AutoLayout約束以適合矩形內部的視圖，並保留一定的縱橫比（以編程方式）

Question

我想將圖像放置在應該具有特定長寬比的矩形內。 不管它是什么，它都應該找到適合矩形內部的形式。 我在情節提要中玩了一下，得到了：

帶有虛線邊框的優先級較低（250）。 這在情節提要中起作用。 但是，我需要以編程方式創建這些約束，所以我這樣嘗試了（我使用的是SnapKit ，它只是提供了更好的AutoLayout語法。這應該是不言自明的）：

let topView = UIView()
topView.translatesAutoresizingMaskIntoConstraints = false
topView.backgroundColor = .gray
view.addSubview(topView)

topView.snp.makeConstraints { (make) in
        make.top.equalToSuperview()
        make.left.equalToSuperview()
        make.trailing.equalToSuperview()
        make.height.equalTo(250)
        }

// This view should have a specific aspect ratio and fit inside topView
let holderView = UIView()
holderView.translatesAutoresizingMaskIntoConstraints = false
holderView.backgroundColor = .red
topView.addSubview(holderView)

holderView.snp.makeConstraints { (make) in
        make.center.equalToSuperview() // If I remove this one, there's no auto-layout issue, but then it's offset
        make.edges.equalToSuperview().priority(250) // sets leading, trailing, top and bottom
        make.edges.greaterThanOrEqualToSuperview().priority(1000)
        make.width.equalTo(holderView.snp.height).multipliedBy(3/2)
        }

如果將其粘貼到一個空的ViewController中並啟動它，則會出現以下問題：

2018-03-16 15:38:50.188867+0100 DemoProject[11298:850932] [LayoutConstraints] Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want. 
Try this: 
    (1) look at each constraint and try to figure out which you don't expect; 
    (2) find the code that added the unwanted constraint or constraints and fix it. 

"<SnapKit.LayoutConstraint:0x6000000a7c80@ViewController.swift#24 UIView:0x7fcd82d12440.left == UIView:0x7fcd82d12640.left>",
"<SnapKit.LayoutConstraint:0x6000000a7ce0@ViewController.swift#25 UIView:0x7fcd82d12440.trailing == UIView:0x7fcd82d12640.trailing>",
"<SnapKit.LayoutConstraint:0x6000000a7d40@ViewController.swift#26 UIView:0x7fcd82d12440.height == 250.0>",
"<SnapKit.LayoutConstraint:0x6000000a7da0@ViewController.swift#35 UIView:0x7fcd8580dad0.centerX == UIView:0x7fcd82d12440.centerX>",
"<SnapKit.LayoutConstraint:0x6000000a7e00@ViewController.swift#35 UIView:0x7fcd8580dad0.centerY == UIView:0x7fcd82d12440.centerY>",
"<SnapKit.LayoutConstraint:0x6000000a8580@ViewController.swift#37 UIView:0x7fcd8580dad0.top >= UIView:0x7fcd82d12440.top>",
"<SnapKit.LayoutConstraint:0x6000000a8a60@ViewController.swift#37 UIView:0x7fcd8580dad0.right >= UIView:0x7fcd82d12440.right>",
"<SnapKit.LayoutConstraint:0x6000000a9360@ViewController.swift#38 UIView:0x7fcd8580dad0.width == UIView:0x7fcd8580dad0.height>",
"<NSLayoutConstraint:0x600000092cf0 'UIView-Encapsulated-Layout-Width' UIView:0x7fcd82d12640.width == 414   (active)>"


Will attempt to recover by breaking constraint <SnapKit.LayoutConstraint:0x6000000a9360@ViewController.swift#38 UIView:0x7fcd8580dad0.width == UIView:0x7fcd8580dad0.height>

當我刪除居中約束make.center.equalToSuperview()時，這不會顯示。 但隨后，它放錯了位置。

情節提要和我的代碼之間有什么區別？ 我不太明白。 我還使用默認的swift語法嘗試了此操作，結果完全相同。 所以我認為SnapKit問題

有任何想法嗎？ 謝謝大家的幫助。 讓我知道您是否需要更多信息。

編輯：我混合了一些東西。 這與圖像及其寬高比無關。 它只是關於一個UIView ，它應該在安裝在矩形內時保持特定的寬高比。 實際圖像將被放入該holderView 。 抱歉

Answer 1

好的-這是一種方法。

取子視圖的“本機”大小，計算“長寬比”比例-即適合寬度或高度到父視圖的比例，並適當縮放其他尺寸。

然后，使用centerXAnchor和centerYAnchor定位子視圖，並使用widthAnchor和heightAnchor其大小。

注意：如果要放置圖像 ，請根據圖像大小計算寬高比合適的大小，將圖像放入圖像視圖，將圖像視圖縮放模式設置為fill ，最后將約束應用於圖像視圖。

您應該能夠按原樣運行此示例。 只需在頂部使用“本機”大小值即可查看它如何將子視圖適合到超級視圖中。

public class AspectFitViewController : UIViewController {

    // "native" size for the holderView
    let hViewWidth: CGFloat = 700.0
    let hViewHeight: CGFloat = 200.0

    let topView: UIView = {
        let v = UIView()
        v.translatesAutoresizingMaskIntoConstraints = false
        v.backgroundColor = UIColor.blue
        return v
    }()

    let holderView: UIView = {
        let v = UIView()
        v.translatesAutoresizingMaskIntoConstraints = false
        v.backgroundColor = UIColor.cyan
        return v
    }()

    public override func viewDidLoad() {
        super.viewDidLoad()
        view.bounds = CGRect(x: 0, y: 0, width: 400, height: 600)
        view.backgroundColor = .yellow

