优化

app/src/main/java/io/legado/app/ui/book/read/page/delegate/CoverPageDelegate.kt

@@ -58,14 +58,11 @@ class CoverPageDelegate(readView: ReadView) : HorizontalPageDelegate(readView) {
     override fun setBitmap() {
         when (mDirection) {
             PageDirection.PREV -> {
-                prevBitmap?.recycle()
-                prevBitmap = prevPage.screenshot()
+                prevBitmap = prevPage.screenshot(prevBitmap, canvas)
             }
             PageDirection.NEXT -> {
-                nextBitmap?.recycle()
-                nextBitmap = nextPage.screenshot()
-                curBitmap?.recycle()
-                curBitmap = curPage.screenshot()
+                nextBitmap = nextPage.screenshot(nextBitmap, canvas)
+                curBitmap = curPage.screenshot(curBitmap, canvas)
             }
             else -> Unit
         }

app/src/main/java/io/legado/app/ui/book/read/page/delegate/HorizontalPageDelegate.kt

@@ -1,6 +1,7 @@
 package io.legado.app.ui.book.read.page.delegate
 
 import android.graphics.Bitmap
+import android.graphics.Canvas
 import android.view.MotionEvent
 import io.legado.app.ui.book.read.page.ReadView
 import io.legado.app.ui.book.read.page.entities.PageDirection
@@ -11,6 +12,7 @@ abstract class HorizontalPageDelegate(readView: ReadView) : PageDelegate(readVie
     protected var curBitmap: Bitmap? = null
     protected var prevBitmap: Bitmap? = null
     protected var nextBitmap: Bitmap? = null
+    protected var canvas: Canvas = Canvas()
     private val slopSquare by lazy { readView.slopSquare * readView.slopSquare }
 
     override fun setDirection(direction: PageDirection) {
@@ -21,17 +23,13 @@ abstract class HorizontalPageDelegate(readView: ReadView) : PageDelegate(readVie
     open fun setBitmap() {
         when (mDirection) {
             PageDirection.PREV -> {
-                prevBitmap?.recycle()
-                prevBitmap = prevPage.screenshot()
-                curBitmap?.recycle()
-                curBitmap = curPage.screenshot()
+                prevBitmap = prevPage.screenshot(prevBitmap, canvas)
+                curBitmap = curPage.screenshot(curBitmap, canvas)
             }
 
             PageDirection.NEXT -> {
-                nextBitmap?.recycle()
-                nextBitmap = nextPage.screenshot()
-                curBitmap?.recycle()
-                curBitmap = curPage.screenshot()
+                nextBitmap = nextPage.screenshot(nextBitmap, canvas)
+                curBitmap = curPage.screenshot(curBitmap, canvas)
             }
 
             else -> Unit

app/src/main/java/io/legado/app/ui/book/read/page/delegate/NoAnimPageDelegate.kt

@@ -12,6 +12,10 @@ class NoAnimPageDelegate(readView: ReadView) : HorizontalPageDelegate(readView)
         stopScroll()
     }
 
+    override fun setBitmap() {
+        // nothing
+    }
+
     override fun onDraw(canvas: Canvas) {
         // nothing
     }

app/src/main/java/io/legado/app/utils/ViewExtensions.kt

@@ -6,6 +6,7 @@ import android.annotation.SuppressLint
 import android.content.Context
 import android.graphics.Bitmap
 import android.graphics.Canvas
+import android.graphics.Color
 import android.os.Build
 import android.text.Html
 import android.view.MotionEvent
@@ -133,12 +134,21 @@ fun View.visible(visible: Boolean) {
     }
 }
 
-fun View.screenshot(): Bitmap? {
+fun View.screenshot(bitmap: Bitmap? = null, canvas: Canvas? = null): Bitmap? {
     return if (width > 0 && height > 0) {
-        val screenshot = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888)
-        val c = Canvas(screenshot)
+        val screenshot = if (bitmap != null && bitmap.width == width && bitmap.height == height) {
+            bitmap.eraseColor(Color.TRANSPARENT)
+            bitmap
+        } else {
+            bitmap?.recycle()
+            Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888)
+        }
+        val c = canvas ?: Canvas()
+        c.setBitmap(screenshot)
+        c.save()
         c.translate(-scrollX.toFloat(), -scrollY.toFloat())
         this.draw(c)
+        c.restore()
         c.setBitmap(null)
         screenshot.prepareToDraw()
         screenshot