@robmayoff有一个很好的答案：http://***.com/a/26066992/1424669

@robmayoff has a great answer for this: http://***.com/a/26066992/1424669

从本质上讲，在iOS8上你再也不能叫 setNeedsUpdateConstraints 和 setNeedsLayout 上一个视图，并期望子视图的限制更新。

Essentially, in iOS8 you can no longer call setNeedsUpdateConstraints and setNeedsLayout on a view and expect the constraints of subviews to update.

您必须要求其约束正在改变视图这些方法。这是为了iOS7向后兼容。

You must call these methods on the view whose constraint is changing. This is backwards compatible to iOS7.

示例：

假设你有根视图 self.view 和一个名为containerView子视图一个的ViewController。 containerView具有附加到它 NSLayoutConstraint 要改变（在这种情况下，顶端空间）。

Suppose you have a ViewController with root view self.view and a subview called containerView. containerView has a NSLayoutConstraint attached to it that you want to change (in this case, top space).

在iOS7你可以通过请求根认为一个新的布局更新一个VC的所有限制：

In iOS7 you could update all constraints in a VC by requesting a new layout for the root view:

self.containerView_TopSpace.constant = 0;
[self.view setNeedsUpdateConstraints];
[self.view setNeedsLayout];

在iOS8上，你需要请求在containerView布局：

In iOS8 you need to request layouts on the containerView:

self.containerView_TopSpace.constant = 0;
[self.containerView setNeedsUpdateConstraints];
[self.containerView setNeedsLayout];