“得到"是什么意思?在协议的属性声明中是什么意思? [英] What does "get" mean in a protocol's property declaration?

问题描述

我正在查看自动布局库中的一些代码.其中有一个UIView采用的协议:

I'm looking at some code from an auto layout library. In it, there is a protocol adopted by UIView:

extension UIView: Constrainable {}
extension UILayoutGuide: Constrainable {
    // LayoutGuide doesn't have baseline anchors, so just use the bottom anchor
    public var firstBaselineAnchor: NSLayoutYAxisAnchor {
        return bottomAnchor
    }
    public var lastBaselineAnchor: NSLayoutYAxisAnchor {
        return bottomAnchor
    }
}

public protocol Constrainable {
    var topAnchor:      NSLayoutYAxisAnchor { get }
    var bottomAnchor:   NSLayoutYAxisAnchor { get }
    var leftAnchor:     NSLayoutXAxisAnchor { get }
    var rightAnchor:    NSLayoutXAxisAnchor { get }
    var leadingAnchor:  NSLayoutXAxisAnchor { get }
    var trailingAnchor: NSLayoutXAxisAnchor { get }

    var centerXAnchor:  NSLayoutXAxisAnchor { get }
    var centerYAnchor:  NSLayoutYAxisAnchor { get }

    var widthAnchor:    NSLayoutDimension { get }
    var heightAnchor:   NSLayoutDimension { get }

    var firstBaselineAnchor : NSLayoutYAxisAnchor { get }
    var lastBaselineAnchor  : NSLayoutYAxisAnchor { get }
}

空的 { get } 有什么作用?

推荐答案

这些是采用协议的类必须为其提供 getter 的属性.该协议没有指定有关 setter 的任何内容，因此类可以提供计算属性而不是存储属性.

These are properties for which the classes that adopt the protocol must supply a getter. The protocol does not specify anything about the setter, so classes could supply a computed property instead of a stored one.

例如，一个采用Constrainable的类可以通过添加

For example, a class that adopts Constrainable could satisfy a requirement of having topAnchor by adding

var topAnchor: NSLayoutYAxisAnchor

或通过添加

var topAnchor: NSLayoutYAxisAnchor {
    ...
    return ...
}

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