NSTimer 常用的方法有三个
+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo;
+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(nullable id)userInfo repeats:(BOOL)yesOrNo;
- (instancetype)initWithFireDate:(NSDate *)date interval:(NSTimeInterval)ti target:(id)t selector:(SEL)s userInfo:(nullable id)ui repeats:(BOOL)rep;
scheduledTimerWithTimeInterval 是创建一个定时器，并加入到当前运行循环[NSRunLoop currentRunLoop]中，而其他两个只是创建定时器，并未添加到当前运行循环中，所以如果是其他两种方式创建的定时器则需要手动添加到currentRunLoop中，示例代码如下：
NSTimer *timer1 = [NSTimer timerWithTimeInterval:3 target:self selector:@selector(doSomeThing1) userInfo:nil repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:timer1 forMode:NSDefaultRunLoopMode];
NSTimer *timer2 = [[NSTimer alloc] initWithFireDate:[NSDate dateWithTimeIntervalSinceNow:5] interval:3 target:self selector:@selector(doSomeThing2) userInfo:nil repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:timer2 forMode:NSDefaultRunLoopMode];
关于fire方法并不是启动一个定时器，只是提前触发而已，详情可查看苹果文档
fire:
You can use this method to fire a repeating timer without interrupting its regular firing schedule. If the timer is non-repeating, it is automatically invalidated after firing, even if its scheduled fire date has not arrived.
定时器如果不再使用的话，需要手动释放，直接调用invalidate即可，千万不要放在dealloc里面调用invalidate，原因我就不说了，不懂可以google
invalidate:
This method is the only way to remove a timer from an NSRunLoop object. The NSRunLoop object removes its strong reference to the timer, either just before the invalidate method returns or at some later point.If it was configured with target and user info objects, the receiver removes its strong references to those objects as well.
