Only layout visible pages on presentation on Android 13 and under

Android 14 seems to not have offscreen culling problems, but my Android 13 tablet gets very laggy with a medium-sized pdf having everything laid out at once.

.idea/kotlinc.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
   <component name="KotlinJpsPluginSettings">
-    <option name="version" value="1.9.0" />
+    <option name="version" value="2.0.0" />
   </component>
 </project>

app/src/main/java/com/lhw/pdf/MyPresentation.kt

@@ -14,8 +14,15 @@ import androidx.core.view.WindowInsetsCompat
 
 class MyPresentation(outerContext: Context, display: Display) : Presentation(outerContext, display) {
     val displayMetrics = DisplayMetrics()
+    private val lazyScroll = android.os.Build.VERSION.SDK_INT < 34  //android.os.Build.VERSION_CODES.UPSIDE_DOWN_CAKE
+    private var lazyIdxStart = -1
+    private var lazyIdxEnd = -1
+
+    private val bitmaps = mutableListOf<Bitmap>()
+    private val bitmapCumWidths = mutableListOf<Int>(0)
     private val layout = LinearLayout(this.context)
     private val scroll = HorizontalScrollView(this.context)
+    private var scrollProgress = 0f
     private val layoutParams = WindowManager.LayoutParams()
     init {
         @Suppress("DEPRECATION")
@@ -33,18 +40,58 @@ class MyPresentation(outerContext: Context, display: Display) : Presentation(out
         println("Updating presentation images")
         println("Layout has scale ${layout.scaleX} ${layout.scaleY}")
         layout.removeAllViewsInLayout()
+        this.bitmaps.clear()
+        bitmapCumWidths.clear()
+        bitmapCumWidths.add(0)
         bitmaps.forEach {
-            val img = ImageView(this.context)
-            img.setImageBitmap(it)
-            layout.addView(img)
-            img.minimumWidth = it.width  // No idea what broke when I refactored this class
-            img.minimumHeight = it.height
-            println("${it.width}x${it.height}")
+            this.bitmaps.add(it)
+            bitmapCumWidths.add(it.width + bitmapCumWidths.last())
         }
+        if (lazyScroll) {
+            lazyIdxStart = -1
+            lazyIdxEnd = -1
+            layoutVisibleBitmaps()
+        } else {
+            layoutAllBitmaps()
+        }
+    }
+
+    private fun layoutBitmap(bitmap: Bitmap) {
+        val img = ImageView(this.context)
+        img.setImageBitmap(bitmap)
+        img.minimumWidth = bitmap.width  // No idea what broke when I refactored this class
+        img.minimumHeight = bitmap.height
+        println("${bitmap.width}x${bitmap.height}")
+        layout.addView(img)
+    }
+
+    private fun layoutAllBitmaps() {
+        bitmaps.forEach(::layoutBitmap)
         println("Presentation heights ${scroll.height} ${layout.height}")
     }
 
+    private fun layoutVisibleBitmaps() {
+        val xStart = (scrollProgress * (bitmapCumWidths.last() - scroll.width)).toInt()
+        val xEnd = xStart + scroll.width
+        // bitmapCumWidths has a prepended 0
+        val idxStart = bitmapCumWidths.indexOfFirst { it > xStart }.let { if (it >= 1) it-1 else 0 }
+        val idxEnd = bitmapCumWidths.indexOfFirst { it > xEnd }.let { if (it >= 0) it else bitmaps.size}
+
+        layout.removeAllViewsInLayout()
+        (idxStart..<idxEnd).map(bitmaps::get).forEach(::layoutBitmap)
+        lazyIdxStart = idxStart
+        lazyIdxEnd = idxEnd
+        println("Laying out pages $idxStart up to $idxEnd - scrollable width ${layout.width}")
+    }
+
     fun setScrollProgress(progress: Float) {
-        scroll.scrollTo((progress * (layout.width - scroll.width)).toInt(), 0)
+        scrollProgress = progress
+        var progressPx = (progress * (bitmapCumWidths.last() - scroll.width)).toInt()
+        if (lazyScroll) {
+            layoutVisibleBitmaps()
+            progressPx -= bitmapCumWidths[lazyIdxStart]
+        }
+        println("Scrolling view to x=$progressPx of scrollable ${layout.width} (visible width ${scroll.width})")
+        scroll.scrollTo(progressPx, 0)
     }
 }

gradle/libs.versions.toml

@@ -1,6 +1,6 @@
 [versions]
 agp = "8.5.2"
-kotlin = "1.9.0"
+kotlin = "2.0.0"
 coreKtx = "1.13.1"
 junit = "4.13.2"
 junitVersion = "1.2.1"