add can flows

Author: aabadaa

UndoRedo/src/main/java/com/abada/undoredo/UndoRedoManager.kt

@@ -2,6 +2,7 @@ package com.abada.undoredo
 
 import androidx.lifecycle.SavedStateHandle
 import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
 
 class UndoRedoManager(
     private val initialStates: Map<String, Any>,
@@ -10,7 +11,18 @@ class UndoRedoManager(
 ) {
 
     private val stack = MutableStateFlow(listOf<UndoRedoItem>())
+
+    private val _canUndoFlow = MutableStateFlow(false)
+    val canUndoFlow: StateFlow<Boolean> = _canUndoFlow
+
+    private val _canRedoFlow = MutableStateFlow(false)
+    val canRedoFlow: StateFlow<Boolean> = _canRedoFlow
     private var index: Int = -1
+        set(value) {
+            field = value
+            _canRedoFlow.value = (field + 1) in stack.value.indices
+            _canUndoFlow.value = (field) >= 0
+        }
 
     init {
         if (maxSize < 5)
@@ -19,14 +31,14 @@ class UndoRedoManager(
     }
 
     fun undo() {
-        index.takeIf { it >= 0 }?.let {
+        index.takeIf { canUndoFlow.value }?.let {
             val undoRedoItem = stack.value[it]
             savedStateHandle[undoRedoItem.key] = undoRedoItem.prevValue
             index = (it - 1)
         }
     }
 
-    fun redo() = index.let {
+    fun redo() = index.takeIf { canRedoFlow.value }?.let {
         if (it + 1 in stack.value.indices) {
             index = it + 1
             val currentState = stack.value[it + 1]

app/src/main/java/com/abada/undoredomanager/MainActivity.kt

@@ -33,6 +33,9 @@ class MainActivity : ComponentActivity() {
                 // A surface container using the 'background' color from the theme
                 val text1 by viewModel.text1.collectAsState("")
                 val text2 by viewModel.text2.collectAsState("")
+
+                val canUndo by viewModel.canUndo.collectAsState()
+                val canRedo by viewModel.canRedo.collectAsState()
                 Surface(
                     modifier = Modifier.fillMaxSize(),
                     color = MaterialTheme.colorScheme.background
@@ -43,10 +46,10 @@ class MainActivity : ComponentActivity() {
                     ) {
                         TextField(value = text1, onValueChange = viewModel::onText1Changed)
                         TextField(value = text2, onValueChange = viewModel::onText2Changed)
-                        Button(onClick = viewModel::undo) {
+                        Button(onClick = viewModel::undo, enabled = canUndo) {
                             Text("Undo")
                         }
-                        Button(onClick = viewModel::redo) {
+                        Button(onClick = viewModel::redo, enabled = canRedo) {
                             Text("redo")
                         }
                     }
@@ -67,7 +70,8 @@ class MyViewModel(
         savedStateHandle
     )
 
-
+    val canUndo = undoRedoManager.canUndoFlow
+    val canRedo = undoRedoManager.canRedoFlow
     val text1 = savedStateHandle.getStateFlow("text1", "")
     val text2 = savedStateHandle.getStateFlow("text2", "")
     fun onText1Changed(text: String) {