为什么在Swift中以编程方式声明约束时会出现错误？ [英] Why do I get an error when declaring a constraint programmatically in Swift?

问题描述

我在 UIViewController 类中声明了一个约束 Home 。代码如下：

I am declaring a constraint inside a UIViewController class called Home. Here's what the code looks like:

class Home: UIViewController {

@IBOutlet weak var buttonUpgrade: UIButton!

var constraint = NSLayoutConstraint (item: buttonUpgrade, 
attribute: NSLayoutAttribute.Bottom, 
relatedBy: NSLayoutRelation.Equal, 
toItem: self.view, 
attribute: NSLayoutAttribute.Bottom, 
multiplier: 1, 
constant: 500)

...
}

但是，我收到此错误： Home.type没有名为buttonUpgrade的成员。为什么？

However, I get this error: Home.type does not have a member named buttonUpgrade. Why?

您可以从这里下载专案： https://www.dropbox.com/s/x7avclri0ijw3pd/DemoConstraints.zip?dl=0 。

You can download the project from here: https://www.dropbox.com/s/x7avclri0ijw3pd/DemoConstraints.zip?dl=0.

推荐答案

如前所述，你不能在函数之外编写这样的代码。但是你可以做的是声明你的变量global（在函数之外）：

As already mentioned, you can't write such code outside a function. But what you can do is declaring your variable global(outside a function) like that:

var constraint:NSLayoutConstraint!

然后你可以初始化它，例如你的 viewDidLoad -method将被调用一次：

Then you can initialize it in for example your viewDidLoad-method which will be called once:

constraint = NSLayoutConstraint (item: buttonUpgrade, 
attribute: NSLayoutAttribute.Bottom, 
relatedBy: NSLayoutRelation.Equal, 
toItem: self.view, 
attribute: NSLayoutAttribute.Bottom, 
multiplier: 1, 
constant: 500)

之后，您可以从类中的任何地方访问约束变量。只要确保在使用变量之前填充变量。

After you did that, you can access the constraint variable from everywhere in your class. Just make sure that the variable is filled before using it.

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