ios - Swift 中的 UIPickerView 子类 : error optional value in viewForRow

Question

我想继承 UIPickerView 以创建自定义 DatePicker。我试过了，但是 Swift 中有一个错误说:

unexpectedly found nil while unwrapping an Optional value

在这一行:(view.viewWithTag(1) as UILabel).text = array[row]

代码如下:

class MyPickerView : UIPickerView, UIPickerViewDataSource, UIPickerViewDelegate{

    let array = ["one", "two", "three"]
    required init(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    override init(frame: CGRect) {
        super.init(frame:frame)
        self.delegate = self
        self.dataSource = self
    }

    //components / row
    func numberOfComponentsInPickerView(pickerView: UIPickerView) -> Int {
        return 1
    }
    func pickerView(pickerView: UIPickerView, numberOfRowsInComponent component: Int) -> Int {
        return array.count
    }

    //view for row
    func pickerView(pickerView: UIPickerView, viewForRow row: Int, forComponent component: Int, reusingView view: UIView!) -> UIView {
        if view == nil {
            var view = UIView(frame: CGRectMake(0,0, 150,50))
            let label = UILabel(frame:CGRectMake(0,0, 80, 40))
            label.tag = 1
            view.addSubview(label)
        }
        (view.viewWithTag(1) as UILabel).text = array[row]
        return view
    }
    //did select row
    func pickerView(pickerView: UIPickerView, didSelectRow row: Int, inComponent component: Int) {
        println("has selected row \(row)")
    }

谢谢

Answer 1

UIPickerView 中有一个已知错误(我认为是从 iOS 7 开始的)，即 View 永远不会被重用。要解决您的问题，请去掉 if 子句(这样就不需要标签)，

func pickerView(pickerView: UIPickerView, viewForRow row: Int, forComponent component: Int, reusingView view: UIView!) -> UIView {
    var view = UIView(frame: CGRectMake(0,0, 150,50))
    let label = UILabel(frame:CGRectMake(0,0, 80, 40))
    view.addSubview(label)
    label.text = array[row]
    return view
}

你得到错误的原因是因为你有两个不同的对象叫做“ View ”。您在 if 子句内创建的 View 在该子句外不可见，因此您在其上调用 viewWithTag 的“ View ”是传递给该方法的 View ，并且该 View 始终为 nil。