refactor: Better custom image resizer

lib/utils/custom_image_resizer.dart

@@ -1,67 +1,103 @@
-import 'dart:typed_data';
 import 'dart:ui';
 
+import 'package:flutter/foundation.dart';
+import 'package:flutter/painting.dart';
+
 import 'package:matrix/matrix.dart';
 import 'package:native_imaging/native_imaging.dart' as native;
 
+(int, int) _scaleToBox(int width, int height, {required int boxSize}) {
+  final fit = applyBoxFit(
+    BoxFit.scaleDown,
+    Size(width.toDouble(), height.toDouble()),
+    Size(boxSize.toDouble(), boxSize.toDouble()),
+  ).destination;
+  return (fit.width.round(), fit.height.round());
+}
+
 Future<MatrixImageFileResizedResponse?> customImageResizer(
   MatrixImageFileResizeArguments arguments,
 ) async {
+  if (kIsWeb) {
+    throw UnsupportedError(
+      'customImageResizer only supports non-web platforms.',
+    );
+  }
+
   await native.init();
-  late native.Image nativeImg;
+
+  var imageBytes = arguments.bytes;
+  String? blurhash;
+
+  var originalWidth = 0;
+  var originalHeight = 0;
+  var width = 0;
+  var height = 0;
 
   try {
-    nativeImg = await native.Image.loadEncoded(arguments.bytes); // load on web
-  } on UnsupportedError {
-    try {
-      // for the other platforms
-      final dartCodec = await instantiateImageCodec(arguments.bytes);
-      final dartFrame = await dartCodec.getNextFrame();
-      final rgbaData = await dartFrame.image.toByteData();
-      if (rgbaData == null) {
-        return null;
-      }
-      final rgba = Uint8List.view(
-        rgbaData.buffer,
-        rgbaData.offsetInBytes,
-        rgbaData.lengthInBytes,
-      );
+    // for the other platforms
+    final dartCodec = await instantiateImageCodec(arguments.bytes);
+    final frameCount = dartCodec.frameCount;
+    final dartFrame = await dartCodec.getNextFrame();
+    final rgbaData = await dartFrame.image.toByteData();
+    if (rgbaData == null) {
+      return null;
+    }
+    final rgba = Uint8List.view(
+      rgbaData.buffer,
+      rgbaData.offsetInBytes,
+      rgbaData.lengthInBytes,
+    );
 
-      final width = dartFrame.image.width;
-      final height = dartFrame.image.height;
+    width = originalWidth = dartFrame.image.width;
+    height = originalHeight = dartFrame.image.height;
 
-      dartFrame.image.dispose();
-      dartCodec.dispose();
+    var nativeImg = native.Image.fromRGBA(width, height, rgba);
 
-      nativeImg = native.Image.fromRGBA(width, height, rgba);
-    } catch (e, s) {
-      Logs().e("Could not generate preview", e, s);
-      rethrow;
-    }
-  }
+    dartFrame.image.dispose();
+    dartCodec.dispose();
 
-  final width = nativeImg.width;
-  final height = nativeImg.height;
+    if (arguments.calcBlurhash) {
+      // scale down image for blurhashing to speed it up
+      final (blurW, blurH) = _scaleToBox(width, height, boxSize: 100);
+      final blurhashImg = nativeImg.resample(
+        blurW, blurH,
+        // nearest is unsupported...
+        native.Transform.bilinear,
+      );
 
-  final max = arguments.maxDimension;
-  if (width > max || height > max) {
-    var w = max, h = max;
-    if (width > height) {
-      h = max * height ~/ width;
-    } else {
-      w = max * width ~/ height;
+      blurhash = blurhashImg.toBlurhash(3, 3);
+
+      blurhashImg.free();
     }
 
-    final scaledImg = nativeImg.resample(w, h, native.Transform.lanczos);
-    nativeImg.free();
-    nativeImg = scaledImg;
+    if (frameCount > 1) {
+      // Don't scale down animated images, since those would lose frames.
+      nativeImg.free();
+    } else {
+      final max = arguments.maxDimension;
+      if (width > max || height > max) {
+        (width, height) = _scaleToBox(width, height, boxSize: max);
+
+        final scaledImg =
+            nativeImg.resample(width, height, native.Transform.lanczos);
+        nativeImg.free();
+        nativeImg = scaledImg;
+      }
+
+      imageBytes = await nativeImg.toJpeg(75);
+      nativeImg.free();
+    }
+  } catch (e, s) {
+    Logs().e("Could not generate preview", e, s);
   }
-  final jpegBytes = await nativeImg.toJpeg(75);
 
   return MatrixImageFileResizedResponse(
-    bytes: jpegBytes,
-    width: nativeImg.width,
-    height: nativeImg.height,
-    blurhash: arguments.calcBlurhash ? nativeImg.toBlurhash(3, 3) : null,
+    bytes: imageBytes,
+    width: width,
+    height: height,
+    originalWidth: originalWidth,
+    originalHeight: originalHeight,
+    blurhash: blurhash,
   );
 }