Restore scroll position within an article on Next/Previous

Next/Previous replaces this screen's own back-stack entry with a fresh one
for the neighboring article, which recreates the WebView from scratch --
going back to an article you'd been partway through always landed you back
at the top. Tracks each article's scroll offset in an in-memory,
process-lifetime map (ReaderScrollPositions) as the WebView scrolls, and
restores it once the freshly-loaded page finishes laying out.

2 files changed, 32 insertions, 2 deletions

diff --git a/app/src/main/java/org/terst/feedreader/ui/ReaderScreen.kt b/app/src/main/java/org/terst/feedreader/ui/ReaderScreen.kt
index 4c74e5a..2bdaf64 100644
--- a/app/src/main/java/org/terst/feedreader/ui/ReaderScreen.kt
+++ b/app/src/main/java/org/terst/feedreader/ui/ReaderScreen.kt
@@ -519,7 +519,12 @@ private fun ReaderContent(
                             ctx,
                             onOpenLink,
                             onLongPressLink = { longPressedLink = it },
-                            onYouTubeVideo = { youtubeVideoId = it }
+                            onYouTubeVideo = { youtubeVideoId = it },
+                            onScrollChanged = { scrollY -> ReaderScrollPositions.set(articleId, scrollY) },
+                            // The page's real content height isn't known until layout finishes
+                            // -- scrolling to a saved offset any earlier would just clamp back
+                            // to 0 on a page that hasn't grown tall enough yet.
+                            onPageFinished = { webView -> webView.scrollTo(0, ReaderScrollPositions.get(articleId)) }
                         ).apply {
                             loadDataWithBaseURL(baseUrl, wrapReaderHtml(content, article?.title, article?.publishedAt, article?.imageUrl), "text/html", "UTF-8", null)
                             loadedContent.value = content
diff --git a/app/src/main/java/org/terst/feedreader/ui/ReaderWebView.kt b/app/src/main/java/org/terst/feedreader/ui/ReaderWebView.kt
index f0873c8..31fcb02 100644
--- a/app/src/main/java/org/terst/feedreader/ui/ReaderWebView.kt
+++ b/app/src/main/java/org/terst/feedreader/ui/ReaderWebView.kt
@@ -23,6 +23,18 @@ import androidx.compose.ui.viewinterop.AndroidView
 import androidx.compose.ui.window.Dialog
 import androidx.compose.ui.window.DialogProperties
 
+// In-memory only (process lifetime, not persisted to disk) -- scoped to "don't lose my
+// place if Next/Previous takes me away and back," not to surviving an app restart. A
+// process-wide singleton rather than screen-scoped state so it survives an Activity
+// recreation (rotation) too, not just Compose recomposition within the same screen instance.
+object ReaderScrollPositions {
+    private val positions = mutableMapOf<String, Int>()
+    fun get(articleId: String): Int = positions[articleId] ?: 0
+    fun set(articleId: String, scrollY: Int) {
+        positions[articleId] = scrollY
+    }
+}
+
 private fun isYouTubeWatchUrl(uri: Uri): Boolean {
     val host = uri.host?.lowercase() ?: return false
     return host == "youtu.be" || host.endsWith(".youtube.com") || host == "youtube.com"
@@ -68,14 +80,27 @@ fun newReaderWebView(
     context: Context,
     onOpenLink: (String) -> Unit,
     onLongPressLink: (String) -> Unit = {},
-    onYouTubeVideo: (String) -> Unit = {}
+    onYouTubeVideo: (String) -> Unit = {},
+    onScrollChanged: (Int) -> Unit = {},
+    onPageFinished: (WebView) -> Unit = {}
 ): WebView =
     WebView(context).apply {
         settings.javaScriptEnabled = false
         overScrollMode = View.OVER_SCROLL_NEVER
         isVerticalScrollBarEnabled = false
         isHorizontalScrollBarEnabled = false
+        // Reports the WebView's own scroll offset as the user reads, so a caller (ReaderScreen)
+        // can remember how far into an article they'd gotten and restore it -- e.g. after
+        // Next/Previous replaces this screen's back-stack entry with a fresh one for the same
+        // article, which recreates the WebView from scratch and would otherwise always land
+        // back at the top.
+        setOnScrollChangeListener { _, _, scrollY, _, _ -> onScrollChanged(scrollY) }
         webViewClient = object : WebViewClient() {
+            override fun onPageFinished(view: WebView, url: String?) {
+                super.onPageFinished(view, url)
+                onPageFinished(view)
+            }
+
             override fun shouldOverrideUrlLoading(view: WebView, request: WebResourceRequest): Boolean {
                 // A subframe (an <iframe> in the article body, e.g. a non-YouTube oEmbed
                 // like Twitter/Instagram, or an ad) fires this same callback the instant