swift - How to set UIStepper value from another cl

问题:

I have a tableview contains some tableviewcell with UIStepper. This is tableviewcell code:

class ShoppingCartItemCell: UITableViewCell {
static let reuseIdentifier = "ShoppingCartItemCell"

@IBOutlet weak var itemImage: UIImageView!
@IBOutlet weak var itemNameLabel: UILabel!
@IBOutlet weak var itemProviderLabel: UILabel!
@IBOutlet weak var itemPriceLabel: UILabel!
@IBOutlet weak var itemQuantityText: UITextField!
@IBOutlet weak var quantityStepper: UIStepper!

var oldValue = 0.0

var itemName = "" {
    didSet {
        itemNameLabel.text = itemName
    }
}

var itemProvider = "" {
    didSet {
        itemProviderLabel.text = itemProvider
    }
}

var itemPrice = "" {
    didSet {
        itemPriceLabel.text = itemPrice
    }
}

var itemQuantity = "" {
    didSet {
        itemQuantityText.text = itemQuantity
    }
}
}

And this is uistepper code in tableview:

class ShoppingCartViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {
     @IBOutlet weak var tableView: UITableView!
     @IBAction func quantityStepper(_ sender: UIStepper) {
     ShoppingCartItemCell.itemQuantity = "\(Int(sender.value))"// this way is not possible...
     // How to update the quantityStepper value in ShoppingCartItemCell class here
     }
}

How to update the quantityStepper value in ShoppingCartViewController class? Some people will tell me to create @IBAction of quantityStepper in ShoppingCartItemCell class but my business logic must to do this way. Please help. Thanks all.

回答1:

I highly recommend to use Key-Value Observation in Swift 4, it's very easy to implement

In the cell create a NSKeyValueObservation property

var observation : NSKeyValueObservation?

In the view controller in cellForRowAt set the values for stepper and label add the observer

let item = dataSource[indexPath.row] // assuming `dataSource` is the data source array
cell.quantityStepper.value = item.quantity
cell.itemQuantityText.text = "\(item.quantity)"
cell.observation = cell.quantityStepper.observe(\.value, options: [.new]) { (stepper, change) in
     cell.itemQuantityText.text = "\(change.newValue!)"
     // update the data model for example
     item.quantity = change.newValue!
}

Finally you have to remove the observer in didEndDisplaying after the cell has left the screen.

func tableView(_ tableView: UITableView, didEndDisplaying cell: UITableViewCell, forRowAt indexPath: IndexPath) {
    (cell as! ShoppingCartItemCell).observation = nil
}

No IBAction, no protocol / delegate. It's very simple and efficient.