取消 NSTimer 和 performSelector

使用 NSTimer 和 perfrormSelectorOnMainThread:withObject:waitUntilDone 做定时并可以取消移除：

-(void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];

    [self performSelectorOnMainThread:@selector(autoRefreshRecentDoctorAdvices) withObject:nil waitUntilDone:NO];
}

//自动刷新
-(void)autoRefreshRecentDoctorAdvices
{
    //_timer 是对象的私有变量，以便取消定时
    _timer = [NSTimer scheduledTimerWithTimeInterval:100 target:self selector:@selector(getRecentDoctorAdvices) userInfo:nil repeats:YES];
}

//取消定时，并移除 performSelector
-(void)   cancelTimer
{
   if( _timer)
    {
        [_timer invalidate];
        [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(autoRefreshRecentDoctorAdvices) object:nil];
        return;
    }  
}