ios - 如何使自动布局约束依赖于多个其他 anchor ？
<p><p>如何使用自动布局使 View 的高度等于其他两个 View 的高度之和？</p>
<p>例如:</p>
<pre><code>viewA.heightAnchor.constraint(equalTo: ...),
viewB.heightAnchor.constraint(equalTo: ...),
viewC.heightAnchor.constraint(equalTo: viewA.heightAnchor + viewB.heightAnchor)
</code></pre>
<p>是否有不涉及设置常量值并在每次 View 边界更改时重新计算它的解决方案？</p></p>
<br><hr><h1><strong>Best Answer-推荐答案</ strong></h1><br>
<p><p>你<em>可以</em>，但我认为只能借助一些辅助 View 。请参阅我刚刚制作的这个示例 Playground ，它实现了这一点:</p>
<pre><code>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
</code></pre>
<p>这个想法是你有两个帮助 View 彼此垂直堆叠，由顶部的 <code>bottomAnchor</code> 和底部的 <code>topAnchor</code> 连接。然后将这些辅助 View 的高度设置为等于 <code>viewA</code> 和 <code>viewB</code> 的高度。然后，您的 <code>viewC</code> 可以附加到顶 View 的 <code>topAnchor</code> 和底 View 的 <code>bottomAnchor</code>，从而得到 <code>viewC</的结果code> 是 <code>viewA</code> 的高度加上 <code>viewB</code> 的高度。</p></p>
<p style="font-size: 20px;">关于ios - 如何使自动布局约束依赖于多个其他 anchor ？，我们在Stack Overflow上找到一个类似的问题：
<a href="https://stackoverflow.com/questions/49060957/" rel="noreferrer noopener nofollow" style="color: red;">
https://stackoverflow.com/questions/49060957/
</a>
</p>
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