add save function of explanation

android/sample-app/src/main/java/cn/learningpad/oraltrainer/sample/MainActivity.kt

@@ -434,6 +434,7 @@ class MainActivity : Activity() {
             explanationOverlay.show(
                 sentence,
                 controller.currentTrainingItem()?.title.orEmpty(),
+                controller.currentTrainingItem()?.id.orEmpty(),
             )
         }
 

android/sample-app/src/main/java/cn/learningpad/oraltrainer/sample/SentenceExplanationOverlay.kt

@@ -18,11 +18,12 @@ internal class SentenceExplanationOverlay(
     private val activity: Activity,
     private val api: SentenceExplanationApi,
 ) {
+    private val store = SentenceExplanationStore(activity)
     private var currentOverlay: View? = null
     private var currentRequest: CancellableExplanationRequest? = null
     private var requestGeneration = 0
 
-    fun show(sentence: SentenceBoundary, courseTitle: String) {
+    fun show(sentence: SentenceBoundary, courseTitle: String, mediaId: String) {
         close()
         val generation = ++requestGeneration
         val root = FrameLayout(activity).apply {
@@ -51,6 +52,31 @@ internal class SentenceExplanationOverlay(
             setTextColor(SampleTheme.textDark)
             textSize = 19f
             typeface = Typeface.DEFAULT_BOLD
+            layoutParams = LinearLayout.LayoutParams(0, ViewGroup.LayoutParams.WRAP_CONTENT, 1f)
+        }
+        val saveHint = TextView(activity).apply {
+            text = HINT_UNSAVED
+            setTextColor(SampleTheme.textSubtle)
+            textSize = 12f
+            setPadding(6.dp, 0, 0, 0)
+        }
+        val saveButton = TextView(activity).apply {
+            text = "保存"
+            setTextColor(Color.WHITE)
+            textSize = 13f
+            typeface = Typeface.DEFAULT_BOLD
+            gravity = Gravity.CENTER
+            background = rounded(SampleTheme.button, 8f)
+            setPadding(12.dp, 6.dp, 12.dp, 6.dp)
+            isEnabled = false
+            alpha = 0.45f
+        }
+        val titleRow = LinearLayout(activity).apply {
+            orientation = LinearLayout.HORIZONTAL
+            gravity = Gravity.CENTER_VERTICAL
+            addView(title)
+            addView(saveButton)
+            addView(saveHint)
         }
         val subtitle = TextView(activity).apply {
             text = sentence.text?.takeIf { it.isNotBlank() } ?: courseTitle
@@ -86,7 +112,7 @@ internal class SentenceExplanationOverlay(
             ).apply { topMargin = 16.dp }
             setOnClickListener { close() }
         }
-        card.addView(title)
+        card.addView(titleRow)
         card.addView(subtitle)
         card.addView(scroll)
         card.addView(close)
@@ -94,6 +120,22 @@ internal class SentenceExplanationOverlay(
 
         addToWindow(root)
         currentOverlay = root
+
+        val cached = store.load(mediaId, sentence.index)
+        if (!cached.isNullOrBlank()) {
+            message.text = cached
+            message.setTextColor(SampleTheme.textDark)
+            markSaved(saveButton, saveHint)
+            return
+        }
+
+        var loadedExplanation: String? = null
+        saveButton.setOnClickListener {
+            val explanation = loadedExplanation ?: return@setOnClickListener
+            if (store.save(mediaId, sentence.index, explanation)) {
+                markSaved(saveButton, saveHint)
+            }
+        }
         currentRequest = api.explain(
             sentence.text.orEmpty(),
             courseTitle,
@@ -101,14 +143,20 @@ internal class SentenceExplanationOverlay(
             sentence.language,
             object : SentenceExplanationCallback {
                 override fun onSuccess(result: SentenceExplanationResult) {
-                    if (generation == requestGeneration) message.text = result.explanation
+                    if (generation != requestGeneration) return
+                    loadedExplanation = result.explanation
+                    message.text = result.explanation
+                    message.setTextColor(SampleTheme.textDark)
+                    saveButton.isEnabled = true
+                    saveButton.alpha = 1f
                 }
 
                 override fun onError(error: Throwable) {
-                    if (generation == requestGeneration) {
-                        message.text = "讲解失败：${error.message ?: "请稍后重试"}"
-                        message.setTextColor(SampleTheme.danger)
-                    }
+                    if (generation != requestGeneration) return
+                    message.text = "讲解失败：${error.message ?: "请稍后重试"}"
+                    message.setTextColor(SampleTheme.danger)
+                    saveButton.isEnabled = false
+                    saveButton.alpha = 0.45f
                 }
             },
         )
@@ -128,6 +176,13 @@ internal class SentenceExplanationOverlay(
         api.release()
     }
 
+    private fun markSaved(saveButton: TextView, saveHint: TextView) {
+        saveButton.text = "已保存"
+        saveButton.isEnabled = false
+        saveButton.alpha = 0.7f
+        saveHint.text = HINT_SAVED
+    }
+
     private fun addToWindow(view: View) {
         (activity.window?.decorView as? ViewGroup)?.addView(view)
     }
@@ -147,4 +202,9 @@ internal class SentenceExplanationOverlay(
         cornerRadius = radiusDp.dp.toFloat()
         strokeColor?.let { setStroke(1.dp, it) }
     }
+
+    private companion object {
+        const val HINT_UNSAVED = "（下次直接看本地）"
+        const val HINT_SAVED = "（已保存本地）"
+    }
 }

android/sample-app/src/main/java/cn/learningpad/oraltrainer/sample/SentenceExplanationStore.kt

@@ -0,0 +1,39 @@
+package cn.learningpad.oraltrainer.sample
+
+import android.content.Context
+import java.io.File
+import java.util.Locale
+
+/** Persists sentence explanations under app filesDir for reuse without another API call. */
+internal class SentenceExplanationStore(context: Context) {
+    private val rootDir = File(context.applicationContext.filesDir, "sentence-explanations")
+
+    fun load(mediaId: String, sentenceIndex: Int): String? {
+        val file = fileFor(mediaId, sentenceIndex) ?: return null
+        if (!file.isFile || file.length() <= 0L) return null
+        return file.readText()
+    }
+
+    fun save(mediaId: String, sentenceIndex: Int, explanation: String): Boolean {
+        val text = explanation.trim()
+        if (text.isEmpty()) return false
+        val file = fileFor(mediaId, sentenceIndex) ?: return false
+        file.parentFile?.mkdirs()
+        file.writeText(text)
+        return file.isFile && file.length() > 0L
+    }
+
+    private fun fileFor(mediaId: String, sentenceIndex: Int): File? {
+        val directoryName = sanitize(mediaId) ?: return null
+        return File(File(rootDir, directoryName), "$sentenceIndex.txt")
+    }
+
+    companion object {
+        fun sanitize(mediaId: String): String? {
+            val filtered = mediaId.trim().lowercase(Locale.US)
+                .replace(Regex("[^a-z0-9._-]"), "_")
+                .trim('_')
+            return filtered.takeIf { it.isNotBlank() && it != "." && it != ".." }
+        }
+    }
+}