UIPickerView确实通过了行

Question

我有一个演示应用程序：

正如你所看到的，背景的alpha正在根据值变为黑色。

但问题是没有平稳过渡：

从GIF中可以看到，只有滚动结束后，背景才会发生变化。我不想让事情变成那样。

这是我的代码：

class ViewController: UIViewController, UIPickerViewDelegate, UIPickerViewDataSource {

    @IBOutlet weak var pickerView: UIPickerView!
    @IBOutlet weak var backView: UIView!
    
    let max = 100
    
    override func viewDidLoad() {
        super.viewDidLoad()

        pickerView.delegate = self
        pickerView.dataSource = self
    }
    
    func numberOfComponents(in pickerView: UIPickerView) -> Int {
        return 1
    }
    
    func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
        return max + 1 // To include '0'
    }
    
    func pickerView(_ pickerView: UIPickerView, viewForRow row: Int, forComponent component: Int, reusing view: UIView?) -> UIView {
        
        let l = UILabel(frame: .zero)
        l.text = String(max - row)
        l.textColor = .white
        
        l.font = UIFont.preferredFont(forTextStyle: .title3)
        l.textAlignment = .center
        
        return l
    }
    
    func pickerView(_ pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) {
        backView.alpha = CGFloat(max - row) / 100
    }


}

我给出了委托uiview而不是String，因为我有一个想法：每次每一行的位置发生变化时都要进行测试，当一行位于屏幕中间时，我应该更新背景。但不幸的是我不知道怎么做。

也许你能帮忙？或者提出其他的想法？

谢谢！

Answer 1

委托方法DidSelectRow仅在滚动停止时调用，因此这不是您应该更新alpha的地方。UIPickerView没有将在滚动期间通知您更改的委托方法，但是UIPickerView将调用您的数据源和委托方法来获取给定行的标题或视图，以便在用户滚动时显示。所以你应该做的就是把你的alpa变化逻辑移到那里：

func pickerView(_ pickerView: UIPickerView, viewForRow row: Int, forComponent component: Int, reusing view: UIView?) -> UIView {
    backView.alpha = CGFloat(max - row) / 100
}

请注意，在加载UIPickerView时将调用此委托方法，因此可能您应该禁用alpha更改，直到视图没有正确地布局出来（可能ViewDidInceure会这样做）。

由于delegate方法有时会发生意外的行为（不仅调用下一行，还调用选取器中的任何一行），我们应该存储并检查该行是比上次保存的值早一步还是晚一步，否则我们应该忽略这一点。我做了一个简单的演示来演示它是如何工作的：

import UIKit

class ViewController: UIViewController, UIPickerViewDelegate, UIPickerViewDataSource {
    private let pickerView = UIPickerView()
    private var isPickerReady = false
    private var lastValue = 0
    private let max = 100

    override func viewDidLoad() {
        super.viewDidLoad()

        view.addSubview(pickerView)
        pickerView.translatesAutoresizingMaskIntoConstraints = false

        pickerView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        pickerView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        pickerView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        pickerView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true


        pickerView.delegate = self
        pickerView.dataSource = self
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        isPickerReady = true
    }

    func numberOfComponents(in pickerView: UIPickerView) -> Int {
        return 1
    }

    func pickerView(_ pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
        return max
    }

    func pickerView(_ pickerView: UIPickerView, titleForRow row: Int, forComponent component: Int) -> String? {
        // Do not update the value until the view is not loaded 
        // Only consider delegate methods which are one step ahead or behind the last value
        if row + 1 == lastValue && isPickerReady || row - 1 == lastValue && isPickerReady {
            lastValue = row
            view.alpha = CGFloat(max - lastValue ) / 100
        }
        return "Tiltle \(row)"
    }
}