1. NSTimer CADisplayLink
   ? 通過(guò) NSTimer 實(shí)現(xiàn)的動(dòng)畫(huà)可能造成卡頓、不連貫的情況（NSTimer 不準(zhǔn)確）
   ? CADisplayLink 表示"顯示連接", 與顯示器的刷新頻率相關(guān)。
   ? 顯示器每秒鐘刷新60次（60HZ）（電視新聞上拍的電腦顯示器就不清楚，原因刷新頻率不一樣）
   ? 通過(guò) CADisplayLink 來(lái)解決

/** 參考代碼:
 // 核心代碼
 CADisplayLink *timer = [CADisplayLink displayLinkWithTarget:self selector:@selector(moveSecond)];
 [timer addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];

 - (void)viewDidLoad {
 [super viewDidLoad];
 // 1. 創(chuàng)建表盤(pán)
 UIView *clockView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 150, 150)];
 clockView.backgroundColor = [UIColor blueColor];
 clockView.center = CGPointMake(self.view.bounds.size.width * 0.5, self.view.bounds.size.height * 0.5);
 [self.view addSubview:clockView];
 self.clockView = clockView;
 
 
 // 2. 創(chuàng)建秒針
 UIView *secondView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 5, 50)];
 secondView.backgroundColor = [UIColor redColor];
 secondView.layer.anchorPoint = CGPointMake(0.5, 1);
 secondView.center = clockView.center;
 [self.view addSubview:secondView];
 self.secondView = secondView;
 
 
 [self moveSecond];
 
 // 3. 啟動(dòng)一個(gè)計(jì)時(shí)器, 每隔一秒鐘計(jì)算一下當(dāng)前秒針的位置, 然后做一次哦旋轉(zhuǎn)操作
 // [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(moveSecond) userInfo:nil repeats:YES];
 
 CADisplayLink *timer = [CADisplayLink displayLinkWithTarget:self selector:@selector(moveSecond)];
 [timer addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];
 }
 
 - (void)moveSecond
 {
 // 1. 計(jì)算當(dāng)前的秒數(shù)
 NSDate *nowDate = [NSDate date];
 NSCalendar *calendar = [NSCalendar currentCalendar];
 NSInteger seconds = [calendar component:NSCalendarUnitSecond fromDate:nowDate];
 // 2. 計(jì)算每一秒的弧度
 CGFloat rotation = M_PI * 2 / 60;
 // 3. 用每一秒的弧度 * 秒數(shù) , 計(jì)算出本次要旋轉(zhuǎn)的弧度
 rotation = seconds * rotation;
 // 4. 旋轉(zhuǎn)秒針
 self.secondView.transform = CGAffineTransformMakeRotation(rotation);
 }
 
 // 每次觸摸一次屏幕秒針轉(zhuǎn)一次
 - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
 {
 CGFloat rotation = M_PI * 2 / 60;
 self.secondView.transform = CGAffineTransformRotate(self.secondView.transform, rotation);
 }
 @end
 */

2. 當(dāng)雙擊 home 鍵的時(shí)候, 動(dòng)畫(huà)不會(huì)暫停

• 注意: 如果當(dāng)動(dòng)畫(huà)正在執(zhí)行的時(shí)候, 將程序退出到后臺(tái), 那么當(dāng)程序再次進(jìn)入前臺(tái)的時(shí)候就不執(zhí)行了。
• 原因: 因?yàn)樵俅芜M(jìn)入前臺(tái)后動(dòng)畫(huà)已經(jīng)被刪除了。
• 解決1: anim.removedOnCompletion = NO;
• 問(wèn)題: 當(dāng)雙擊 home 鍵的時(shí)候, 動(dòng)畫(huà)不會(huì)暫停。
• 解決:
  /** 參考:

// 暫停
- (void)applicationWillResignActive:(UIApplication *)application {
ViewController *vc =  (ViewController *)self.window.rootViewController;
[vc pause];
}

// 恢復(fù)
- (void)applicationDidBecomeActive:(UIApplication *)application {
ViewController *vc =  (ViewController *)self.window.rootViewController;
[vc resume];
}
*/