feat: добавил визуальную индикацию ожидающей операции

app/src/main/java/com/heller/calculator/CalculatorScreen.kt

@@ -54,21 +54,41 @@ fun CalculatorScreen(viewModel: CalculatorViewModel) {
                     .padding(16.dp),
                 verticalArrangement = Arrangement.SpaceBetween,
             ) {
-                DisplaySection(display = uiState.display)
+                DisplaySection(
-                ButtonGrid(viewModel = viewModel)
+                    display = uiState.display,
+                    pendingExpression = uiState.pendingExpression,
+                )
+                ButtonGrid(
+                    viewModel = viewModel,
+                    pendingOperation = uiState.pendingOperation,
+                )
             }
         }
     }
 }
 
 @Composable
-private fun DisplaySection(display: String) {
+private fun DisplaySection(
+    display: String,
+    pendingExpression: String?,
+) {
     Column(
         modifier = Modifier
             .fillMaxWidth()
             .padding(vertical = 24.dp),
         horizontalAlignment = Alignment.End,
     ) {
+        if (pendingExpression != null) {
+            Text(
+                text = pendingExpression,
+                style = MaterialTheme.typography.titleMedium,
+                color = MaterialTheme.colorScheme.onBackground.copy(alpha = 0.6f),
+                textAlign = TextAlign.End,
+                maxLines = 1,
+                overflow = TextOverflow.Ellipsis,
+                modifier = Modifier.fillMaxWidth(),
+            )
+        }
         Text(
             text = display,
             style = MaterialTheme.typography.headlineLarge,
@@ -82,7 +102,10 @@ private fun DisplaySection(display: String) {
 }
 
 @Composable
-private fun ButtonGrid(viewModel: CalculatorViewModel) {
+private fun ButtonGrid(
+    viewModel: CalculatorViewModel,
+    pendingOperation: CalculatorOperation?,
+) {
     val rows = listOf(
         listOf("C" to CalculatorButtonType.Clear, "÷" to CalculatorButtonType.Operator),
         listOf("7" to CalculatorButtonType.Digit, "8" to CalculatorButtonType.Digit, "9" to CalculatorButtonType.Digit, "×" to CalculatorButtonType.Operator),
@@ -111,6 +134,7 @@ private fun ButtonGrid(viewModel: CalculatorViewModel) {
                 label = "÷",
                 type = CalculatorButtonType.Operator,
                 onClick = { viewModel.onOperation(CalculatorOperation.Divide) },
+                isSelected = pendingOperation == CalculatorOperation.Divide,
                 modifier = Modifier
                     .weight(1f)
                     .aspectRatio(1f),
@@ -128,6 +152,7 @@ private fun ButtonGrid(viewModel: CalculatorViewModel) {
                         label = label,
                         type = type,
                         onClick = { handleButtonClick(label, viewModel) },
+                        isSelected = labelToOperation(label) == pendingOperation,
                         modifier = Modifier
                             .weight(weight)
                             .aspectRatio(if (label == "0") 2.15f else 1f),
@@ -139,6 +164,14 @@ private fun ButtonGrid(viewModel: CalculatorViewModel) {
     }
 }
 
+private fun labelToOperation(label: String): CalculatorOperation? = when (label) {
+    "+" -> CalculatorOperation.Add
+    "−" -> CalculatorOperation.Subtract
+    "×" -> CalculatorOperation.Multiply
+    "÷" -> CalculatorOperation.Divide
+    else -> null
+}
+
 private fun handleButtonClick(label: String, viewModel: CalculatorViewModel) {
     when (label) {
         "C" -> viewModel.onClear()

app/src/main/java/com/heller/calculator/CalculatorState.kt

@@ -1,10 +1,10 @@
 package com.heller.calculator
 
-enum class CalculatorOperation(val symbol: String) {
+enum class CalculatorOperation(val symbol: String, val displaySymbol: String) {
-    Add("+"),
+    Add("+", "+"),
-    Subtract("-"),
+    Subtract("-", "−"),
-    Multiply("*"),
+    Multiply("*", "×"),
-    Divide("/");
+    Divide("/", "÷");
 
     fun apply(a: Double, b: Double): Double = when (this) {
         Add -> a + b
@@ -17,4 +17,6 @@ enum class CalculatorOperation(val symbol: String) {
 data class CalculatorUiState(
     val display: String = "0",
     val error: String? = null,
+    val pendingOperation: CalculatorOperation? = null,
+    val pendingExpression: String? = null,
 )

