Support DisposalMethod::kRestorePrevious in MultiFrameCodec and fix the apng problem.

lib/ui/painting/image_generator_apng.cc

@@ -536,6 +536,13 @@ bool APNGImageGenerator::DemuxNextImageInternal() {
           SkCodecAnimation::DisposalMethod::kKeep) {
     // Mark the required frame as the previous frame in all cases.
     image->frame_info->required_frame = images_.size() - 1;
+  } else if (images_.size() > (first_frame_index_ + 1) &&
+             images_.back().frame_info->disposal_method ==
+                 SkCodecAnimation::DisposalMethod::kRestorePrevious) {
+    // Mark the required frame as the last previous frame
+    // It is not valid if there are 2 or above frames set |disposal_method| to
+    // |kRestorePrevious|. But it also works in MultiFrameCodec.
+    image->frame_info->required_frame = images_.size() - 2;
   }
 
   // Calling SkCodec::getInfo at least once prior to decoding is mandatory.

lib/ui/painting/multi_frame_codec.cc

@@ -116,10 +116,16 @@ MultiFrameCodec::State::GetNextFrameImage(
   std::optional<unsigned int> prior_frame_index = std::nullopt;
 
   if (requiredFrameIndex != SkCodec::kNoFrame) {
-    // We currently assume that frames can only ever depend on the immediately
-    // previous frame, if any. This means that
-    // `DisposalMethod::kRestorePrevious` is not supported.
-    if (lastRequiredFrame_ == nullptr) {
+    // We are here when the frame said |disposal_method| is
+    // `DisposalMethod::kKeep` or `DisposalMethod::kRestorePrevious` and
+    // |requiredFrameIndex| is set to ex-frame or ex-ex-frame.
+    if (requiredFrameIndex == exExRequiredFrameIndex_ && exExRequiredFrame_) {
+      if (exExRequiredFrame_->getPixels() &&
+          CopyToBitmap(&bitmap, exExRequiredFrame_->colorType(),
+                       *exExRequiredFrame_)) {
+        prior_frame_index = requiredFrameIndex;
+      }
+    } else if (lastRequiredFrame_ == nullptr) {
       FML_DLOG(INFO)
           << "Frame " << nextFrameIndex_ << " depends on frame "
           << requiredFrameIndex
@@ -146,12 +152,20 @@ MultiFrameCodec::State::GetNextFrameImage(
     return std::make_pair(nullptr, decode_error);
   }
 
-  // Hold onto this if we need it to decode future frames.
-  if (frameInfo.disposal_method == SkCodecAnimation::DisposalMethod::kKeep ||
-      lastRequiredFrame_) {
-    lastRequiredFrame_ = std::make_unique<SkBitmap>(bitmap);
-    lastRequiredFrameIndex_ = nextFrameIndex_;
-  }
+  // Always keep the last frame to support kRestorePrevious disposal op.
+  //
+  // Example below shows we must keep the frame whatever disposal method is
+  // |kRestoreBGColor| or |kRestorePrevious| or |kKeep|.
+  //
+  // Frame 1          ,   Frame 2           ,  Frame 3          ,  Frame 4
+  // complete decode  ,   complete decode   ,  reuse frame 1    ,  reuse frame 2
+  // OP_BACKGROUND    ,   OP_PREVIOUS       ,  OP_PREVIOUS      ,  OP_KEEP
+  // kRestoreBGColor  ,   kRestorePrevious  ,  kRestorePrevious ,  kKeep
+  exExRequiredFrame_ = std::move(lastRequiredFrame_);
+  exExRequiredFrameIndex_ = lastRequiredFrameIndex_;
+
+  lastRequiredFrame_ = std::make_unique<SkBitmap>(bitmap);
+  lastRequiredFrameIndex_ = nextFrameIndex_;
 
 #if IMPELLER_SUPPORTS_RENDERING
   if (is_impeller_enabled_) {

lib/ui/painting/multi_frame_codec.h

@@ -58,6 +58,12 @@ class MultiFrameCodec : public Codec {
     // The index of the last decoded required frame.
     int lastRequiredFrameIndex_ = -1;
 
+    // The frame before last frame
+    std::unique_ptr<SkBitmap> exExRequiredFrame_;
+
+    // The index of the frame before last frame
+    int exExRequiredFrameIndex_ = -1;
+
     std::pair<sk_sp<DlImage>, std::string> GetNextFrameImage(
         fml::WeakPtr<GrDirectContext> resourceContext,
         const std::shared_ptr<const fml::SyncSwitch>& gpu_disable_sync_switch,