ios - 原型(prototype) tableviewcell 内的 UIPickerView 没有选择指示器

Question

我有一个 TableView Controller ，它有多个原型(prototype)单元格用于不同的输入控件，如 UISwitch、UITextField 和 UIPickerView，当加载 View 时，我确定需要哪个控件并只显示一个原型(prototype)单元格。

但是对于带有 UIPickerView 的单元格，我无法让选择指示器出现并且 pickerview 最终看起来像这样:

在我的 tableView:cellForRowAtIndexPath: 方法中，我明确启用了选择指示器:

UIPickerView * pickerView = [cell viewWithTag:InputTagPicker];
pickerView.dataSource = self;
pickerView.delegate = self;
[pickerView selectRow:self.pickerInitialIndex inComponent:0 animated:YES];
[pickerView setShowsSelectionIndicator:YES];

为什么选择指示器不显示？

Answer 1

我遇到了类似的问题。我注意到每次我滑动到不同的选项卡时，这个问题都会消失。这意味着当 tableview(cell) 出现在 View 中时发生了一些有趣的事情。

由于 UITableViewCell 没有 viewWillAppear() 方法，我设法通过从单元格的 subview 列表中强制删除 pickerView 然后再将其添加回来来解决这个问题。这模拟了在将pickerView作为 subview 添加到单元格之前设置选择的过程，并修复了问题。

以下是您可以尝试的方法:

UIPickerView * pickerView = [cell viewWithTag:InputTagPicker];
pickerView.dataSource = self;
pickerView.delegate = self;
[pickerView selectRow:self.pickerInitialIndex inComponent:0 animated:YES];
[pickerView setShowsSelectionIndicator:YES];

// hack to make sure the selection indicator shows
// first remove the picker
[pickerView removeFromSuperview];

// then add it back
[cell.contentView addSubview:pickerView];

但是，这会创建粘在单元格左边缘的选择器，因此您需要以编程方式设置约束

// finally set the constraints to make sure it fits horizontally centered
//Trailing    
NSLayoutConstraint *trailing =[NSLayoutConstraint
                                constraintWithItem:pickerView
                                attribute:NSLayoutAttributeTrailing
                                relatedBy:NSLayoutRelationEqual
                                toItem:cell.contentView   
                                attribute:NSLayoutAttributeTrailing
                                multiplier:1.0f
                                constant:0.f];

//Leading
NSLayoutConstraint *leading = [NSLayoutConstraint
                                   constraintWithItem:pickerView
                                   attribute:NSLayoutAttributeLeading
                                   relatedBy:NSLayoutRelationEqual
                                   toItem:cell.contentView
                                   attribute:NSLayoutAttributeLeading
                                   multiplier:1.0f
                                   constant:0.f];

//Center Y
NSLayoutConstraint *centerY = [NSLayoutConstraint
                                   constraintWithItem:pickerView
                                   attribute:NSLayoutAttributeCenterY
                                   relatedBy:NSLayoutRelationEqual
                                   toItem:cell.contentView
                                   attribute:NSLayoutAttributeCenterY
                                   multiplier:1.0f
                                   constant:0.f];

[cell.contentView addConstraints:trailing];
[cell.contentView addConstraints:leading];
[cell.contentView addConstraints:centerY];

这是一个 Swift 4 版本:

// first remove the picker
pickerView?.removeFromSuperview()

// then add it back
cell.contentView.addSubview(pickerView!)

// and finally set the constraints to make sure it fits horizontally centered
cell.contentView.addConstraints([NSLayoutConstraint.init(item: pickerView as Any, attribute: .centerY, relatedBy: .equal, toItem: cell.contentView, attribute: .centerY, multiplier: 1, constant: 0),
                                 NSLayoutConstraint.init(item: pickerView as Any, attribute: .trailing, relatedBy: .equal, toItem: cell.contentView, attribute: .trailing, multiplier: 1, constant: 0),
                                 NSLayoutConstraint.init(item: pickerView as Any, attribute: .leading, relatedBy: .equal, toItem: cell.contentView, attribute: .leading, multiplier: 1, constant: 0)])