MPSCopyAllocator can't initialize

Question

In Objective-C I try to use this method to encode commandbuffer in-place.

-(BOOL)    encodeToCommandBuffer: (nonnull id <MTLCommandBuffer>)commandBuffer  
                  inPlaceTexture: (__nonnull id <MTLTexture> __strong * __nonnull) texture  
           fallbackCopyAllocator: (nullable MPSCopyAllocator) copyAllocator  
                MPS_SWIFT_NAME(encode(commandBuffer:inPlaceTexture:fallbackCopyAllocator:));  

and i would like to create a new MPSCopyAllocator .

i used the following Code from the Document.

MPSCopyAllocator myAllocator = ^id <MTLTexture> _Nonnull (MPSKernel * __nonnull filter, __nonnull id <MTLCommandBuffer> cmdBuf, __nonnull id <MTLTexture> sourceTexture)  
{  
    MTLPixelFormat format = sourceTexture.pixelFormat;  
    MTLTextureDescriptor *d = [MTLTextureDescriptor texture2DDescriptorWithPixelFormat: format width: sourceTexture.width height: sourceTexture.height mipmapped: NO];  

    id <MTLTexture> result = [cmdBuf.device newTextureWithDescriptor: d];  

    return result;  
    /  
};  

but i got a semantic issue in the issue navigator

Incompatible block pointer types initializing '__strong MPSCopyAllocator' (aka 'id<MTLTexture>  _Nonnull (^__strong)(MPSKernel * _Nonnull __strong, id<MTLCommandBuffer>  _Nonnull __strong, id<MTLTexture>  _Nonnull __strong)') with an expression of type 'id<MTLTexture>  _Nonnull (^)(MPSKernel * _Nonnull __strong, id<MTLCommandBuffer>  _Nonnull __strong, id<MTLTexture>  _Nonnull __strong)'  

the MPSCopyAllocator define

typedef id <MTLTexture> __nonnull NS_RETURNS_RETAINED (^MPSCopyAllocator)( MPSKernel * __nonnull filter,  
                                                                          id <MTLCommandBuffer> __nonnull commandBuffer,  
                                                                          id <MTLTexture> __nonnull sourceTexture);

what's the right way to create a MPSCopyAllocator?

Answer 1

Regrettably, you need to include NS_RETURNS_RETAINED in your block definition when assigning it to a variable:

MPSCopyAllocator allocator = ^id <MTLTexture> NS_RETURNS_RETAINED(MPSKernel *filter,
                                                                  id <MTLCommandBuffer> commandBuffer,
                                                                  id <MTLTexture> sourceTexture)
{
    // ...
};

For terseness, I've omitted the nullability annotations here, as they're optional.