UIKit：如何使用以編程方式創建的約束為約束更改設定影片？

我已按照此答案從 XIB 檔案實體化視圖：

extension UIView {    
    class func fromNib(named: String? = nil) -> Self {
        let name = named ?? "\(Self.self)"
        guard let nib = Bundle.main.loadNibNamed(name, owner: nil, options: nil) else {
            fatalError("missing expected nib named: \(name)")
        }
        guard let view = nib.first as? Self else {
            fatalError("view of type \(Self.self) not found in \(nib)")
        }
        return view
    }
}

現在我想根據其內容創建一個具有自適應高度的視圖：

class CustomTopView: UIView {
    @IBOutlet weak var titleLabel: UILabel!
    @IBOutlet weak var subtitleLabel: UILabel!
    
    static func instance(withTitle title: NSAttributedString, subtitle: NSAttributedString) -> CustomTopView {
        let customTopView = CustomTopView.fromNib()
        customTopView.titleLabel.attributedText = title
        customTopView.subtitleLabel.attributedText = subtitle
        return customTopView
    }
}

我想要做的是從螢屏頂部影片顯示此視圖，就像通知彈出視窗一樣。我是這樣寫的：

extension UIView {
    func showCustomTopView(withTitle title: NSAttributedString, subtitle: NSAttributedString) {
        let customTopView = CustomTopView.instance(withTitle: title, subtitle: subtitle)
        addSubview(customTopView)
        customTopView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            customTopView.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 20.0),
            customTopView.constraint(equalTo: customTopView.trailingAnchor, constant: 20.0),
        ])
        customTopView.layoutIfNeeded() // *
        NSLayoutConstraint.activate([customTopView.topAnchor.constraint(equalTo: topAnchor, constant: -customTopView.bounds.height)])
    }
}

我這里有兩個問題。主要的一點是，我不知道如何從這里開始執行我想要的影片，以便視圖最終可見。我在最后一次之后嘗試了這個NSLayoutConstraint.activate(...)：

UIView.animate(withDuration: 0.5) {
    self.topAnchor.constraint(equalTo: customTopView.topAnchor, constant: 100.0).isActive = true
    self.layoutIfNeeded()
}

但不是按預期顯示，彈出視窗從螢屏頂部開始 100 像素，然后上升并消失。我究竟做錯了什么？

另外，我不確定標有// *: 我寫這一行來獲取正確customTopView高度的行，但我不確定這是正確的方法。

感謝您的幫助！

uj5u.com熱心網友回復：

一種方法：

• 在視圖頂部上方創建一個約束設定底部customTopView
• 創建第二個約束，設定customTopView視圖頂部下方的頂部
• 添加customTopView為子視圖，激活第一個約束
• customTopView通過停用第一個約束并激活第二個約束來制作影片

這是您的showCustomTopView(...)擴展的修改版本，使用UILabelas customTopView-- 應該適用于您的CustomTopView.instance(withTitle: ...)：

extension UIView {
    func showCustomTopView(withTitle title: NSAttributedString, subtitle: NSAttributedString) {
        let customTopView = UILabel()
        customTopView.text = title.string
        customTopView.backgroundColor = .green
        addSubview(customTopView)
        customTopView.translatesAutoresizingMaskIntoConstraints = false
        let g = self.safeAreaLayoutGuide
        // constraint: Bottom of customTopView to Top of self Minus 8-pts
        let cvBot = customTopView.bottomAnchor.constraint(equalTo: topAnchor, constant: -8.0)
        // constraint: Top of customTopView to Top of self Plus 8-pts
        let cvTop = customTopView.topAnchor.constraint(equalTo: g.topAnchor, constant: 8.0)
        NSLayoutConstraint.activate([
            customTopView.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 20.0),
            customTopView.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -20.0),
            // activate cvBot, so customTopView is 8-pts above the top of self
            cvBot,
        ])
        // execute this async, so the initial view position is set
        DispatchQueue.main.async {
            // deactivate the Bottom constraint
            cvBot.isActive = false
            // activate the Top constraint
            cvTop.isActive = true
            // animate it into view
            UIView.animate(withDuration: 0.5, animations: {
                self.layoutIfNeeded()
            })
        }
    }
}

標籤：ios 工具包 nslayout约束

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