When building a design system for the Android app at inDrive, we implemented a reusable TextArea component using androidx.compose.foundation.text.BasicTextField. TextArea androidx.compose.foundation.text.BasicTextField One of the basic requirements was straightforward: limit the maximum number of characters a user can enter. Jetpack Compose seems to already support this through InputTransformation. However, after exploring the implementation more closely, we discovered a subtle behavior that can lead to a completely unusable TextField in certain scenarios. This article explores: InputTransformation how Jetpack Compose TextField max length works internally
why the default implementation may cause issues
and how we solved it in production. how Jetpack Compose TextField max length works internally why the default implementation may cause issues and how we solved it in production. Understanding InputTransformation in BasicTextField BasicTextField exposes a parameter called inputTransformation. BasicTextField inputTransformation @Suppress("ComposableLambdaParameterPosition")
@Composable
fun BasicTextField(
    //...
    inputTransformation: InputTransformation? = null,
    //...
) @Suppress("ComposableLambdaParameterPosition")
@Composable
fun BasicTextField(
    //...
    inputTransformation: InputTransformation? = null,
    //...
) InputTransformation works only for user-generated input. InputTransformation user-generated This includes: typing on the keyboard
pasting text
drag & drop
accessibility input
automated tests typing on the keyboard pasting text drag & drop accessibility input automated tests However, it does NOT apply when the state changes programmatically. does NOT apply when the state changes programmatically This becomes important when the UI contains a custom Paste button that inserts text from the clipboard directly into TextFieldState. custom Paste button TextFieldState The Built-in maxLength Transformation Jetpack Compose provides a helper function: fun InputTransformation.maxLength(maxLength: Int): InputTransformation =
    this.then(MaxLengthFilter(maxLength)) fun InputTransformation.maxLength(maxLength: Int): InputTransformation =
    this.then(MaxLengthFilter(maxLength)) But when we looked deeper into the implementation, things became more interesting. The MaxLengthFilter Implementation // This is a very naive implementation for now, not intended to be production-ready.
private data class MaxLengthFilter(private val maxLength: Int) : InputTransformation {

   override fun TextFieldBuffer.transformInput() {
       if (length > maxLength) {
           revertAllChanges()
       }
   }
} // This is a very naive implementation for now, not intended to be production-ready.
private data class MaxLengthFilter(private val maxLength: Int) : InputTransformation {

   override fun TextFieldBuffer.transformInput() {
       if (length > maxLength) {
           revertAllChanges()
       }
   }
} The most interesting part is the comment: “This is a very naive implementation for now, not intended to be production-ready.” “This is a very naive implementation for now, not intended to be production-ready.” Finding such a comment inside a foundation library is always surprising. It also highlights an important lesson: comments are not rules. comments are not rules The Scenario That Breaks Everything Let’s walk through a real scenario. The user presses a Paste button
The app programmatically inserts a long text into the TextField
InputTransformation is not applied
The user tries to edit the text
InputTransformation runs
All changes are reverted The user presses a Paste button Paste button The app programmatically inserts a long text into the TextField InputTransformation is not applied InputTransformation The user tries to edit the text InputTransformation runs InputTransformation All changes are reverted All changes are reverted At this point, the TextField becomes effectively unusable. unusable Every attempt to modify the text triggers a revert. Why This Happens The difference lies in how TextField state changes are applied. Programmatic change inline fun edit(block: TextFieldBuffer.() -> Unit) {
   val mutableValue = startEdit()
   try {
       mutableValue.block()
       commitEdit(mutableValue)
   } finally {
       finishEditing()
   }
} inline fun edit(block: TextFieldBuffer.() -> Unit) {
   val mutableValue = startEdit()
   try {
       mutableValue.block()
       commitEdit(mutableValue)
   } finally {
       finishEditing()
   }
} User input internal inline fun editAsUser(
   inputTransformation: InputTransformation?,
   restartImeIfContentChanges: Boolean = true,
   block: TextFieldBuffer.() -> Unit,
) internal inline fun editAsUser(
   inputTransformation: InputTransformation?,
   restartImeIfContentChanges: Boolean = true,
   block: TextFieldBuffer.() -> Unit,
) Only editAsUser applies InputTransformation. editAsUser InputTransformation A Practical Fix The simplest and most reliable solution is to observe the TextField state and trim the text manually. LaunchedEffect(textFieldState.text) {
   if (textFieldState.text.length > maxCharacters) {
       textFieldState.edit {
           delete(maxCharacters, length)
       }
   }
} LaunchedEffect(textFieldState.text) {
   if (textFieldState.text.length > maxCharacters) {
       textFieldState.edit {
           delete(maxCharacters, length)
       }
   }
} This approach: prevents the TextField from becoming blocked
guarantees the text length limit
provides a better user experience prevents the TextField from becoming blocked guarantees the text length limit provides a better user experience Final Thoughts Always look deeper into the frameworks you rely on. Even foundational libraries may contain: unexpected edge cases
experimental implementations
or design tradeoffs that surface only in real-world usage. unexpected edge cases experimental implementations or design tradeoffs that surface only in real-world usage. In many ways, software engineers are a bit like entomologists — studying strange behaviors hidden deep inside complex systems. entomologists Library version used: androidx.compose.foundation:foundation-android:1.10.1

This story contains new, firsthand information uncovered by the writer.

A Hidden Problem in Jetpack Compose TextField Max Length

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