        // add topView
        view.addSubview(topView)

        // pin topView to leading / top / trailing
        topView.topAnchor.constraint(equalTo: view.topAnchor, constant: 0.0).isActive = true
        topView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 0.0).isActive = true
        topView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: 0.0).isActive = true

        // explicit height for topView
        topView.heightAnchor.constraint(equalToConstant: 250.0).isActive = true

        // add holderView to topView
        topView.addSubview(holderView)

        // center X and Y
        holderView.centerXAnchor.constraint(equalTo: topView.centerXAnchor, constant: 0.0).isActive = true
        holderView.centerYAnchor.constraint(equalTo: topView.centerYAnchor, constant: 0.0).isActive = true

        // holderView's width and height will be calculated in viewDidAppear
        // after topView has been laid-out by the auto-layout engine

    }

    public override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        let aspectWidth  = topView.bounds.size.width / hViewWidth
        let aspectHeight = topView.bounds.size.height / hViewHeight

        let aspectFit = min(aspectWidth, aspectHeight)

        let newWidth = hViewWidth * aspectFit
        let newHeight = hViewHeight * aspectFit

        holderView.widthAnchor.constraint(equalToConstant: newWidth).isActive = true
        holderView.heightAnchor.constraint(equalToConstant: newHeight).isActive = true

    }

}

---

編輯：

在澄清之后……這只能通過約束來實現。 關鍵是“ Priority 1000” top和leading約束必須為.greaterThanOrEqual等於零，而bottom和trailing約束則必須為.lessThanOrEqual等於零。

public class AspectFitViewController : UIViewController {

    // "native" size for the holderView
    let hViewWidth: CGFloat = 700.0
    let hViewHeight: CGFloat = 200.0

    let topView: UIView = {
        let v = UIView()
        v.translatesAutoresizingMaskIntoConstraints = false
        v.backgroundColor = UIColor.blue
        return v
    }()

    let holderView: UIView = {
        let v = UIView()
        v.translatesAutoresizingMaskIntoConstraints = false
        v.backgroundColor = UIColor.cyan
        return v
    }()

    public override func viewDidLoad() {
        super.viewDidLoad()
        view.bounds = CGRect(x: 0, y: 0, width: 400, height: 600)
        view.backgroundColor = .yellow

        // add topView
        view.addSubview(topView)

        // pin topView to leading / top / trailing
        topView.topAnchor.constraint(equalTo: view.topAnchor, constant: 0.0).isActive = true
        topView.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 0.0).isActive = true
        topView.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: 0.0).isActive = true

        // explicit height for topView
        topView.heightAnchor.constraint(equalToConstant: 250.0).isActive = true

        // add holderView to topView
        topView.addSubview(holderView)

        // center X and Y
        holderView.centerXAnchor.constraint(equalTo: topView.centerXAnchor, constant: 0.0).isActive = true
        holderView.centerYAnchor.constraint(equalTo: topView.centerYAnchor, constant: 0.0).isActive = true

        // aspect ratio size
        holderView.widthAnchor.constraint(equalTo: holderView.heightAnchor, multiplier: hViewWidth / hViewHeight).isActive = true

        // two constraints for each side...
        // the .equal constraints need .defaultLow priority
        // top and leading constraints must be .greaterThanOrEqual to 0
        // bottom and trailing constraints must be .lessThanOrEqual to 0

        let topA = NSLayoutConstraint(item: holderView, attribute: .top, relatedBy: .greaterThanOrEqual, toItem: topView, attribute: .top, multiplier: 1.0, constant: 0.0)
        let topB = NSLayoutConstraint(item: holderView, attribute: .top, relatedBy: .equal, toItem: topView, attribute: .top, multiplier: 1.0, constant: 0.0)

        let bottomA = NSLayoutConstraint(item: holderView, attribute: .bottom, relatedBy: .lessThanOrEqual, toItem: topView, attribute: .bottom, multiplier: 1.0, constant: 0.0)
        let bottomB = NSLayoutConstraint(item: holderView, attribute: .bottom, relatedBy: .equal, toItem: topView, attribute: .bottom, multiplier: 1.0, constant: 0.0)

        let leadingA = NSLayoutConstraint(item: holderView, attribute: .leading, relatedBy: .greaterThanOrEqual, toItem: topView, attribute: .leading, multiplier: 1.0, constant: 0.0)
        let leadingB = NSLayoutConstraint(item: holderView, attribute: .leading, relatedBy: .equal, toItem: topView, attribute: .leading, multiplier: 1.0, constant: 0.0)

        let trailingA = NSLayoutConstraint(item: holderView, attribute: .trailing, relatedBy: .lessThanOrEqual, toItem: topView, attribute: .trailing, multiplier: 1.0, constant: 0.0)
        let trailingB = NSLayoutConstraint(item: holderView, attribute: .trailing, relatedBy: .equal, toItem: topView, attribute: .trailing, multiplier: 1.0, constant: 0.0)

        topB.priority = .defaultLow
        bottomB.priority = .defaultLow
        leadingB.priority = .defaultLow
        trailingB.priority = .defaultLow

        NSLayoutConstraint.activate([
            topA, topB,
            bottomA, bottomB,
            leadingA, leadingB,
            trailingA, trailingB
            ])

    }

}