Clamp currentAlpha to 0...255

Author: eisukekusachi

TextureLayerView/Sources/TextureLayerView/TextureLayerView.swift

@@ -53,7 +53,7 @@ public struct TextureLayerView: View {
 
     /// Updates the alpha slider without changing the selected layer.
     public func updateAlpha(_ alpha: Int) {
-        viewModel.currentAlpha = alpha
+        viewModel.setCurrentAlpha(alpha)
     }
 }
 

TextureLayerView/Sources/TextureLayerView/TextureLayerViewModel.swift

@@ -13,6 +13,8 @@ import UIKit
 @MainActor
 open class TextureLayerViewModel: ObservableObject {
 
+    static let alphaRange: ClosedRange<Int> = 0...255
+
     @Published public var currentAlpha: Int = 0
 
     @Published public var isAlphaSliderDragging: Bool = false
@@ -143,10 +145,17 @@ open class TextureLayerViewModel: ObservableObject {
 
     open func onChangeCurrentAlpha(_ alpha: Int) {
         guard let selectedLayerId = selectedLayer?.id else { return }
-        textureLayers.updateAlpha(selectedLayerId, alpha: alpha)
-        updateCurrentAlpha()
+        let clamped = Self.clampedAlpha(alpha)
+        textureLayers.updateAlpha(selectedLayerId, alpha: clamped)
+        setCurrentAlpha(clamped)
         onLayersChanged?(.changeLayerAlpha)
     }
+
+    func setCurrentAlpha(_ alpha: Int) {
+        let clamped = Self.clampedAlpha(alpha)
+        guard currentAlpha != clamped else { return }
+        currentAlpha = clamped
+    }
 }
 
 public extension TextureLayerViewModel {
@@ -199,13 +208,16 @@ public extension TextureLayerViewModel {
 
 extension TextureLayerViewModel {
 
+    static func clampedAlpha(_ alpha: Int) -> Int {
+        min(max(alphaRange.lowerBound, alpha), alphaRange.upperBound)
+    }
+
     private func updateCurrentAlpha() {
         guard
             let selectedLayerId = selectedLayer?.id,
-            let layer = textureLayers.layer(selectedLayerId),
-            currentAlpha != layer.alpha
+            let layer = textureLayers.layer(selectedLayerId)
         else { return }
 
-        currentAlpha = layer.alpha
+        setCurrentAlpha(layer.alpha)
     }
 }

TextureLayerView/Tests/TextureLayerViewTests/TextureLayerViewModelTests.swift

@@ -0,0 +1,60 @@
+//
+//  TextureLayerViewModelTests.swift
+//  TextureLayerViewTests
+//
+//  Created by Eisuke Kusachi on 2026/08/23.
+//
+
+import CoreGraphics
+import Testing
+
+@testable import TextureLayerView
+
+@MainActor
+struct TextureLayerViewModelTests {
+
+    private typealias Subject = TextureLayerViewModel
+
+    private let textureSize: CGSize = .init(width: 1, height: 1)
+
+    @Test
+    func `setCurrentAlpha clamps values below 0`() {
+        let subject = makeSubject()
+
+        subject.setCurrentAlpha(-1)
+
+        #expect(subject.currentAlpha == 0)
+    }
+
+    @Test
+    func `setCurrentAlpha clamps values above 255`() {
+        let subject = makeSubject()
+
+        subject.setCurrentAlpha(256)
+
+        #expect(subject.currentAlpha == 255)
+    }
+
+    @Test
+    func `onChangeCurrentAlpha clamps the selected layer alpha`() {
+        let subject = makeSubject()
+
+        subject.onChangeCurrentAlpha(-1)
+
+        #expect(subject.currentAlpha == 0)
+        #expect(subject.textureLayers.selectedLayer?.alpha == 0)
+
+        subject.onChangeCurrentAlpha(300)
+
+        #expect(subject.currentAlpha == 255)
+        #expect(subject.textureLayers.selectedLayer?.alpha == 255)
+    }
+
+    private func makeSubject() -> Subject {
+        Subject(
+            textureLayers: TextureLayersState(
+                textureLayers: .init(textureSize: textureSize)
+            )
+        )
+    }
+}