CPAnimationSequenceiOS视图动画效果

编写iOS视图UIView的动画效果需要很多代码，虽然iOS 4支持块定义，对视图动画的定义减少了不少代码，但语法依旧不好看。CPAnimationSequence是一个开源代码库，可以一种间接快捷的方式来定义描述动画队列，克服了写繁多的代码。

下面一个使用例子：

CPAnimationSequence* shakespeare = [CPAnimationSequence sequenceWithSteps:
    [CPAnimationStep           for:0.2 animate:^{ self.romeo.alpha = 1.0; }],
    [CPAnimationStep           for:0.2 animate:^{ self.julia.alpha = 1.0;  }],
    [CPAnimationStep after:1.0 for:0.7 animate:^{
                            CGPoint love = CGPointMake((self.romeo.center.x + self.julia.center.x)/2,
                                                       (self.romeo.center.y + self.julia.center.y)/2);
                            self.romeo.center = love;
                            self.julia.center = love;
    }],
    [CPAnimationStep after:2.0 for:0.5 animate:^{ self.romeo.alpha = 0.0;
                                                  self.julia.alpha = 0.0; }]
    nil];
[shakespeare run];