<p style="margin: 0px; font-size: 16px; line-height: normal; font-family: Menlo;"><span style="font-variant-ligatures: no-common-ligatures">willMovetoSuperview</span></p>
记录父视图，必须是UIScrollView的子类
KVO监听父视图的contentOffset
<p style="margin: 0px; font-size: 16px; line-height: normal; font-family: Menlo;"><span style="font-variant-ligatures: no-common-ligatures">removeFromSuperview</span></p>
通过路径移除父视图<span style="font-family: Menlo; font-size: 16px; font-variant-ligatures: no-common-ligatures; color: rgb(112, 61, 170);">superview</span><span style="font-family: Menlo; font-size: 16px; font-variant-ligatures: no-common-ligatures;">?.</span><span style="font-family: Menlo; font-size: 16px; font-variant-ligatures: no-common-ligatures; color: rgb(61, 29, 129);">removeObserver</span><span style="font-family: Menlo; font-size: 16px; font-variant-ligatures: no-common-ligatures;">(</span><span style="font-family: Menlo; font-size: 16px; font-variant-ligatures: no-common-ligatures; color: rgb(187, 44, 162);">self</span><span style="font-family: Menlo; font-size: 16px; font-variant-ligatures: no-common-ligatures;">, forKeyPath: </span><span style="font-family: Menlo; font-size: 16px; font-variant-ligatures: no-common-ligatures; color: rgb(209, 47, 27);">"contentOffSet"</span><span style="font-family: Menlo; font-size: 16px; font-variant-ligatures: no-common-ligatures;">)</span>
注：移除路径的代码要添加在父类方法之前
