cancelPreviousPerformRequestsWithTarget.m

cancelPreviousPerformRequestsWithTarget.m

[NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(overtimeRemind) object:nil];
先看这段代码：
- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(showLeft) object:nil];
    [self performSelector:@selector(showLeft)];
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(showLeft) object:nil];
    [self performSelector:@selector(showLeft)];
}
为什么[self performSelector:@selector(showLeft)];前面还需要调用cancelPreviousPerformRequestsWithTarget:self方法呢？

那是因为有时候我们通过[self performSelector:@selector(showLeft)];来实现showLeft方法，可能会出现延迟执行或内存泄漏的问题，而前面加上这句话：

[NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(showLeft) object:nil];

可以先将这在执行的方法取消，在继续执行，有效避免了上述两个问题。