如何将变量从类传递到另一个？ [英] How to pass a variable from a class to another?

问题描述

目标

我需要我的应用传递变量的值 myVariable 仅当变量更改其值时，才从类 firstClass 到另一个 secondClass 。

I need my app to pass the value of a variable myVariable from a class firstClass to another secondClass only when the variable changed its value.

代码

为此，我想到了使用 willSet 属性。虽然，在Swift中你不能在声明变量之后使用它。

To do so, I thought of using the willSet property. Though, in Swift you can't use it after the declaration of the variable.

class firstClass: NSObject {
    var myVariable = 0

    func myFunction {
       myVariable = 5
    }
}

class secondClass: NSObject {
    var otherClass = firstClass()
    // How do I retrive the value of the variable right after its value changed?
} 

我还想过添加 NSNotification ，但这没有用，因为它没有传递值。 NSNotification 仅提醒其更改。

I also thought of adding a NSNotification, but that wouldn't help because it doesn't pass a value. NSNotification only alerts about its changes.

 let myVariableNotification = NSNotification(name: "myVariableNotification", object: nil)

 class firstClass: NSObject {
    var myVariable = 0

    func myFunction {
       myVariable = 5 
       notificationCenter.postNotification(myVariableNotification)
    }
}

class secondClass: NSObject {
    var otherClass = firstClass()
       NSNotificationCenter.defaultCenter().addObserverForName("myVariableNotification", 
         object: nil, 
         queue: NSOperationQueue.mainQueue()
         usingBlock: { notification in
         println("The variable has been updated!")
       })
}

问题

一旦变量改变了值，我似乎无法传递变量。我该怎么做？

I seem to find no way to pass a variable once that variable changed its value. How can I do that?

推荐答案

您应该使用委托协议。有关详细信息，请查看此文档。

You should use delegate protocols. For more information check out this document.

在 secondClass 中设置协议，对在 import 语句之后，如下所示：

Set up a protocol in the secondClass, right after the import statements, like so:

protocol InformingDelegate {
    func valueChanged() -> CGFloat
}

在同一个 secondClass 创建一个委托变量（有人建议它应该标记为 weak ）：

Inside the same secondClass create a delegate variable (some suggest that it should be marked as weak):

var delegate: InformingDelegate?

然后，创建一些方法，您将在其中访问更改的值。您可以将其分配给值，例如：

Then, create some method in which you will access the changed value. You can assign it to value for example:

func callFromOtherClass() {
    value = self.delegate?.valueChanged()
}

这适用于 secondClass 。现在进入 firstClass 。

这里你只需要通过添加 InformingDelegate 来符合协议在类定义之后，如下所示：

This is it for the secondClass. Now onto the firstClass.
Here you only need to conform to the protocol by adding InformingDelegate after the class definition, like this:

class firstClass: UIViewController, InformingDelegate {
    ...
}

然后，通过创建它来通知编译器您将成为另一个类的委托实例，并将自己设置为委托：

Then, inform the compiler that you are going to be a delegate for the other class by creating its instance, and setting yourself to be the delegate:

var secondVC : secondClass = secondClass()
secondClass.delegate = self
secondClass.callFromOtherClass() // This will call the method in the secondClass  
// which will then ask its delegate to trigger a method valueChanged() -   
// Who is the delegate? Well, your firstClass, so you better implement  
// this method!

最后一件事是通过实施其方法来实际符合协议：

The last thing is to actually conform to the protocol by implementing its method:

func valueChanged() -> CGFloat {
    return myVariable // which is 5 in your case (value taken from a question)
}

这会将 myVariable 值（本例中为5）分配给另一个值 class。

This will assign myVariable value (5 in this example) to the value in the other class.

这篇关于如何将变量从类传递到另一个？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！