延迟加载

1.performSelector
 [self performSelector:@selector(removeViews) withObject:nil afterDelay:5.0];
2.NSTimer 触发定时任务
NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(removeViews) userInfo:nil repeats:NO];  
[timer setFireDate:[NSDate dateWithTimeIntervalSinceNow:5.0]];  
3.使用GCD
dispatch_time_t delayTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5.0 * NSEC_PER_SEC));  
dispatch_after(delayTime, dispatch_get_main_queue(), ^{  
              [self removeViews];  
        });
4.NSThread  阻塞子线程
[NSThread sleepForTimeInterval:5];  
[self removeViews];