It also center aligns the view and superview.

它还让视图的中心跟父视图的中心对齐。

youdao

It may not be obvious, but the second and third layouts produce identical behaviors: They both maintain a fixed margin between the view and its superview as the superview's width changes.

或许不是很明显，但是第二第三个布局产生一样的行为：即使父视图的尺寸发生改变，它们都让视图和父视图的边距保持不变。

youdao

Changing the bounds origin of the superview does not change the position of the view.

ENGADGET: Apple seeds iOS 6 beta 4 to developers: here's the changelog

That is, if you pin a view to the left edge of its superview, you're really pinning it to the minimum x-value of the superview's bounds.

ENGADGET: Apple seeds iOS 6 beta 4 to developers: here's the changelog

Note that you can make a subview of the scroll view appear to float (not scroll) over the other scrolling content by creating constraints between the view and a view outside the scroll view's subtree, such as the scroll view's superview.

ENGADGET: Apple seeds iOS 6 beta 4 to developers: here's the changelog