use a gl context pool to provide glcontext objects to multiple panels

engine/src/shell/platform/unity/darwin/ios/unity_surface_manager.h

 #pragma once
 
+#include <vector>
 #include <OpenGLES/EAGL.h>
 #include <OpenGLES/ES2/gl.h>
 #include <OpenGLES/ES2/glext.h>
 #include "flutter/fml/macros.h"
 
 namespace uiwidgets {
+
+struct GLContextPair
+{
+  EAGLContext *gl_context_;
+  EAGLContext *gl_resource_context_;
+
+  GLContextPair(EAGLContext* gl, EAGLContext* gl_resource)
+  {
+    gl_context_ = gl;
+    gl_resource_context_ = gl_resource;
+  }
+};
+
-  //openGLES contexts
-  static EAGLContext *gl_context_;
-  static EAGLContext *gl_resource_context_;
+  //openGL contexts pool
+  static std::vector<GLContextPair> gl_context_pool_;
+
 
   void* CreateRenderTexture(size_t width, size_t height);
 
   uint32_t GetFbo();
 
  private:
+  GLContextPair GetFreeOpenGLContext();
+  void RecycleOpenGLContext(EAGLContext* gl, EAGLContext* gl_resource);
+  EAGLContext *gl_context_;
+  EAGLContext *gl_resource_context_;
   GLuint default_fbo_ = 0;
   GLuint gl_tex_ = 0;
   CVOpenGLESTextureCacheRef gl_tex_cache_ref_ = nullptr;

engine/src/shell/platform/unity/darwin/ios/unity_surface_manager.mm

 
 namespace uiwidgets {
 
-EAGLContext* UnitySurfaceManager::gl_context_ = nullptr;
-EAGLContext* UnitySurfaceManager::gl_resource_context_ = nullptr;
+std::vector<GLContextPair> UnitySurfaceManager::gl_context_pool_;
+
+GLContextPair UnitySurfaceManager::GetFreeOpenGLContext()
+{
+  if (gl_context_pool_.size() == 0)
+  {
+    EAGLContext* gl = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
+    EAGLContext* gl_resource = [[EAGLContext alloc] initWithAPI:[gl API] sharegroup: [gl sharegroup]];
+    gl_context_pool_.push_back(GLContextPair(gl, gl_resource));
+  }
+  auto context_pair = gl_context_pool_.back();
+  gl_context_pool_.pop_back();
+
+  return context_pair;
+}
+
+void UnitySurfaceManager::RecycleOpenGLContext(EAGLContext* gl, EAGLContext* gl_resource)
+{
+  MakeCurrentContext();
+  ClearCurrentContext();
+  MakeCurrentResourceContext();
+  ClearCurrentContext();
+
+  gl_context_pool_.push_back(GLContextPair(gl, gl_resource));
+}
 
 UnitySurfaceManager::UnitySurfaceManager(IUnityInterfaces* unity_interfaces)
 {
 
   //create opengl context
   if (gl_context_ == nullptr && gl_resource_context_ == nullptr) {
-    gl_context_ = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
-    gl_resource_context_ = [[EAGLContext alloc] initWithAPI:[gl_context_ API] sharegroup: [gl_context_ sharegroup]];
+    auto new_context = GetFreeOpenGLContext();
+    gl_context_ = new_context.gl_context_;
+    gl_resource_context_ = new_context.gl_resource_context_;
   }
 
   FML_DCHECK(gl_context_ != nullptr && gl_resource_context_ != nullptr);
 void UnitySurfaceManager::ReleaseNativeRenderContext()
 {
   FML_DCHECK(gl_resource_context_);
-
+  RecycleOpenGLContext(gl_context_, gl_resource_context_);
+  gl_context_ = nullptr;
+  gl_resource_context_ = nullptr;
 
   [EAGLContext setCurrentContext:nil];
 

engine/src/shell/platform/unity/darwin/macos/unity_surface_manager.h

 #include <AppKit/AppKit.h>
 #include <Metal/Metal.h>
 #include <CoreVideo/CoreVideo.h>
+#include <vector>
+
+struct GLContextPair
+{
+  NSOpenGLContext *gl_context_;
+  NSOpenGLContext *gl_resource_context_;
+
+  GLContextPair(NSOpenGLContext* gl, NSOpenGLContext* gl_resource)
+  {
+    gl_context_ = gl;
+    gl_resource_context_ = gl_resource;
+  }
+};
+
-  //openGL contexts
-  static NSOpenGLContext *gl_context_;
-  static NSOpenGLContext *gl_resource_context_;
+  //openGL contexts pool
+  static std::vector<GLContextPair> gl_context_pool_;
 
