Add an extension on TextController that returns a mutable state of TextFieldValue

This makes it easy to use it with text fields while persisting selection state.

Author: darshanparajuli

workflow-ui/compose/src/main/java/com/squareup/workflow1/ui/compose/TextControllerAsMutableState.kt

@@ -6,6 +6,8 @@ import androidx.compose.runtime.MutableState
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.snapshotFlow
+import androidx.compose.ui.text.TextRange
+import androidx.compose.ui.text.input.TextFieldValue
 import com.squareup.workflow1.ui.TextController
 import kotlinx.coroutines.launch
 
@@ -42,3 +44,66 @@ import kotlinx.coroutines.launch
     }
   }
 }
+
+/**
+ * Exposes the [textValue][TextController.textValue] of a [TextController]
+ * as a remembered [MutableState] of [TextFieldValue], suitable for use from `@Composable`
+ * functions.
+ *
+ * Usage:
+ *
+ *    ```
+ *    var fooText by fooTextController.asMutableTextFieldValueState()
+ *    BasicTextField(
+ *      value = fooText,
+ *      onValueChange = { fooText = it },
+ *    )
+ *    ```
+ *
+ * @param initialSelection The initial range of selection. If [TextRange.start] equals
+ * [TextRange.end], then nothing is selected, and the cursor is placed at
+ * [TextRange.start]. By default, the cursor will be placed at the end of the text.
+ */
+@Composable public fun TextController.asMutableTextFieldValueState(
+  initialSelection: TextRange = TextRange(textValue.length),
+): MutableState<TextFieldValue> {
+  val textFieldValue = remember(this) {
+    val actualStart = initialSelection.start.coerceIn(0, textValue.length)
+    val actualEnd = initialSelection.end.coerceIn(actualStart, textValue.length)
+    mutableStateOf(
+      TextFieldValue(
+        text = textValue,
+        // We need to set the selection manually when creating  new `TextFieldValue` whenever
+        // `TextController` changes because the text inside may not be empty.
+        selection = TextRange(actualStart, actualEnd),
+      )
+    )
+  }
+
+  LaunchedEffect(this) {
+    launch {
+      // This is to address the case when value of `TextController` is updated within the workflow.
+      // By subscribing directly to `onTextChanged` we can use this to also update the textFieldValue.
+      onTextChanged
+        .collect { newText ->
+          // Only update the `textFieldValue` if the new text is different from the current text.
+          // This ensures the selection is maintained when the text is updated from the UI side,
+          // and is only reset when the text is changed via `TextController`.
+          if (textFieldValue.value.text != newText) {
+            textFieldValue.value = TextFieldValue(
+              text = newText,
+              selection = TextRange(newText.length),
+            )
+          }
+        }
+    }
+
+    // Update this `TextController`'s text whenever the `textFieldValue` changes.
+    snapshotFlow { textFieldValue.value }
+      .collect { newText ->
+        textValue = newText.text
+      }
+  }
+
+  return textFieldValue
+}