Rudimentary thumbnail pixels-based autocrop detection.

A better solution would be using element bounds, but that requires a higher API level so I need a working fallback.

app/build.gradle.kts

@@ -9,7 +9,7 @@ android {
 
     defaultConfig {
         applicationId = "com.lhw.pdf"
-        minSdk = 25
+        minSdk = 26
         targetSdk = 35
         versionCode = 1
         versionName = "1.0"

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

@@ -28,51 +28,33 @@ import androidx.core.view.WindowInsetsCompat
 import com.lhw.pdf.databinding.ActivityMainBinding
 import java.io.File
 import java.io.FileOutputStream
+import java.io.InputStream
+import kotlin.math.min
 
 class MainActivity : AppCompatActivity() {
 
     private lateinit var appBarConfiguration: AppBarConfiguration
     private lateinit var binding: ActivityMainBinding
-    private lateinit var fileCached: File
-    private lateinit var renderer: PdfRenderer
     private val thumbnailWidth = 500
     private val thumbnailHeight = 700
-    private val bitmapThumbnails = mutableMapOf<Int, Bitmap>()
-//    private val bitmapPagesMain = mutableMapOf<Int, Bitmap>()
-    private val bitmapPagesPresentation = mutableMapOf<Int, Bitmap>()
-    private val testPdf = R.raw.testpdf
+    private lateinit var pdfDocument: PdfDocument
     private lateinit var presentationScroll: HorizontalScrollView
     private lateinit var presentationLayout: LinearLayout
 
-    private fun initializeRenderer() {
+    private fun inputStreamToCache(outputFilename: String, inputStream: InputStream): File {
+        val fileCached = File(cacheDir, outputFilename)
         val output = FileOutputStream(fileCached)
-        val inputStream = resources.openRawResource(testPdf)
         while (inputStream.available() > 0) {
             inputStream.copyTo(output)
         }
         inputStream.close()
         output.close()
-        val pfd = ParcelFileDescriptor.open(fileCached, ParcelFileDescriptor.MODE_READ_ONLY)
-        renderer = PdfRenderer(pfd)
+        return fileCached
     }
 
-    private fun renderPagesToMap(map: MutableMap<Int, Bitmap>, width: Int, height: Int) {
-        map.clear()
-        val numPages = renderer.pageCount
-        for (i in 0..<numPages) {
-            map[i] = renderPage(i, width, height)
-        }
-    }
-    private fun renderThumbnails() {
-        renderPagesToMap(bitmapThumbnails, thumbnailWidth, thumbnailHeight)
-    }
-
-    private fun renderPage(index: Int, width: Int, height: Int): Bitmap {
-        val page = renderer.openPage(index)
-        val bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888)
-        page.render(bitmap, null, null, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY)
-        page.close()
-        return bitmap
+    private fun initializePdfDocument() {
+        val inputStream = resources.openRawResource(R.raw.testpdf)
+        pdfDocument = PdfDocument(inputStreamToCache("cached.pdf", inputStream), true)
     }
 
     class MyPresentation(outerContext: Context, display: Display) : Presentation(outerContext, display) {
@@ -93,11 +75,11 @@ class MainActivity : AppCompatActivity() {
 
             val metrics = DisplayMetrics()
             presentationDisplay.getMetrics(metrics)
-            val maxHeight = metrics.heightPixels
-            val maxWidth = maxHeight * 5 / 7
-            renderPagesToMap(bitmapPagesPresentation, maxWidth, maxHeight)
+            val maxHeight: Int = metrics.heightPixels
+            val maxWidth: Int = min(maxHeight*5/7, metrics.widthPixels)
+            pdfDocument.renderPagesPresentation(maxWidth, maxHeight)
             presentationLayout = LinearLayout(presentation.context)
-            for (bitmap in bitmapPagesPresentation.values) {
+            for (bitmap in pdfDocument.bitmapPagesPresentation.values) {
                 val img = ImageView(this)
                 img.setImageBitmap(bitmap)
                 presentationLayout.addView(img)
@@ -138,11 +120,10 @@ class MainActivity : AppCompatActivity() {
         setupActionBarWithNavController(navController, appBarConfiguration)
         navView.setupWithNavController(navController)
 
-        fileCached = File(cacheDir, "cached.pdf")
-        initializeRenderer()
-        renderThumbnails()
+        initializePdfDocument()
+        pdfDocument.renderThumbnails(thumbnailWidth, thumbnailHeight)
         val container = findViewById<LinearLayout>(R.id.thumbnails_layout)
-        for (bitmap in bitmapThumbnails.values) {
+        for (bitmap in pdfDocument.bitmapThumbnails.values) {
             val img = ImageView(this)
             img.setImageBitmap(bitmap)
             container.addView(img)

app/src/main/java/com/lhw/pdf/ui/gallery/GalleryFragment.kt

@@ -23,7 +23,7 @@ class GalleryFragment : Fragment() {
         savedInstanceState: Bundle?
     ): View {
         val galleryViewModel =
-            ViewModelProvider(this).get(GalleryViewModel::class.java)
+            ViewModelProvider(this)[GalleryViewModel::class.java]
 
         _binding = FragmentGalleryBinding.inflate(inflater, container, false)
         val root: View = binding.root

app/src/main/java/com/lhw/pdf/ui/home/HomeFragment.kt

@@ -23,7 +23,7 @@ class HomeFragment : Fragment() {
         savedInstanceState: Bundle?
     ): View {
         val homeViewModel =
-            ViewModelProvider(this).get(HomeViewModel::class.java)
+            ViewModelProvider(this)[HomeViewModel::class.java]
 
         _binding = FragmentHomeBinding.inflate(inflater, container, false)
         val root: View = binding.root

app/src/main/java/com/lhw/pdf/ui/slideshow/SlideshowFragment.kt

@@ -23,7 +23,7 @@ class SlideshowFragment : Fragment() {
         savedInstanceState: Bundle?
     ): View {
         val slideshowViewModel =
-            ViewModelProvider(this).get(SlideshowViewModel::class.java)
+            ViewModelProvider(this)[SlideshowViewModel::class.java]
 
         _binding = FragmentSlideshowBinding.inflate(inflater, container, false)
         val root: View = binding.root