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.

1 files changed, 6 insertions, 1 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