NSTimer創(chuàng)建分兩種情況：

1、通過(guò) scheduledTimerWithTimeInterval 創(chuàng)建的NSTimer, 默認(rèn)就會(huì)添加到RunLoop的DefaultMode中。

_timer = [NSTimer scheduledTimerWithTimeInterval:2.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"%@",[NSThread currentThread]);
    }];
// 雖然默認(rèn)已經(jīng)添加到DefaultMode中, 但是我們也可以自己修改它的模式 
[[NSRunLoop currentRunLoop] addTimer:_timer forMode:NSRunLoopCommonModes];

2、通過(guò) timerWithTimeInterval 和 alloc 創(chuàng)建的NSTimer, 不會(huì)被添加到RunLoop的Mode中，我們要調(diào)用 [[NSRunLoop currentRunLoop] addTimer:方法。

_timer = [NSTimer timerWithTimeInterval:2.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"%@",[NSThread currentThread]);
    }];
_timer = [[NSTimer alloc]initWithFireDate:[NSDate dateWithTimeIntervalSinceNow:2.0] interval:2.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"timer:%@",[NSThread currentThread]);
    }];
[[NSRunLoop currentRunLoop] addTimer:_timer forMode:NSDefaultRunLoopMode];

NSTimer執(zhí)行分兩種情況：

主線程：

NSRunLoop默認(rèn)開(kāi)啟，不用調(diào)用 [[NSRunLoop currentRunLoop] run];

NSLog(@"主線程：%@",[NSThread currentThread]);
_timer = [NSTimer scheduledTimerWithTimeInterval:2.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"%@",[NSThread currentThread]);
    }];

子線程：

NSRunLoop需要自主開(kāi)啟，要調(diào)用 [[NSRunLoop currentRunLoop] run];

__weak typeof(self) weakSelf = self;
dispatch_queue_t queue = dispatch_queue_create("zixiancheng", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
      NSLog(@"子線程:%@",[NSThread currentThread]);
      weakSelf.timer = [NSTimer scheduledTimerWithTimeInterval:2.0 repeats:YES block:^(NSTimer * _Nonnull timer) {
          NSLog(@"%@",[NSThread currentThread]);
      }];
      [[NSRunLoop currentRunLoop] run];
   });