在这种情况下，您应该使用TextFieldValue作为TextField的状态，当它接收焦点时，您使用TextFieldValue状态设置selection。

val state = remember {
    mutableStateOf(TextFieldValue(""))
}
TextField(
    value = state.value,
    onValueChange = { text -> state.value = text },
    modifier = Modifier
        .onFocusChanged { focusState ->
            if (focusState.isFocused) {
                val text = state.value.text
                state.value = state.value.copy(
                    selection = TextRange(0, text.length)
                )
            }
        }
)

结果如下：

请注意，根据您的触摸，光标会转到触摸的位置，而不是选择整个文本。您可以尝试找出这是一个bug还是一个功能：）

@nglauber解决方案似乎不再起作用了。
调试显示onFocusChanged在onValueChange之前调用，并且在一个视图生命周期内。在onFocusChanged期间更改的选择对TextField没有影响，因为它在onValueChange期间被覆盖。
以下是可能的解决方法：

var state by remember {
    mutableStateOf(TextFieldValue("1231"))
}
var keepWholeSelection by remember { mutableStateOf(false) }
if (keepWholeSelection) {
    // in case onValueChange was not called immediately after onFocusChanged
    // the selection will be transferred correctly, so we don't need to redefine it anymore
    SideEffect {
        keepWholeSelection = false
    }
}
TextField(
    value = state,
    onValueChange = { newState ->
        if (keepWholeSelection) {
            keepWholeSelection = false
            state = newState.copy(
                selection = TextRange(0, newState.text.length)
            )
        } else {
            state = newState
        }
    },
    modifier = Modifier
        .onFocusChanged { focusState ->
            if (focusState.isFocused) {
                val text = state.text
                state = state.copy(
                    selection = TextRange(0, text.length)
                )
                keepWholeSelection = true
            }
        }
)

我认为应该可以使它更容易，所以我创建了this question的组成问题跟踪。

我没有100%成功地使用@nglauber答案。你应该添加一个小延迟，它的效果很好。例如：

val state = remember {
            mutableStateOf(TextFieldValue(""))
        }

        // get coroutine scope from composable
        val scope = rememberCoroutineScope()

        TextField(
            value = state.value,
            onValueChange = { text -> state.value = text },
            modifier = Modifier
                .onFocusChanged {
                    if (it.hasFocus) {
                        // start coroutine
                        scope.launch {
                            // add your preferred delay
                            delay(10)
                            val text = state.value.text
                            state.value = state.value.copy(
                                selection = TextRange(0, text.length)
                            )
                        }
                    }
                }
        )

我写了一个Modifier扩展函数，尽管@Pylyp指出了bug，它仍然工作

fun Modifier.onFocusSelectAll(textFieldValueState: MutableState<TextFieldValue>): Modifier =
  composed(
    inspectorInfo = debugInspectorInfo {
      name = "textFieldValueState"
      properties["textFieldValueState"] = textFieldValueState
    }
  ) {
    var triggerEffect by remember {
      mutableStateOf<Boolean?>(null)
    }
    if (triggerEffect != null) {
      LaunchedEffect(triggerEffect) {
        val tfv = textFieldValueState.value
        textFieldValueState.value = tfv.copy(selection = TextRange(0, tfv.text.length))
      }
    }
    Modifier.onFocusChanged { focusState ->
      if (focusState.isFocused) {
        triggerEffect = triggerEffect?.let { bool ->
          !bool
        } ?: true
      }
    }
  }

用法

@Composable
fun SelectAllOnFocusDemo() {
  var tfvState = remember {
    mutableStateOf(TextFieldValue("initial text"))
  }
  TextField(
    modifier = Modifier.onFocusSelectAll(tfvState),
    value = tfvState.value,
    onValueChange = { tfvState.value = it },
  )
}

我想添加到Phil's answer中，我想动态更新状态，结果是：

var state by remember(textVal) {
    mutableStateOf(TextFieldValue(text = textVal, selection = TextRange(textVal.length)))
}

它做了两件事，首先，如果textVal发生变化，它会更新字段，还将光标放在末尾。