systemLayoutSizeFitting始终返回零 [英] systemLayoutSizeFitting returns zero always

问题描述

基于Apple的 doc ，应该遵守UIView元素上的当前约束.但是，每当我运行以下代码时，对于UIView.layoutFittingCompressedSize输入，我将得到{0, 0}，对于UIView.layoutFittingExpandedSizeSize输入，我将得到{1000, 1000}.

Based on Apple's doc, systemLayoutSizeFitting is supposed to respect current constraints on the UIView element when returning the optimal size. However, whenever I run following code, I would get {0, 0} for UIView.layoutFittingCompressedSize and {1000, 1000} for UIView.layoutFittingExpandedSizeSize input.

let mainView = UIView(frame: CGRect(origin: .zero, size: CGSize(width: 375, height: 50)))
mainView.backgroundColor = .red
PlaygroundPage.current.liveView = mainView

let subview = UIView()
subview.backgroundColor = .yellow
mainView.addSubview(subview)
subview.snp.makeConstraints { make in
    make.width.equalToSuperview().dividedBy(3.0)
    make.left.top.bottom.equalToSuperview()
}
mainView.setNeedsLayout()
mainView.layoutIfNeeded()

subview.frame

subview.systemLayoutSizeFitting(UIView.layoutFittingCompressedSize)

我注意到，如果将width约束更改为常量，那么我将从systemLayoutSizeFitting获得有效值.试图了解为什么会发生这种行为，以及是否有可能从systemLayoutSizeFittingSize(_ size: CGSize)获取正确的值.

I noticed that if I change the width constraint to something constants, then I'd get a valid value from systemLayoutSizeFitting. Trying to understand why such behavior is happening and if it's possible to get a right value from systemLayoutSizeFittingSize(_ size: CGSize).

推荐答案

此文档似乎缺少文档.

看来.systemLayoutSizeFitting高度依赖于元素的.intrinsicContentSize.对于UIView，它没有固有的内容大小(除非您已覆盖它).

It appears that .systemLayoutSizeFitting is highly dependent on the .intrinsicContentSize of the element. In the case of a UIView, it has no intrinsic content size (unless you've overridden it).

因此，如果相关约束是另一个约束的 percentage ，则.systemLayoutSizeFitting(UIView.layoutFittingCompressedSize)将返回{0, 0}.我认为这是因为相关约束可能会更改(变为零)，因此最小值实际上是零.

So, if the related constraint is a percentage of another constraint, .systemLayoutSizeFitting(UIView.layoutFittingCompressedSize) will return {0, 0}. I gather this is because the related constraint could change (to Zero), therefor the minimum value is, in fact, Zero.

如果将.width约束更改为常量(例如mainView.frame.width * 0.3333)，则由于常量宽度约束变为固有宽度，您将获得有效的大小值.

If you change your .width constraint to a constant (such as mainView.frame.width * 0.3333) then you'll get a valid size value, as the constant width constraint becomes the intrinsic width.

例如，如果子视图是UILabel，则该元素将具有固有尺寸，而.systemLayoutSizeFitting应返回您期望的尺寸值.

If your subview is a UILabel, for example, that element will have an intrinsic size, and .systemLayoutSizeFitting should return a size value that you'd expect.

以下是使用UILabel的示例，该示例将演示:

Here's an example using a UILabel that will demonstrate:

import UIKit
import PlaygroundSupport

let mainView = UIView(frame: CGRect(origin: .zero, size: CGSize(width: 375, height: 50)))
mainView.backgroundColor = .red
PlaygroundPage.current.liveView = mainView

let v = UILabel()
v.text = "Testing"
v.translatesAutoresizingMaskIntoConstraints = false
v.backgroundColor = .green
mainView.addSubview(v)

NSLayoutConstraint.activate([
    v.widthAnchor.constraint(equalTo: mainView.widthAnchor, multiplier: 3.0 / 10.0),
    v.leftAnchor.constraint(equalTo: mainView.leftAnchor),
    v.topAnchor.constraint(equalTo: mainView.topAnchor),
    v.bottomAnchor.constraint(equalTo: mainView.bottomAnchor),
    ])

mainView.setNeedsLayout()
mainView.layoutIfNeeded()

v.frame

v.systemLayoutSizeFitting(UIView.layoutFittingCompressedSize)

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