   void* CreateRenderTexture(size_t width, size_t height);
 
   uint32_t GetFbo();
 
  private:
+  GLContextPair GetFreeOpenGLContext();
+  void RecycleOpenGLContext(NSOpenGLContext* gl, NSOpenGLContext* gl_resource);
+  NSOpenGLContext *gl_context_;
+  NSOpenGLContext *gl_resource_context_;
   GLuint default_fbo_ = 0;
   GLuint gl_tex_ = 0;
   CVOpenGLTextureCacheRef gl_tex_cache_ref_ = nullptr;

engine/src/shell/platform/unity/darwin/macos/unity_surface_manager.mm

 
 namespace uiwidgets {
 
-NSOpenGLContext* UnitySurfaceManager::gl_context_ = nullptr;
-NSOpenGLContext* UnitySurfaceManager::gl_resource_context_ = nullptr;
+std::vector<GLContextPair> UnitySurfaceManager::gl_context_pool_;
+
+GLContextPair UnitySurfaceManager::GetFreeOpenGLContext()
+{
+  if (gl_context_pool_.size() == 0)
+  {
+    NSOpenGLPixelFormatAttribute attrs[] =
+    {
+      NSOpenGLPFAAccelerated,
+      0
+    };
+
+    NSOpenGLPixelFormat *pixelFormat = [[NSOpenGLPixelFormat alloc] initWithAttributes:attrs];
+
+    NSOpenGLContext* gl_resource;
+    NSOpenGLContext* gl;
+
+    while(gl_resource == nil) {
+      gl = [[NSOpenGLContext alloc] initWithFormat:pixelFormat shareContext:nil];
+      gl_resource = [[NSOpenGLContext alloc] initWithFormat:pixelFormat shareContext:gl];  
+    
+      if (gl_resource == nil) {
+        CGLReleaseContext(gl.CGLContextObj);
+        gl = nullptr;
+      }
+    }
+
+
+
+    gl_context_pool_.push_back(GLContextPair(gl, gl_resource));
+  }
+
+  auto context_pair = gl_context_pool_.back();
+  gl_context_pool_.pop_back();
+
+  return context_pair;
+}
+
+void UnitySurfaceManager::RecycleOpenGLContext(NSOpenGLContext* gl, NSOpenGLContext* gl_resource)
+{
+  MakeCurrentContext();
+  ClearCurrentContext();
+  MakeCurrentResourceContext();
+  ClearCurrentContext();
+
+  gl_context_pool_.push_back(GLContextPair(gl, gl_resource));
+}
 
 UnitySurfaceManager::UnitySurfaceManager(IUnityInterfaces* unity_interfaces)
 {
 
   //create opengl context
   if (gl_context_ == nullptr && gl_resource_context_ == nullptr) {
-    NSOpenGLPixelFormatAttribute attrs[] =
-    {
-      NSOpenGLPFAAccelerated,
-      0
-    };
-
-    NSOpenGLPixelFormat *pixelFormat = [[NSOpenGLPixelFormat alloc] initWithAttributes:attrs];
-    
-    //gl_context_ may be created unproperly, we can check this using the relevant gl_resource_context_
-    //loop until the created gl_context_ is ok, otherwise the program will crash anyway
-    while(gl_resource_context_ == nil) {
-      gl_context_ = [[NSOpenGLContext alloc] initWithFormat:pixelFormat shareContext:nil];
-      gl_resource_context_ = [[NSOpenGLContext alloc] initWithFormat:pixelFormat shareContext:gl_context_];  
-    
-      if (gl_resource_context_ == nil) {
-        CGLReleaseContext(gl_context_.CGLContextObj);
-        gl_context_ = nullptr;
-      }
-    }
+    auto new_context = GetFreeOpenGLContext();
+    gl_context_ = new_context.gl_context_;
+    gl_resource_context_ = new_context.gl_resource_context_;
   }
 
   FML_DCHECK(gl_context_ != nullptr && gl_resource_context_ != nullptr);
 void UnitySurfaceManager::ReleaseNativeRenderContext()
 {
   FML_DCHECK(gl_resource_context_);
+  FML_DCHECK(gl_context_);
-  FML_DCHECK(gl_context_);
+  RecycleOpenGLContext(gl_context_, gl_resource_context_);
+  gl_context_ = nullptr;
+  gl_resource_context_ = nullptr;
 
   FML_DCHECK(metal_device_ != nullptr);
   metal_device_ = nullptr;