refactor: isolate input method switching and recovery

CONTEXT.md

@@ -0,0 +1,15 @@
+# TypeSwitch
+
+TypeSwitch 根据前台应用的规则选择输入法，并在输入法可用性变化后恢复所需的切换。
+
+## 领域语言
+
+**输入法切换与恢复**：根据当前前台应用及其规则确定目标输入法，并处理切换尝试、输入法目录恢复与前台重试的完整过程。
+
+**切换尝试**：针对某个应用和目标输入法发起的一次程序切换。已经选中目标输入法与实际完成一次切换是不同的结果。
+
+**输入法目录**：TypeSwitch 获取到的可用输入法集合。尚未获取或获取失败，不等于集合为空，也不等于某个输入法已被删除。
+
+**前台重试**：重新确认当前前台应用，再按该应用当时的规则尝试切换；不是无条件重放此前应用的切换。
+
+**跟随上次**：记住某个应用上次使用的输入法，并在该应用再次成为前台应用时将其作为切换目标。程序切换自身的目标通知不能被误当作新的手动选择。

TypeSwitch/Sources/App/AppFeature+MenuState.swift

@@ -1,6 +1,11 @@
 import Foundation
 
 extension AppFeature.State {
+    var currentFrontmostBundleId: String? { switching.currentFrontmostBundleId }
+    var inputMethodCatalogStatus: SwitchingFeature.State.InputMethodCatalogStatus { switching.inputMethodCatalogStatus }
+    var inputMethods: [InputMethod] { switching.inputMethods }
+    var lastSwitchAttempt: SwitchingFeature.State.LastSwitchAttempt? { switching.lastSwitchAttempt }
+
     struct InputMethodDiagnostic: Equatable {
         enum Kind: Equatable {
             case catalogEmpty
@@ -70,14 +75,7 @@ extension AppFeature.State {
         }
     }
 
-    var fallbackStrategy: InputMethodStrategy {
-        switch fallbackRuleStore.strategy {
-        case .followLast, .ignored:
-            return .none
-        case .none, .fixed:
-            return fallbackRuleStore.strategy
-        }
-    }
+    var fallbackStrategy: InputMethodStrategy { switching.fallbackStrategy }
 
     var fallbackSelectedLabel: String? {
         if fallbackStrategy == .none {
@@ -112,7 +110,7 @@ extension AppFeature.State {
 
         guard let lastSwitchAttempt,
               currentFrontmostBundleId == lastSwitchAttempt.bundleId,
-              isCurrentTarget(lastSwitchAttempt),
+              switching.isCurrentTarget(lastSwitchAttempt),
               case .failed(let error) = lastSwitchAttempt.outcome
         else {
             return nil
@@ -399,28 +397,6 @@ extension AppFeature.State {
         inputMethods.first(where: { $0.id == inputMethodId })?.name
     }
 
-    func isCurrentTarget(_ attempt: LastSwitchAttempt) -> Bool {
-        let appStrategy = strategy(for: attempt.bundleId)
-        switch attempt.ruleSource {
-        case .app:
-            return inputMethodId(for: appStrategy) == attempt.inputMethodId
-        case .fallback:
-            return appStrategy == .none
-                && inputMethodId(for: fallbackStrategy) == attempt.inputMethodId
-        }
-    }
-
-    private func inputMethodId(for strategy: InputMethodStrategy) -> String? {
-        switch strategy {
-        case .fixed(let inputMethodId):
-            return inputMethodId
-        case .followLast(let lastInputMethodId):
-            return lastInputMethodId
-        case .ignored, .none:
-            return nil
-        }
-    }
-
     private var unavailableInputMethodLabel: String {
         inputMethodCatalogStatus == .ready
             ? TypeSwitchStrings.InputMethod.deletedOption