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Where To Put The Auto Layout Code将AutoLayout的代码放在哪里?参考文档A参考文档B结论原文链接

Where To Put The Auto Layout Code将AutoLayout的代码放在哪里?

Auto Layout is kind of magic. Like a sorcerer you tell the elements where to position and how to behave. You don’t put the elements to those positions yourself. The universe moves them because of your spells. Kind of.(AutoLayout就像一种魔法.好像只要你念几下咒语,巫师就可以帮你把元素布局好.)

But when should the spells be spoken? In other words when not using Interface Builder, where should the Auto Layout code go?(问题是:在不使用IB的时候,该什么时候念咒语?)

参考文档A

From the documentation of the UIView method updateConstraints:

Custom views that set up constraints themselves should do so by overriding this method.

Because of this sentence I always thought Apple asks me to put the layout code in this method. But here is the problem: This method is called more than once by UIKit and adding the same constraint more than once is an error. The suggested approach I found in the Internet? is to add a boolean to the view class and set it in updateConstraints() to make sure to only run the layout code once.(我以为Apple让我把AutoLayout的代码放在这里,但是在UIKit中这个方法会调用多次,解决的办法是添加一个布尔变量,让这段代码仅仅执行一次.)

参考文档B

From the documentation again:

When your custom view notes that a change has been made to the view that invalidates one of its constraints, it should immediately remove that constraint, and then call setNeedsUpdateConstraints to note that constraints need to be updated.

This means updateConstraints() is meant to be used in the case then the constraints within a view change because of some events. For this scenario the name of the method makes much more sense.

In nearly all of my layouts the constraints are fixed. Sometimes I need to change the constant of a constraint. This can be done without removing and re-adding the constraint. (The constant of a NSLayoutConstraint is the only thing that can be changed after creation.)(改变一个约束的常数.)

Because of this, I started to put all the layout code in init(frame:). But I was not really comfortable with it because of the documentation mentioned above.(把所有的AutoLayout的代码都放在了init方法中.)

But then I heard in a session video of the WWDC this year an Apple engineer suggesting exactly that. And just yesterday I received an answer to a bug report in which I described the ‘bug’ that updateConstraints() isn’t called. Turns out it wasn’t a bug.

The Apple engineer wrote:

In general, if the constraints will only be created once, it should be done in an initialization method (such as -init or -viewDidLoad, and so forth). Save -updateConstraints for things that are expected to change over the course of running the App.

一般的,如果一个约束只被创建一次,这段约束的代码应该放在初始化方法中(例如init,viewDidLoad)

Nice. I love when my feeling about how code should be written is suggested by Apple.

An example of where I put my layout code can be found here.

Happy layouting!

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Update: Ole Begemann wrote about When should you implement updateConstraints on his great blog.

No related posts.

结论

一般的,如果一个约束只被创建一次,这段约束的代码应该放在初始化方法中(例如init,viewDidLoad)

原文链接

http://swiftandpainless.com/where-to-put-the-auto-layout-code/