer {if(self.target) {[self.target performSelector:self.selector withObject:timer.userInfo];
} else {[self.timer invalidate];
}
}@end然后我们再封装个假的scheduledTimerWithTimeInterval方法，但是在调用的时候已经偷梁换柱了：+ (NSTimer *) scheduledTimerWithTimeInterval:(NSTimeInterval)interval
target:(id)aTarget
selector:(SEL)aSelector
userInfo:(id)userInfo
repeats:(BOOL)repeats {
HWWeakTimerTarget* timerTarget = [[HWWeakTimerTarget alloc] init];
timerTarget.target= aTarget;
timerTarget.selector= aSelector;
timerTarget.timer= [NSTimer scheduledTimerWithTimeInterval:interval
target:timerTarget
selector:@selector(fire:)
userInfo:userInfo
repeats:repeats];returntimerTarget.timer;
}再次运行，问题解决。block如果能用block来调用NSTimer那岂不是更好了。我们可以这样来实现：+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)interval
block:(HWTimerHandler)block
userInfo:(id)userInfo
repeats:(BOOL)repeats {return[selfscheduledTimerWithTimeInterval:interval
target:selfselector:@selector(_timerBlockInvoke:)
userInfo:@[[blockcopy], userInfo]
repeats:repeats];
}
+ (void)_timerBlockInvoke:(NSArray*)userInfo {
HWTimerHandler block = userInfo[0];idinfo = userInfo[1];// or `!block ?: block();` @sunnyxxif(block) {
block(info);
}
}这样我们就可以直接在block里写相关逻辑了：- (IBAction)fireButtonPressed:(id)sender {
_timer = [HWWeakTimerscheduledTimerWithTimeInterval:3.0fblock:^(id userInfo) {
NSLog(@"%@", userInfo);
}userInfo:@"Fire"repeats:YES];
[_timer fire];
}嗯就是这样。More把上面的的代码简单的封装到了HWWeakTimer中，欢迎试用。参考文献：NStimerHow to stop/invalidate NStimerWeak Reference to NSTimer Target To Prevent Retain CycleperformSelector may cause a leak because its selector is unknown