Updating UILabel text when variable changes

问题

I've got a UIControl class and need to do some calculation based on UIImageView location which can be moved with touchesBegan and touchesMoved (everything inside this class). Than I would like to display it as a UILabel which I've created programmatically.

class control : UIControl{
   ...
   let leftControl: UIImageView = UIImageView(image: UIImage(named: "left-control"))
   ...
   func leftValue() -> String{
       var leftValue : String = "0.0"
       leftValue = "\(leftControl.center.x)"
       return leftValue
   }
}

and my ViewController.swift

class ViewController: UIViewController {

let ctrl : Control = Control()
let leftLabel : UILabel = UILabel(frame: CGRect(x: 40, y: 300, width: 150, height: 30))

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    ctrl.frame.origin = CGPoint(x: 40, y: 400)

    leftLabel.text = "\(ctrl.leftValue())" //displays only starting value

    view.addSubview(slider)
    view.addSubview(leftLabel)
    view.addSubview(rightLabel)
}

I know that it's inside the viewDidLoad so it's not updating properly. I was wondering about scheduledTimer but don't know if it's good solution.

回答1:

You can achieve this using protocols and delegation - in the file for your Control add this :

protocol ControlDelegate: class {
    func controlPositionDidChange(leftValue: String)
}

And add a weak var delegate: ControlDelegate? inside Control class.

In the file for view controller make following changes :

class ViewController: UIViewController, ControllDelegate {

let ctrl : Control = Control() 
let leftLabel : UILabel = UILabel(frame: CGRect(x: 40, y: 300, width: 150, height: 30))

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    ctrl.frame.origin = CGPoint(x: 40, y: 400)
    ctrl.delegate = self

    leftLabel.text = "\(ctrl.leftValue())" //displays only starting value

    view.addSubview(slider)
    view.addSubview(leftLabel)
    view.addSubview(rightLabel) 
}

func controlPositionDidChange(leftValue: String) {
    leftLabel.text = leftValue
}
}

Now, whenever you want to inform the delegate that your control has changed the position, simply call self.delegate?.controlPositionDidChange(self.leftValue()) in appropriate places.

As usually, there is more in the docs. I highly suggest reading through them as delegation and protocols are widely used in CocoaTouch.

回答2:

The answer of @Losiowaty describes the solution that most developers choose. But there are (in my opinion) much better ways to achieve it. I prefer the object oriented solution. It might look like more code, but its a much better maintainable way with more reusable code.

A real object oriented solution of your problem might look like that:

// reusable protocol set

protocol OOString: class {
    var value: String { get set }
}

// reusable functional objects

final class RuntimeString: OOString {

    init(initialValue: String = "") {
        self.value = initialValue
    }

    var value: String

}

final class ViewUpdatingString: OOString {

    init(_ decorated: OOString, view: UIView) {
        self.decorated = decorated
        self.view = view
    }

    var value: String {
        get {
            return decorated.value
        }
        set(newValue) {
            decorated.value = newValue
            view.setNeedsLayout()
        }
    }

    private let decorated: OOString
    private let view: UIView
}

// reusable ui objects

final class MyLabel : UILabel {

    init(frame: CGRect, imageXCenter: OOString) {
        self.imageXCenter = imageXCenter
        super.init(frame: frame)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("Not supported")
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        text = imageXCenter.value
    }

    private let imageXCenter: OOString

}

final class MyControl : UIControl {

    init(frame: CGRect, imageXCenter: OOString) {
        self.imageXCenter = imageXCenter
        super.init(frame: frame)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("Not supported")
    }

    private let imageXCenter: OOString
    private let leftControl = UIImageView(image: UIImage(named: "left-control"))

    // call this at change
    private func updateValue() {
        imageXCenter.value = "\(leftControl.center.x)"
    }

}

// non reusable business logic

final class MyViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let dependency = RuntimeString(initialValue: "unset")
        let leftLabel = MyLabel(
            frame: CGRect(x: 40, y: 300, width: 150, height: 30),
            imageXCenter: dependency
        )
        let control = MyControl(
            frame: CGRect(x: 40, y: 400, width: 400, height: 400),
            imageXCenter: ViewUpdatingString(dependency, view: leftLabel)
        )
        view.addSubview(leftLabel)
        view.addSubview(control)
    }

}

The main idea is to extract the dependency of both objects into another object and use a decorator then to automatically update the ui on every set value.

Note:

this approach follows the rules of object oriented coding (clean coding, elegant objects, decorator pattern, ...)
reusable classes are very simple constructed and fullfill exactly one task
classes communicate by protocols with each other
dependencies are given by dependency injection as far as possible
internal object functionality is private (loose coupling)
everything (except the business logic) is designed for reuse -> if you code like that the portfolio of reusable code grows with every day you code
the business logic of your app is more concentrated in one place (in my real coding its even outside the UIViewController)
unittesting of reusable objects is very simple when using fake implementations for the protocols (even mocking is not needed in most cases)
lesser problems with retain cycles, in most cases you do not need weak properties any more
avoiding Null, nil and Optionals (they pollute your code)
...

来源：https://stackoverflow.com/questions/43644972/updating-uilabel-text-when-variable-changes

标签

ios

swift

uilabel

uicontrol