app/src/main/java/com/heller/calculator/CalculatorViewModel.kt

@@ -57,6 +57,7 @@ class CalculatorViewModel : ViewModel() {
 
         pendingOperation = operation
         waitingForOperand = true
+        syncUiState()
     }
 
     fun onEquals() {
@@ -69,7 +70,7 @@ class CalculatorViewModel : ViewModel() {
         storedValue = null
         pendingOperation = null
         waitingForOperand = true
-        updateDisplay(formatResult(result))
+        syncUiState(display = formatResult(result))
     }
 
     fun onClear() {
@@ -106,6 +107,33 @@ class CalculatorViewModel : ViewModel() {
     }
 
     private fun updateDisplay(display: String) {
-        _uiState.update { it.copy(display = display) }
+        syncUiState(display = display)
+    }
+
+    private fun syncUiState(
+        display: String? = null,
+        clearError: Boolean = false,
+        setError: String? = null,
+    ) {
+        _uiState.update { state ->
+            val newDisplay = display ?: state.display
+            val newError = when {
+                setError != null -> setError
+                clearError -> null
+                else -> state.error
+            }
+            val expression = if (pendingOperation != null && storedValue != null) {
+                "${formatResult(storedValue!!)} ${pendingOperation!!.displaySymbol}"
+            } else {
+                null
+            }
+
+            state.copy(
+                display = newDisplay,
+                error = newError,
+                pendingOperation = pendingOperation,
+                pendingExpression = expression,
+            )
+        }
     }
 }

app/src/main/java/com/heller/calculator/ui/components/CalculatorButton.kt

@@ -1,5 +1,6 @@
 package com.heller.calculator.ui.components
 
+import androidx.compose.foundation.BorderStroke
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.shape.RoundedCornerShape
@@ -26,19 +27,22 @@ fun CalculatorButton(
     type: CalculatorButtonType,
     onClick: () -> Unit,
     modifier: Modifier = Modifier,
+    isSelected: Boolean = false,
 ) {
-    val containerColor = when (type) {
+    val containerColor = when {
-        CalculatorButtonType.Digit -> MaterialTheme.colorScheme.surfaceVariant
+        isSelected -> MaterialTheme.colorScheme.primary
-        CalculatorButtonType.Operator -> MaterialTheme.colorScheme.primaryContainer
+        type == CalculatorButtonType.Digit -> MaterialTheme.colorScheme.surfaceVariant
-        CalculatorButtonType.Equals -> MaterialTheme.colorScheme.primary
+        type == CalculatorButtonType.Operator -> MaterialTheme.colorScheme.primaryContainer
-        CalculatorButtonType.Clear -> MaterialTheme.colorScheme.errorContainer
+        type == CalculatorButtonType.Equals -> MaterialTheme.colorScheme.primary
+        else -> MaterialTheme.colorScheme.errorContainer
     }
 
-    val contentColor = when (type) {
+    val contentColor = when {
-        CalculatorButtonType.Digit -> MaterialTheme.colorScheme.onSurfaceVariant
+        isSelected -> MaterialTheme.colorScheme.onPrimary
-        CalculatorButtonType.Operator -> MaterialTheme.colorScheme.onPrimaryContainer
+        type == CalculatorButtonType.Digit -> MaterialTheme.colorScheme.onSurfaceVariant
-        CalculatorButtonType.Equals -> MaterialTheme.colorScheme.onPrimary
+        type == CalculatorButtonType.Operator -> MaterialTheme.colorScheme.onPrimaryContainer
-        CalculatorButtonType.Clear -> MaterialTheme.colorScheme.onErrorContainer
+        type == CalculatorButtonType.Equals -> MaterialTheme.colorScheme.onPrimary
+        else -> MaterialTheme.colorScheme.onErrorContainer
     }
 
     Button(
@@ -49,7 +53,14 @@ fun CalculatorButton(
             containerColor = containerColor,
             contentColor = contentColor,
         ),
-        elevation = ButtonDefaults.buttonElevation(defaultElevation = 2.dp),
+        border = if (isSelected) {
+            BorderStroke(2.dp, MaterialTheme.colorScheme.onPrimary)
+        } else {
+            null
+        },
+        elevation = ButtonDefaults.buttonElevation(
+            defaultElevation = if (isSelected) 6.dp else 2.dp,
+        ),
     ) {
         Box(
             modifier = Modifier.fillMaxSize(),