有时候会遇到UILabel中的内容超出长度，显示不完全的问题。有一种解决方法是通过动画字幕来实现，比如：
1.字幕向左或者右滚动
2.字幕来回滚动
本文以后者为例来说明吧。这里先介绍UIView的通过Block实现的Animation以及其参数控制，最后是实现滚动字幕的代码。
1. UIView有方便的动画实现方式，SDK4.0以上，提供了三个Block的动画方式：
+ (void)animateWithDuration:(NSTimeInterval)duration delay:(NSTimeInterval)delay options:(UIViewAnimationOptions)options animations:(void (^)(void))animations completion:(void (^)(BOOL finished))completion __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_4_0);
+ (void)animateWithDuration:(NSTimeInterval)duration animations:(void (^)(void))animations completion:(void (^)(BOOL finished))completion __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_4_0); // delay = 0.0, options = 0
+ (void)animateWithDuration:(NSTimeInterval)duration animations:(void (^)(void))animations __OSX_AVAILABLE_STARTING(__MAC_NA,__IPHONE_4_0); // delay = 0.0, options = 0, completion = NULL
其中第一个最全面，可以设置UIViewAnimationOptions来控制动画的参数，比如重复，自动reverse之类的。
2. UIViewAnimationOptions具体定义如下：
enum {
UIViewAnimationOptionLayoutSubviews = 1 << 0,
UIViewAnimationOptionAllowUserInteraction = 1 << 1, // turn on user interaction while animating
UIViewAnimationOptionBeginFromCurrentState = 1 << 2, // start all views from current value, not initial value
UIViewAnimationOptionRepeat = 1 << 3, // repeat animation indefinitely
UIViewAnimationOptionAutoreverse = 1 << 4, // if repeat, run animation back and forth
UIViewAnimationOptionOverrideInheritedDuration = 1 << 5, // ignore nested duration
UIViewAnimationOptionOverrideInheritedCurve = 1 << 6, // ignore nested curve
UIViewAnimationOptionAllowAnimatedContent = 1 << 7, // animate contents (applies to transitions only)
UIViewAnimationOptionShowHideTransitionViews = 1 << 8, // flip to/from hidden state instead of adding/removing
UIViewAnimationOptionCurveEaseInOut = 0 << 16, // default
UIViewAnimationOptionCurveEaseIn = 1 << 16,
UIViewAnimationOptionCurveEaseOut = 2 << 16,
UIViewAnimationOptionCurveLinear = 3 << 16,
UIViewAnimationOptionTransitionNone = 0 << 20, // default
UIViewAnimationOptionTransitionFlipFromLeft = 1 << 20,
UIViewAnimationOptionTransitionFlipFromRight = 2 << 20,
UIViewAnimationOptionTransitionCurlUp = 3 << 20,
UIViewAnimationOptionTransitionCurlDown = 4 << 20,
UIViewAnimationOptionTransitionCrossDissolve = 5 << 20,
UIViewAnimationOptionTransitionFlipFromTop = 6 << 20,
UIViewAnimationOptionTransitionFlipFromBottom = 7 << 20,
};
typedef NSUInteger UIViewAnimationOptions;
3. 本文实现方法就是使用Animation with Block的方式来实现UILabel来回滚动。代码如下：
-(void)startAnimationIfNeeded{
//取消、停止所有的动画
[self.aUILabel.layer removeAllAnimations];
CGSize textSize = [self.aUILabel.text sizeWithFont:self.aUILabel.font];
CGRect lframe = self.aUILabel.frame;
lframe.size.width = textSize.width;
self.aUILabel.frame = lframe;
const float oriWidth = 180;
if (textSize.width > oriWidth) {
float offset = textSize.width - oriWidth;
[UIView animateWithDuration:3.0
delay:0
options:UIViewAnimationOptionRepeat //动画重复的主开关
|UIViewAnimationOptionAutoreverse //动画重复自动反向，需要和上面这个一起用
|UIViewAnimationOptionCurveLinear //动画的时间曲线，滚动字幕线性比较合理
animations:^{
self.aUILabel.transform = CGAffineTransformMakeTranslation(-offset, 0);
}
completion:^(BOOL finished) {
}
];
}
}