- (void)viewDidLoad {
    [super viewDidLoad];
    
    _timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(timeEvent:) userInfo:nil repeats:YES];
}
 
- (void)timeEvent:(NSTimer *)sender{
    NSLog(@"&sender before = %p",&sender);
    NSLog(@"sender before = %p",sender);
    [sender invalidate];
    sender = nil;
    NSLog(@"&sender after = %p",&sender);
    NSLog(@"&_timer = %p",&_timer);
    NSLog(@"sender after = %p",sender);
    NSLog(@"_timer = %p",_timer);
}
NSLog(@"sender before = %p",&sender);  这个是指针的地址
NSLog(@"sender before = %p",sender);  这个是指针指向值的地址

打印结果：
&sender before = 0x7fff5dc516f8
sender before = 0x6000003615c0
&sender after = 0x7fff5dc516f8
&_timer = 0x7ff586707d88
sender after = 0x0
_timer = 0x6000003615c0

结论：将定时器事件中参数置空，但是不能将self.timer的地址置空。