Bar button item 使用 button 作为 custom view，初始化 isEnabled 为 false，注意顺序
需要设置 bar button item 的 custom view 为 button，但一开始 isEnabled 要为 false。
生成一个 button
let leftButton = UIButton(frame: CGRect(x: 0, y: 0, width: 80, height: 44))
leftButton.setTitleColor(UIColor.green, for: .normal)
leftButton.setTitleColor(UIColor.red, for: .disabled)
leftButton.setTitle("Enabled", for: .normal)
leftButton.setTitle("Disabled", for: .disabled)
leftButton.addTarget(self, action: #selector(leftButtonClicked(_:)), for: .touchUpInside)
如果先设置 isEnabled，后设置 bar button item
leftButton.isEnabled = false
navigationItem.leftBarButtonItem = UIBarButtonItem(customView: leftButton)
结果 isEnabled 还是 true
正确的顺序
navigationItem.leftBarButtonItem = UIBarButtonItem(customView: leftButton)
leftButton.isEnabled = false // or navigationItem.leftBarButtonItem?.isEnabled = false
结果 isEnabled 是 false
改变 navigation bar isTranslucent 属性会改变 view 的坐标
放置两个 label。其中， frameLabel 没有添加约束（NSLayoutConstraint），constraintLabel 左、右、下都有约束，与 view 相接。
设置右上角按钮动作
navigationItem.rightBarButtonItem = UIBarButtonItem(title: "Change", style: .plain, target: self, action: #selector(rightButtonClicked(_:)))
改变 navigation bar isTranslucent 属性，显示 label 的坐标
@objc private func rightButtonClicked(_ sender: AnyObject) {
navigationController?.navigationBar.isTranslucent = !navigationController!.navigationBar.isTranslucent
updateLabelContent()
}
private func updateLabelContent() {
let frameLabelOrigin = frameLabel.frame.origin
frameLabel.text = "Frame label. x = (frameLabelOrigin.x), y = (frameLabelOrigin.y)"
let constraintLabelOrigin = constraintLabel.frame.origin
constraintLabel.text = "Constraint label. x = (constraintLabelOrigin.x), y = (constraintLabelOrigin.y)"
print("(title)")
print("Status bar frame:", UIApplication.shared.statusBarFrame) // (0.0, 0.0, 375.0, 20.0)
print("Navigation bar frame:", navigationController!.navigationBar.frame) // (0.0, 20.0, 375.0, 44.0)
}
通过点击右上角按钮，来查看变化。
