iOS（Swift）：使用for循环更改UILabel文本

Question

I have a for loop as follows:

@objc private func resetValue() {
    for i in stride(from: value, to: origValue, by: (value > origValue) ? -1 : 1) {
        value = i
    }
    value = origValue
}

And when value is set it updates a label :

private var value = 1 {
    didSet {
        updateLabelText()
    }
}

private func updateLabelText() {

    guard let text = label.text else { return }

    if let oldValue = Int(text) { // is of type int?
        let options: UIViewAnimationOptions = (value > oldValue) ? .transitionFlipFromTop : .transitionFlipFromBottom
        UIView.transition(with: label, duration: 0.5, options: options, animations: { self.label.text = "\(value)" }, completion: nil)
    } else {
        label.text = "\(value)"
    }
}

I was hoping that if value=5 and origValue=2 , then the label would flip through the numbers 5,4,3,2 . However, this is not happening - any suggestions why, please?

I've tried using a delay function:

func delay(_ delay:Double, closure: @escaping ()->()) {
    DispatchQueue.main.asyncAfter(
        deadline: DispatchTime.now() + Double(Int64(delay * Double(NSEC_PER_SEC))) / Double(NSEC_PER_SEC), execute: closure)
}

and then placing the following within the stride code:

delay(2.0) { self.value = i }

However, this doesn't seem to work either.

Thanks for any help offered.

Answer 1

UIKit won't be able to update the label until your code is finished with the main thread, after the loop completes. Even if UIKit could update the label after each iteration of the loop, the loop is going to complete in a fraction of a second.

The result is that you only see the final value.

When you attempted to introduce the delay, you dispatched the update to the label asynchronously after 0.5 second; Because it is asynchronous, the loop doesn't wait for the 0.5 second before it continues with the next iteration. This means that all of the delayed updates will execute after 0.5 seconds but immediately one after the other, not 0.5 seconds apart. Again, the result is you only see the final value as the other values are set too briefly to be visible.

You can achieve what you want using a Timer :

func count(fromValue: Int, toValue: Int) {
    let stride = fromValue > toValue ? -1 : 1
    self.value = fromValue
    let timer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats:true) { [weak self] (timer) in
        guard let strongSelf = self else {
            return
        }
        strongSelf.value += stride
        if strongSelf.value == toValue {
            timer.invalidate()
        }
    }
}

I would also update the didSet to send the oldValue to your updateLabelText rather than having to try and parse the current text.

private var value = 1 {
    didSet {
        updateLabelText(oldValue: oldValue, value: value)
    }
}

private func updateLabelText(oldValue: Int, value: Int) {
    guard oldValue != value else {
        self.label.text = "\(value)"
        return
    }

    let options: UIViewAnimationOptions = (value > oldValue) ? .transitionFlipFromTop : .transitionFlipFromBottom
    UIView.transition(with: label, duration: 0.5, options: options, animations: { self.label.text = "\(value)" }, completion: nil)
}