Downsample offscreen pages option

app/src/main/kotlin/org/koitharu/kotatsu/core/prefs/AppSettings.kt

@@ -109,6 +109,9 @@ class AppSettings @Inject constructor(@ApplicationContext context: Context) {
 	val isReaderTapsAdaptive: Boolean
 		get() = !prefs.getBoolean(KEY_READER_TAPS_LTR, false)
 
+	val isReaderOptimizationEnabled: Boolean
+		get() = prefs.getBoolean(KEY_READER_OPTIMIZE, false)
+
 	var isTrafficWarningEnabled: Boolean
 		get() = prefs.getBoolean(KEY_TRAFFIC_WARNING, true)
 		set(value) = prefs.edit { putBoolean(KEY_TRAFFIC_WARNING, value) }
@@ -506,6 +509,7 @@ class AppSettings @Inject constructor(@ApplicationContext context: Context) {
 		const val KEY_READER_SCREEN_ON = "reader_screen_on"
 		const val KEY_SHORTCUTS = "dynamic_shortcuts"
 		const val KEY_READER_TAPS_LTR = "reader_taps_ltr"
+		const val KEY_READER_OPTIMIZE = "reader_optimize"
 		const val KEY_LOCAL_LIST_ORDER = "local_order"
 		const val KEY_HISTORY_ORDER = "history_order"
 		const val KEY_WEBTOON_ZOOM = "webtoon_zoom"

app/src/main/kotlin/org/koitharu/kotatsu/core/ui/list/lifecycle/RecyclerViewLifecycleDispatcher.kt

@@ -0,0 +1,31 @@
+package org.koitharu.kotatsu.core.ui.list.lifecycle
+
+import androidx.core.view.children
+import androidx.recyclerview.widget.LinearLayoutManager
+import androidx.recyclerview.widget.RecyclerView
+import androidx.recyclerview.widget.RecyclerView.NO_POSITION
+
+class RecyclerViewLifecycleDispatcher : RecyclerView.OnScrollListener() {
+
+	private var prevFirst = NO_POSITION
+	private var prevLast = NO_POSITION
+
+	override fun onScrolled(recyclerView: RecyclerView, dx: Int, dy: Int) {
+		super.onScrolled(recyclerView, dx, dy)
+		val lm = recyclerView.layoutManager as? LinearLayoutManager ?: return
+		val first = lm.findFirstVisibleItemPosition()
+		val last = lm.findLastVisibleItemPosition()
+		if (first == prevFirst && last == prevLast) {
+			return
+		}
+		prevFirst = first
+		prevLast = last
+		if (first == NO_POSITION || last == NO_POSITION) {
+			return
+		}
+		for (child in recyclerView.children) {
+			val wh = recyclerView.getChildViewHolder(child) ?: continue
+			(wh as? LifecycleAwareViewHolder)?.setIsCurrent(wh.absoluteAdapterPosition in first..last)
+		}
+	}
+}

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/config/ReaderSettings.kt

@@ -36,6 +36,9 @@ class ReaderSettings(
 	val colorFilter: ReaderColorFilter?
 		get() = colorFilterFlow.value?.takeUnless { it.isEmpty }
 
+	val isReaderOptimizationEnabled: Boolean
+		get() = settings.isReaderOptimizationEnabled
+
 	val bitmapConfig: Bitmap.Config
 		get() = if (settings.is32BitColorsEnabled) {
 			Bitmap.Config.ARGB_8888
@@ -104,7 +107,8 @@ class ReaderSettings(
 				key == AppSettings.KEY_ZOOM_MODE ||
 				key == AppSettings.KEY_PAGES_NUMBERS ||
 				key == AppSettings.KEY_READER_BACKGROUND ||
-				key == AppSettings.KEY_32BIT_COLOR
+				key == AppSettings.KEY_32BIT_COLOR ||
+				key == AppSettings.KEY_READER_OPTIMIZE
 			) {
 				notifyChanged()
 			}

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/pager/BasePageHolder.kt

@@ -7,6 +7,7 @@ import androidx.viewbinding.ViewBinding
 import org.koitharu.kotatsu.core.exceptions.resolve.ExceptionResolver
 import org.koitharu.kotatsu.core.os.NetworkState
 import org.koitharu.kotatsu.core.ui.list.lifecycle.LifecycleAwareViewHolder
+import org.koitharu.kotatsu.core.util.ext.isLowRamDevice
 import org.koitharu.kotatsu.databinding.LayoutPageInfoBinding
 import org.koitharu.kotatsu.reader.domain.PageLoader
 import org.koitharu.kotatsu.reader.ui.config.ReaderSettings
@@ -67,4 +68,10 @@ abstract class BasePageHolder<B : ViewBinding>(
 	open fun onRecycled() {
 		delegate.onRecycle()
 	}
+
+	protected fun getBackgroundDownsampling() = when {
+		!settings.isReaderOptimizationEnabled -> 1
+		context.isLowRamDevice() -> 8
+		else -> 4
+	}
 }

