设置 alpha 对 UITableView 标题视图没有影响？答案

【问题标题】：Setting alpha has no effect on UITableView header view?设置 alpha 对 UITableView 标题视图没有影响？
【发布时间】：2020-02-01 18:59:07
【问题描述】：

设置 alpha 对 UITableView 标题视图没有影响？

tableView.tableHeaderView? = nil //will remove the header completely
tableView.tableHeaderView?.alpha = 0.5 //has no effect, it has a stackview and just a label inside

【问题讨论】：

标签： uitableview uiview uikit

【解决方案1】：

对我来说似乎工作得很好......

class AlphaHeaderTableViewController: UITableViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let vv = UIView()
        vv.backgroundColor = .blue
        vv.frame.size.height = 60

        let v = UILabel()
        v.textAlignment = .center
        v.text = "Testing"
        v.backgroundColor = .green
        v.font = UIFont.systemFont(ofSize: 40.0)
        v.translatesAutoresizingMaskIntoConstraints = false

        vv.addSubview(v)

        v.centerXAnchor.constraint(equalTo: vv.centerXAnchor).isActive = true
        v.centerYAnchor.constraint(equalTo: vv.centerYAnchor).isActive = true

        tableView.tableHeaderView = vv

        tableView.tableHeaderView?.alpha = 0.1

        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "cell")
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 5
    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath)
        cell.textLabel?.text = "Row \(indexPath.row + 1)"
        return cell
    }
}

tableView.tableHeaderView?.alpha = 0.1 的结果：

为了比较，结果与tableView.tableHeaderView?.alpha = 1.0:

编辑

当表更新时，tableHeaderView.alpha 属性似乎也被重置了1.0。

要解决这个问题，请将用作标题视图的当前视图嵌入到具有清晰背景的视图中，并设置子视图的.alpha 属性。

例如：

    let innerView = UIView()
    innerView.backgroundColor = .blue

    let vLabel = UILabel()
    vLabel.textAlignment = .center
    vLabel.text = "Testing"
    vLabel.backgroundColor = .green
    vLabel.font = UIFont.systemFont(ofSize: 40.0)
    vLabel.translatesAutoresizingMaskIntoConstraints = false

    innerView.addSubview(vLabel)

    vLabel.centerXAnchor.constraint(equalTo: innerView.centerXAnchor).isActive = true
    vLabel.centerYAnchor.constraint(equalTo: innerView.centerYAnchor).isActive = true

    let containerView = UIView()
    containerView.addSubview(innerView)

    innerView.translatesAutoresizingMaskIntoConstraints = false
    NSLayoutConstraint.activate([
        innerView.topAnchor.constraint(equalTo: containerView.topAnchor),
        innerView.bottomAnchor.constraint(equalTo: containerView.bottomAnchor),
        innerView.leadingAnchor.constraint(equalTo: containerView.leadingAnchor),
        innerView.trailingAnchor.constraint(equalTo: containerView.trailingAnchor),
    ])

    containerView.frame.size.height = 60

    tableView.tableHeaderView = containerView

    // desired alpha value on innerView
    innerView.alpha = 0.1

【讨论】：