From 2e738ee40eeab407f31723fafd0336706cf889c1 Mon Sep 17 00:00:00 2001
From: Chinmay Garde <chinmaygarde@google.com>
Date: Wed, 29 Apr 2020 13:27:00 -0700
Subject: [PATCH] [Metal] On context loss with a paused texture source,
 preserve external pixel buffer. (#18022)

Fixes https://github.com/flutter/flutter/issues/55783
Requires follow-up https://github.com/flutter/flutter/issues/55926
---
 .../darwin/ios/ios_external_texture_metal.h     |  4 +++-
 .../darwin/ios/ios_external_texture_metal.mm    | 17 ++++++++++++++---
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/shell/platform/darwin/ios/ios_external_texture_metal.h b/shell/platform/darwin/ios/ios_external_texture_metal.h
index efedc9f19f4..9bcce844d39 100644
--- a/shell/platform/darwin/ios/ios_external_texture_metal.h
+++ b/shell/platform/darwin/ios/ios_external_texture_metal.h
@@ -31,6 +31,7 @@ class IOSExternalTextureMetal final : public Texture {
   fml::CFRef<CVMetalTextureCacheRef> texture_cache_;
   fml::scoped_nsobject<NSObject<FlutterTexture>> external_texture_;
   std::atomic_bool texture_frame_available_;
+  fml::CFRef<CVPixelBufferRef> last_pixel_buffer_;
   sk_sp<SkImage> external_image_;
 
   // |Texture|
@@ -48,7 +49,8 @@ class IOSExternalTextureMetal final : public Texture {
   // |Texture|
   void OnTextureUnregistered() override;
 
-  sk_sp<SkImage> WrapExternalPixelBuffer(GrContext* context) const;
+  sk_sp<SkImage> WrapExternalPixelBuffer(fml::CFRef<CVPixelBufferRef> pixel_buffer,
+                                         GrContext* context) const;
 
   FML_DISALLOW_COPY_AND_ASSIGN(IOSExternalTextureMetal);
 };
diff --git a/shell/platform/darwin/ios/ios_external_texture_metal.mm b/shell/platform/darwin/ios/ios_external_texture_metal.mm
index e9871327123..a5ea79fb99b 100644
--- a/shell/platform/darwin/ios/ios_external_texture_metal.mm
+++ b/shell/platform/darwin/ios/ios_external_texture_metal.mm
@@ -30,11 +30,17 @@ void IOSExternalTextureMetal::Paint(SkCanvas& canvas,
   const bool needs_updated_texture = (!freeze && texture_frame_available_) || !external_image_;
 
   if (needs_updated_texture) {
+    auto pixel_buffer = fml::CFRef<CVPixelBufferRef>([external_texture_ copyPixelBuffer]);
+    if (!pixel_buffer) {
+      pixel_buffer = std::move(last_pixel_buffer_);
+    }
+
     // If the application told us there was a texture frame available but did not provide one when
     // asked for it, reuse the previous texture but make sure to ask again the next time around.
-    if (auto wrapped_texture = WrapExternalPixelBuffer(context)) {
+    if (auto wrapped_texture = WrapExternalPixelBuffer(pixel_buffer, context)) {
       external_image_ = wrapped_texture;
       texture_frame_available_ = false;
+      last_pixel_buffer_ = std::move(pixel_buffer);
     }
   }
 
@@ -48,8 +54,9 @@ void IOSExternalTextureMetal::Paint(SkCanvas& canvas,
   }
 }
 
-sk_sp<SkImage> IOSExternalTextureMetal::WrapExternalPixelBuffer(GrContext* context) const {
-  auto pixel_buffer = fml::CFRef<CVPixelBufferRef>([external_texture_ copyPixelBuffer]);
+sk_sp<SkImage> IOSExternalTextureMetal::WrapExternalPixelBuffer(
+    fml::CFRef<CVPixelBufferRef> pixel_buffer,
+    GrContext* context) const {
   if (!pixel_buffer) {
     return nullptr;
   }
@@ -126,6 +133,10 @@ void IOSExternalTextureMetal::OnGrContextCreated() {
 }
 
 void IOSExternalTextureMetal::OnGrContextDestroyed() {
+  // The image must be reset because it is tied to the onscreen context. But the pixel buffer that
+  // created the image is still around. In case of context reacquisition, that last pixel
+  // buffer will be used to materialize the image in case the application fails to provide a new
+  // one.
   external_image_.reset();
   CVMetalTextureCacheFlush(texture_cache_,  // cache
                            0                // options (must be zero)