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/pager/PageHolderDelegate.kt

@@ -42,7 +42,9 @@ class PageHolderDelegate(
 	private var error: Throwable? = null
 
 	init {
-		callback.onConfigChanged()
+		scope.launch(Dispatchers.Main) { // the same as post() -- wait until child fields init
+			callback.onConfigChanged()
+		}
 	}
 
 	fun isLoading() = job?.isActive == true

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/pager/standard/PageHolder.kt

@@ -45,12 +45,22 @@ open class PageHolder(
 		binding.textViewNumber.isVisible = settings.isPagesNumbersEnabled
 	}
 
+	override fun onResume() {
+		super.onResume()
+		binding.ssiv.downsampling = 1
+	}
+
+	override fun onPause() {
+		super.onPause()
+		binding.ssiv.downsampling = getBackgroundDownsampling()
+	}
+
 	override fun onConfigChanged() {
 		super.onConfigChanged()
-		@Suppress("SENSELESS_COMPARISON")
-		if (settings.applyBitmapConfig(binding.ssiv) && delegate != null) {
+		if (settings.applyBitmapConfig(binding.ssiv)) {
 			delegate.reload()
 		}
+		binding.ssiv.downsampling = if (isResumed()) 1 else getBackgroundDownsampling()
 	}
 
 	@SuppressLint("SetTextI18n")

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/pager/webtoon/WebtoonHolder.kt

@@ -40,10 +40,20 @@ class WebtoonHolder(
 
 	override fun onConfigChanged() {
 		super.onConfigChanged()
-		@Suppress("SENSELESS_COMPARISON")
-		if (settings.applyBitmapConfig(binding.ssiv) && delegate != null) {
+		if (settings.applyBitmapConfig(binding.ssiv)) {
 			delegate.reload()
 		}
+		binding.ssiv.downsampling = if (isResumed()) 1 else getBackgroundDownsampling()
+	}
+
+	override fun onResume() {
+		super.onResume()
+		binding.ssiv.downsampling = 1
+	}
+
+	override fun onPause() {
+		super.onPause()
+		binding.ssiv.downsampling = getBackgroundDownsampling()
 	}
 
 	override fun onBind(data: ReaderPage) {

app/src/main/kotlin/org/koitharu/kotatsu/reader/ui/pager/webtoon/WebtoonReaderFragment.kt

@@ -11,6 +11,7 @@ import kotlinx.coroutines.coroutineScope
 import kotlinx.coroutines.yield
 import org.koitharu.kotatsu.R
 import org.koitharu.kotatsu.core.os.NetworkState
+import org.koitharu.kotatsu.core.ui.list.lifecycle.RecyclerViewLifecycleDispatcher
 import org.koitharu.kotatsu.core.util.ext.findCenterViewPosition
 import org.koitharu.kotatsu.core.util.ext.firstVisibleItemPosition
 import org.koitharu.kotatsu.core.util.ext.observe
@@ -44,6 +45,7 @@ class WebtoonReaderFragment : BaseReaderFragment<FragmentReaderWebtoonBinding>()
 			setHasFixedSize(true)
 			adapter = readerAdapter
 			addOnPageScrollListener(PageScrollListener())
+			addOnScrollListener(RecyclerViewLifecycleDispatcher())
 		}
 		viewModel.isWebtoonZooEnabled.observe(viewLifecycleOwner) {
 			binding.frame.isZoomEnable = it

app/src/main/res/values/strings.xml

@@ -527,4 +527,6 @@
 	<string name="available_d">Available: %1$d</string>
 	<string name="disable_nsfw_summary">Disable NSFW sources and hide adult manga from list if possible</string>
 	<string name="state_paused">Paused</string>
+	<string name="reader_optimize">Reduce memory consumption (beta)</string>
+	<string name="reader_optimize_summary">Reduce offscreen pages quality to use less memory</string>
 </resources>

app/src/main/res/xml/pref_reader.xml

@@ -60,6 +60,12 @@
 		android:summary="@string/enhanced_colors_summary"
 		android:title="@string/enhanced_colors" />
 
+	<SwitchPreferenceCompat
+		android:defaultValue="false"
+		android:key="reader_optimize"
+		android:summary="@string/reader_optimize_summary"
+		android:title="@string/reader_optimize" />
+
 	<SwitchPreferenceCompat
 		android:defaultValue="true"
 		android:key="reader_bar"