Synchronization mechanism on Mixing Metal and OpenGL Rendering

ios: 14.2 iphone xr

I download the demo from "https://developer.apple.com/documentation/metal/mixing_metal_and_opengl_rendering_in_a_view?language=objc"

it play normally.

and then i change the code as followed:

the purpose is switching "mix render" and "just opengl rendering" and re-create AAPLMetalRenderer each time when re-entry "mix render"

there come a 'bug' phenomenon: the first frame each time from "just opengl" to "mix render" will display the old picture ( it was the last picture when "mix render" to "just opengl " ) .

the first frame it will re-create the AAPLMetalRenderer and call drawToInteropTexture, but it seems that the "InteropTexture" do not 'update' yet ( or opengl's draw do not wait for metal finish rendering to 'InteropTexture' ? )

so I have a question about how metal and opengl sync??

int counter = 0 ;
bool currentMetal = false ;
- (void)draw:(id)sender
{
  [EAGLContext setCurrentContext:_context];
	
	counter++;
	counter = counter % 180;
	
	if (counter < 90)
	{
		
		bool waitForFinish = false ;
		
		if (!currentMetal)  // re-entry "mix render"
		{
             // create MetalRender 
			_metalRenderer = nil;

			_metalRenderer = [[AAPLMetalRenderer alloc] initWithDevice:_metalDevice colorPixelFormat:AAPLOpenGLViewInteropPixelFormat];

			[_metalRenderer useTextureFromFileAsBaseMap];

			[_metalRenderer resize:AAPLInteropTextureSize];
			
 
		}
		
		currentMetal = true ;
	 	
		[_metalRenderer drawToInteropTexture:_interopTexture.metalTexture waitForFinish:waitForFinish];
		
		
		[_openGLRenderer draw];
	}
	else
	{
		[_metalRenderer justUpdate]; // not metal render
		[_openGLRenderer justClear]; // just clean opengl's fbo
	 
		currentMetal = false ;
	}

  glBindRenderbuffer(GL_RENDERBUFFER, _colorRenderbuffer);
  [_context presentRenderbuffer:GL_RENDERBUFFER];
}

_openGLRenderer justClear is below:

- (void) justClear
{
	glBindFramebuffer(GL_FRAMEBUFFER, _defaultFBOName);
	glClearColor(0.5, 0.5, 0.5, 1);
	glClear(GL_COLOR_BUFFER_BIT);
}
 

_metalRenderer justUpdate is below:

- (void)justUpdate
{
	[self updateState];
}