Tracking value sources to prevent recursive SwiftUI updates

When wrapping a UITextView or NSTextView for use in SwiftUI, delegate updates are usually written back to a SwiftUI binding. SwiftUI then calls updateUIView(_:context:) or updateNSView(_:context:) with the same value on the next run loop iteration. Applying unchanged text can perform unnecessary layout work and move the insertion point while the user is typing, so a typical UIViewRepresentable compares the strings before updating the text view.

// UITextViewDelegate callback
func textViewDidChange(_ textView: UITextView) {
    text = textView.text
}

// UIViewRepresentable property
@Binding var text: String

func updateUIView(_ textView: UITextView, context: Context) {
    // Comparing large strings on every update can be expensive.
    guard textView.text != text else { return }
    textView.text = text
}

For large text values, we can avoid the string comparison by using a custom SwiftUI transaction value to record where the update originated. First, define an optional ObjectIdentifier on Transaction using the @Entry macro.

extension Transaction {
    @Entry var originatingTextView: ObjectIdentifier? = nil
}

In textViewDidChange(_:), wrap the binding write in withTransaction(::_:) and store the identity of the text view that produced the update.

func textViewDidChange(_ textView: UITextView) {
    withTransaction(
        \.originatingTextView,
        ObjectIdentifier(textView)
    ) {
        text = textView.text
    }
}

The representable can then compare the transaction value with the identity of the text view being updated.

func updateUIView(_ textView: UITextView, context: Context) {
    // Comparing fixed-size identity values is inexpensive.
    guard context.transaction.originatingTextView != ObjectIdentifier(textView) else {
        return
    }
    textView.text = text
}

Updates made elsewhere that omit the custom withTransaction call use the default nil value and continue to update the wrapped view normally.

Work with SwiftUI. Not against it.$35

A field guide to SwiftUI patterns and anti-patterns

The SwiftUI Wayby Natalia Panferova

• Avoid common SwiftUI pitfalls
• Build deeper intuition for the framework
• Gain insights from a former SwiftUI Engineer at Apple

Work with SwiftUI. Not against it.

A field guide to SwiftUI patterns and anti-patterns

The SwiftUI Way

by Natalia Panferova

$35