Captured previous value in onChange is equal to the new value not the old value

Question

When I try to replicate the example from SwiftUI documentation on onChange modifier and capture the previous value, inside the closure the previous value is equal to the new value, not the old one.

Code Block swiftstruct ContentView: View {
  @State private var isSubscribed = false
  var body: some View {
    Toggle("Subscribe", isOn: $isSubscribed)
       
    .onChange(of: isSubscribed) { [isSubscribed] newState in
      print(isSubscribed)
      print(newState)
    }
  }
}

In the documentation example, the onChange modifier is attached to the body property, not to a view inside it. Is this how it should be used? Attaching it to the body property in Xcode 12.0 beta (12A6159) doesn't compile.

SwiftUI

6.8k

Posted by

natpan

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To anyone who find this question after iOS 17, here is a new modifier that solves exactly this problem: onChange(of:initial:_:)

—
cht1995

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Answer 1

I am having the same issue, onChange is not capturing the previous value.

Note, I am able to attach .onChange to the body view and have it compile.

Posted by

tarasis

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Answer 2

I submitted feedback FB7824176

Posted by

natpan

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Answer 3

A good alternative to get old values is to use 'willSet'

@State private var isSubscribed = false {
    willSet {
       print("oldValue: \(isSubscribed)")
    }
  }

Posted by

Thomas994y

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Answer 4

In my case I was missing the part in square brackets which captures the old value.

Wrong

.onChange(of: isSubscribed) { newState in      
  print("debug", isSubscribed, newState) // will print the same value twice      
}

Correct

.onChange(of: isSubscribed) { [isSubscribed] newState in      
  print("debug", isSubscribed, newState) // will print two values - old, then new
}

Posted by

nmecld

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Answer 5

That's an old thread, but this doesn't compile:

.onChange(of: isSubscribed) { [isSubscribed] newState in      
  print("debug", isSubscribed, newState) &#x2F;&#x2F; will print two values - old, then new
}

I needed to write:

.onChange(of: isSubscribed) { [oldValue = isSubscribed] newState in      
  print("debug", oldValue, newState) &#x2F;&#x2F; will print two values - old, then new
}

Posted by

Claude31

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Captured previous value in onChange is equal to the new value not the old value

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