Metal offscreen rendering - metal

Question

I need to implement offscreen rendering in Metal with copying to a system memory. Without drawing on the screen.
This code works fine, but I'm not sure that it's a correct code:
// rendering to offscreen texture
auto commandQueue = [device newCommandQueue];
auto commandBuffer = [commandQueue commandBuffer];
//[commandBuffer enqueue]; // Do I need this command?
id<MTLRenderCommandEncoder> renderEncoder = [commandBuffer renderCommandEncoderWithDescriptor:mtlDescriptor];
// perform encoding
[renderEncoder endEncoding];
[commandBuffer commit];
auto commandBuffer = [commandQueue commandBuffer];
id<MTLBlitCommandEncoder> blitEncoder = [commandBuffer blitCommandEncoder];
// Copying offscreen texture to a new managed texture
[blitEncoder copyFromTexture:drawable.texture sourceSlice:0 sourceLevel:level sourceOrigin:region.origin sourceSize:region.size toTexture:_mtlTexture destinationSlice:0 destinationLevel:level destinationOrigin:{xOffset, yOffset, 0}];
[blitEncoder endEncoding];
[commandBuffer commit];
[commandBuffer WaitUntilCompleted]; // I add waiting to get a fully completed texture for copying.
// Final stage - we copy a texture to our buffer in system memory
getBytes_bytesPerRow_fromRegion_mipmapLevel()
Do I need to call commandBuffer.enqueue ?
Also if I remove commandBuffer.WaitUntilCompleted I can get only a half of a frame. It seems that getBytes_bytesPerRow_fromRegion_mipmapLevel doesn't check that rendering is finished.
Or should I create offscreen texture "managed" instead of "private" and then copy it directly to my buffer:
// creating offscreen texture "managed"
// rendering to offscreen texture
auto commandQueue = [device newCommandQueue];
auto commandBuffer = [commandQueue commandBuffer];
//[commandBuffer enqueue]; // Do I need this command?
id<MTLRenderCommandEncoder> renderEncoder = [commandBuffer renderCommandEncoderWithDescriptor:mtlDescriptor];
// perform encoding
[renderEncoder endEncoding];
[commandBuffer commit];
[commandBuffer waitUntilCompleted];
// Copying "managed" offscreen texture to my buffer
getBytes_bytesPerRow_fromRegion_mipmapLevel()

Answer 1

1) You don't need to call enqueue on the command buffer. This is used in situations where you want to explicitly specify the order of command buffers in a multithreaded scenario, which is irrelevant here. Your command buffer will be implicitly enqueued upon being committed.
2) You do indeed need to wait until the command buffer has completed before copying its contents to system memory. Normally, it's essential for the GPU and CPU to be able to run asynchronously without waiting on one another, but in your use case, you want the opposite, and waiting is how you keep them in lockstep.
3) If you don't need a copy of the rendered image as a texture for further work on the GPU, you should be able to omit the full-on blit entirely, provided the texture you're rendering to is in the managed storage mode. You can call synchronizeResource: on the blit encoder instead, which will make the results of the rendering work visible to the copy of the texture in system memory, from which you can then copy directly.
If for some reason the render target can't be in managed storage (I noticed you're using drawables—are these actually MTLDrawables provided by a view or layer, and if so, why?), you will in fact need to blit to either a managed texture or a shared/managed buffer in order to copy the bits on the CPU side.