updateConstraints、needsUpdateConstraints、setNeedsUpdateConstraints、updateConstraintsIfNeeded

官方文档的阐述

updateConstraints

Override this method to optimize changes to your constraints.

Note

It is almost always cleaner and easier to update a constraint immediately after the affecting change has occurred. For example, if you want to change a constraint in response to a button press, make that change directly in the button’s action method.

You should only override this method when changing constraints in place is too slow, or when a view is producing a number of redundant changes.
To schedule a change, set the view’s needsUpdateConstraints property to YES. The system then calls your implementation of updateConstraints before the layout occurs. This lets you verify that all necessary constraints for your content are in place at a time when your custom view’s properties are not changing.

Your implementation must be as efficient as possible. Do not deactivate all your constraints, then reactivate the ones you need. Instead, your app must have some way of tracking your constraints, and validating them during each update pass. Only change items that need to be changed. During each update pass, you must ensure that you have the appropriate constraints for the app’s current state.

Do not set the needsUpdateConstraints property inside your implementation. Setting needsUpdateConstraints to YES schedules another update pass, creating a feedback loop.

Important

Call [super updateConstraints] as the final step in your implementation.

重写此方法以优化对约束的更改。

请注意

在影响更改发生后立即更新约束几乎总是更干净、更容易。例如，如果您希望更改一个约束以响应按钮按下，请直接在按钮的操作方法中更改该约束。

只有在更改适当的约束太慢或视图产生大量冗余更改时，才应该重写此方法。

若要安排更改，请将视图的needsUpdateConstraints属性设置为YES。然后，系统在出现布局之前调用您的updateConstraints实现。这使您可以验证，当自定义视图的属性没有更改时，内容的所有必要约束都已就绪。

您的实现必须尽可能地高效。不要破坏你所有的限制，然后重新激活你需要的。相反，应用程序必须有某种方式来跟踪约束，并在每次更新时验证它们。只更改需要更改的项。在每次更新过程中，必须确保对应用程序的当前状态有适当的约束。

不要在实现中设置needsUpdateConstraints属性。设置必要的条件，以确定另一个更新通过，创建一个反馈循环。

重要

调用[super updateconstr]作为实现的最后一步。

needsUpdateConstraints

The constraint-based layout system uses the return value of this method to determine whether it needs to call updateConstraints on your view as part of its normal layout pass.

基于约束的布局系统使用此方法的返回值来确定是否需要在视图中调用updateConstraints作为其正常布局传递的一部分。

setNeedsUpdateConstraints

When a property of your custom view changes in a way that would impact constraints, you can call this method to indicate that the constraints need to be updated at some point in the future. The system will then call updateConstraints as part of its normal layout pass. Use this as an optimization tool to batch constraint changes. Updating constraints all at once just before they are needed ensures that you don’t needlessly recalculate constraints when multiple changes are made to your view in between layout passes.

当您的自定义视图的属性以一种会影响约束的方式发生变化时，您可以调用此方法来指示在将来某个时点需要更新约束。然后，系统将调用updateConstraints作为常规布局传递的一部分。使用它作为一个优化工具来批处理约束更改。在需要约束之前立即更新所有约束，确保在布局传递之间对视图进行多次更改时，不需要重新计算约束。

updateConstraintsIfNeeded

When a new layout pass is triggered for a window, the system invokes this method to ensure that any constraints for views in the window are updated with information from the current view hierarchy and its constraints. This method is called automatically by the system, but may be invoked manually if you need to examine the most up to date constraints.

Subclasses should not override this method.

当为窗口触发新的布局传递时，系统调用此方法以确保窗口中视图的任何约束都使用来自当前视图层次结构及其约束的信息进行更新。系统自动调用此方法，但如果需要检查最新的约束条件，则可以手动调用该方法。

子类不应重写这个方法。