minor reader bugs: (#2491)

- fix preload on last page for R2L reader
 - page 3 bug

(cherry picked from commit 8b0458cdf6f9ff733074945d5e8f4209f73474f5)

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/pager/PagerViewer.kt

@@ -70,14 +70,7 @@ abstract class PagerViewer(val activity: ReaderActivity) : BaseViewer {
         pager.adapter = adapter
         pager.addOnPageChangeListener(object : ViewPager.SimpleOnPageChangeListener() {
             override fun onPageSelected(position: Int) {
-                val page = adapter.items.getOrNull(position)
-                if (page != null && currentPage != page) {
-                    currentPage = page
-                    when (page) {
-                        is ReaderPage -> onPageSelected(page, position)
-                        is ChapterTransition -> onTransitionSelected(page)
-                    }
-                }
+                onPageChange(position)
             }
 
             override fun onPageScrollStateChanged(state: Int) {
@@ -129,25 +122,38 @@ abstract class PagerViewer(val activity: ReaderActivity) : BaseViewer {
     }
 
     /**
-     * Called from the ViewPager listener when a [page] is marked as active. It notifies the
+     * Called when a new page (either a [ReaderPage] or [ChapterTransition]) is marked as active
+     */
+    private fun onPageChange(position: Int) {
+        val page = adapter.items.getOrNull(position)
+        if (page != null && currentPage != page) {
+            currentPage = page
+            when (page) {
+                is ReaderPage -> onReaderPageSelected(page)
+                is ChapterTransition -> onTransitionSelected(page)
+            }
+        }
+    }
+
+    /**
+     * Called when a [ReaderPage] is marked as active. It notifies the
      * activity of the change and requests the preload of the next chapter if this is the last page.
      */
-    private fun onPageSelected(page: ReaderPage, position: Int) {
+    private fun onReaderPageSelected(page: ReaderPage) {
         val pages = page.chapter.pages!! // Won't be null because it's the loaded chapter
-        Timber.d("onPageSelected: ${page.number}/${pages.size}")
+        Timber.d("onReaderPageSelected: ${page.number}/${pages.size}")
         activity.onPageSelected(page)
 
         if (page === pages.last()) {
             Timber.d("Request preload next chapter because we're at the last page")
-            val transition = adapter.items.getOrNull(position + 1) as? ChapterTransition.Next
-            if (transition?.to != null) {
-                activity.requestPreloadChapter(transition.to)
+            adapter.nextTransition?.to?.let {
+                activity.requestPreloadChapter(it)
             }
         }
     }
 
     /**
-     * Called from the ViewPager listener when a [transition] is marked as active. It request the
+     * Called when a [ChapterTransition] is marked as active. It request the
      * preload of the destination chapter of the transition.
      */
     private fun onTransitionSelected(transition: ChapterTransition) {
@@ -194,10 +200,15 @@ abstract class PagerViewer(val activity: ReaderActivity) : BaseViewer {
      * Tells this viewer to move to the given [page].
      */
     override fun moveToPage(page: ReaderPage) {
-        Timber.d("moveToPage")
+        Timber.d("moveToPage ${page.number}")
         val position = adapter.items.indexOf(page)
         if (position != -1) {
+            val currentPosition = pager.currentItem
             pager.setCurrentItem(position, true)
+            // manually call onPageChange since ViewPager listener is not triggered in this case
+            if (currentPosition == position) {
+                onPageChange(position)
+            }
         } else {
             Timber.d("Page $page not found in adapter")
         }

app/src/main/java/eu/kanade/tachiyomi/ui/reader/viewer/pager/PagerViewerAdapter.kt

@@ -20,6 +20,9 @@ class PagerViewerAdapter(private val viewer: PagerViewer) : ViewPagerAdapter() {
     var items: List<Any> = emptyList()
         private set
 
+    var nextTransition: ChapterTransition.Next? = null
+        private set
+
     /**
      * Updates this adapter with the given [chapters]. It handles setting a few pages of the
      * next/previous chapter to allow seamless transitions and inverting the pages if the viewer
@@ -46,7 +49,8 @@ class PagerViewerAdapter(private val viewer: PagerViewer) : ViewPagerAdapter() {
         }
 
         // Add next chapter transition and pages.
-        newItems.add(ChapterTransition.Next(chapters.currChapter, chapters.nextChapter))
+        nextTransition = ChapterTransition.Next(chapters.currChapter, chapters.nextChapter)
+                .also { newItems.add(it) }
         if (chapters.nextChapter != null) {
             // Add at most two pages, because this chapter will be selected before the user can
             // swap more pages.