如何使自动布局约束依赖于多个其他锚点？

Question

如何使用自动布局使视图的高度等于其他两个视图的高度之和？

例如：

viewA.heightAnchor.constraint(equalTo: ...),
viewB.heightAnchor.constraint(equalTo: ...),
viewC.heightAnchor.constraint(equalTo: viewA.heightAnchor + viewB.heightAnchor)

是否有一种解决方案不涉及设置常量值并在每次视图边界更改时重新计算它？

Answer 1

我认为您可以，但只能借助一些辅助视图。请参阅我刚刚编写的这个示例游乐场，它实现了这一目标：

import Foundation
import UIKit
import PlaygroundSupport

let vc = UIViewController()
vc.view = UIView()
vc.view.backgroundColor = .white

let viewA = UIView()
let viewB = UIView()
let viewC = UIView()

viewA.backgroundColor = .red
viewB.backgroundColor = .green
viewC.backgroundColor = .blue

viewA.translatesAutoresizingMaskIntoConstraints = false
viewB.translatesAutoresizingMaskIntoConstraints = false
viewC.translatesAutoresizingMaskIntoConstraints = false

vc.view.addSubview(viewA)
vc.view.addSubview(viewB)
vc.view.addSubview(viewC)

let helperViewA = UIView()
let helperViewB = UIView()

helperViewA.translatesAutoresizingMaskIntoConstraints = false
helperViewB.translatesAutoresizingMaskIntoConstraints = false

helperViewA.isHidden = true
helperViewB.isHidden = true

vc.view.addSubview(helperViewA)
vc.view.addSubview(helperViewB)

viewA.bottomAnchor.constraint(equalTo: vc.view.bottomAnchor).isActive = true
viewA.leadingAnchor.constraint(equalTo: vc.view.leadingAnchor).isActive = true
viewA.trailingAnchor.constraint(equalTo: viewB.leadingAnchor).isActive = true
viewA.heightAnchor.constraint(equalToConstant: 20.0).isActive = true

viewB.bottomAnchor.constraint(equalTo: vc.view.bottomAnchor).isActive = true
viewB.widthAnchor.constraint(equalTo: viewA.widthAnchor, multiplier: 1.0).isActive = true
viewB.trailingAnchor.constraint(equalTo: vc.view.trailingAnchor).isActive = true
viewB.heightAnchor.constraint(equalToConstant: 40.0).isActive = true

viewA.heightAnchor.constraint(equalTo: helperViewA.heightAnchor, multiplier: 1.0).isActive = true
viewB.heightAnchor.constraint(equalTo: helperViewB.heightAnchor, multiplier: 1.0).isActive = true

helperViewA.bottomAnchor.constraint(equalTo: helperViewB.topAnchor).isActive = true

viewC.widthAnchor.constraint(equalToConstant: 100).isActive = true
viewC.topAnchor.constraint(equalTo: helperViewA.topAnchor).isActive = true
viewC.bottomAnchor.constraint(equalTo: helperViewB.bottomAnchor).isActive = true


helperViewA.leadingAnchor.constraint(equalTo: viewC.leadingAnchor).isActive = true
helperViewA.trailingAnchor.constraint(equalTo: viewC.trailingAnchor).isActive = true
helperViewB.leadingAnchor.constraint(equalTo: viewC.leadingAnchor).isActive = true
helperViewB.trailingAnchor.constraint(equalTo: viewC.trailingAnchor).isActive = true

viewC.centerXAnchor.constraint(equalTo: vc.view.centerXAnchor).isActive = true
viewC.centerYAnchor.constraint(equalTo: vc.view.centerYAnchor).isActive = true

vc.view.frame.size = CGSize(width: 375, height: 667)
PlaygroundPage.current.liveView = vc.view

这个想法是，你有两个互相垂直堆叠的辅助视图，通过顶部的bottomAnchor和底部的来连接topAnchor。然后，您将每个辅助视图的高度设置为等于您的viewA高度viewB。然后，您viewC可以将其附加到顶视图topAnchor和底视图bottomAnchor，这给出的结果viewC是 的高度viewA加上的viewB高度。