246 lines
11 KiB
Markdown
246 lines
11 KiB
Markdown
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## About
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**metal-cpp** is a low overhead and header only C++ interface for Metal that helps developers add Metal functionality to graphics applications that are written in C++ (such as game engines). **metal-cpp** removes the need to create a shim and allows developers to call Metal functions directly from anywhere in their existing C++ code.
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## Highlights
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- Drop in C++ alternative interface to the Metal Objective-C headers.
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- Direct mapping of all Metal Objective-C classes, constants and enums to C++ in the MTL C++ namespace.
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- No measurable overhead compared to calling Metal Objective-C headers, due to inlining of C++ function calls.
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- No usage of wrapper containers that require additional allocations.
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- Requires C++17 due to the usage of `constexpr` in `NS::Object`.
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- Identical header files and function/constant/enum availability for iOS, macOS and tvOS.
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- Backwards compatibility: All `bool MTL::Device::supports...()` functions check if their required selectors exist and automatically return `false` if not.
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- String (`ErrorDomain`) constants are weak linked and automatically set to `nullptr` if not available.
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## Memory Allocation Policy
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**metal-cpp** follows the object allocation policies of Cocoa and Cocoa Touch. Understanding those rules is especially important when using metal-cpp, as C++ objects are not eligible for automatic reference counting (ARC).
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**metal-cpp** objects are reference counted. To help convey and manage object lifecycles, the following conventions are observed:
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1. *You own any object returned by methods whose name begins with* `alloc` *,* `new` *,* `copy` *, or* `mutableCopy`. The method returns these objects with `retainCount` equals to `1`.
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2. *You can take ownership of an object by calling its* ```retain()``` *method*. A received object is normally guaranteed to remain valid within the method it was received in. You use `retain` in two situations: (1) In the implementation of an accessor method (a setter) or to take ownership of an object; and (2) To prevent an object from being deallocated as a side-effect of some other operation.
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3. *When you no longer need it, you must relinquish ownership of an object you own*. You relinquish ownership by calling its `release()` or `autorelease()` method.
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4. *You must not relinquish ownership of an object you do not own*.
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When an object's `retainCount` reaches `0`, the object is immediately deallocated. It is illegal to call methods on a deallocated object and it may lead to an application crash.
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### AutoreleasePools and Objects
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Several methods that create temporary objects in **metal-cpp** add them to an `AutoreleasePool` to help manage their lifetimes. In these situations, after **metal-cpp** creates the object, it adds it to an `AutoreleasePool`, which will release its objects when you release (or drain) it.
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By adding temporary objects to an AutoreleasePool, you do not need to explicitly call `release()` to deallocate them. Instead, you can rely on the `AutoreleasePool` to implicitly manage those lifetimes.
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If you create an object with a method that does not begin with `alloc`, `new`, `copy`, or `mutableCopy`, the creating method adds the object to an autorelease pool.
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The typical scope of an `AutoreleasePool` is one frame of rendering for the main thread of the program. When the thread returns control to the RunLoop (an object responsible for receiving input and events from the windowing system), the pool is *drained*, releasing its objects.
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You can create and manage additional `AutoreleasePool`s at smaller scopes to reduce your program's working set, and you are required to do so for any additional threads your program creates.
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If an object's lifecycle needs to be extended beyond the `AutoreleasePool`'s scope, you can claim ownership of it (avoiding its release beyond the pool's scope) by calling its `retain()` method before its pool is drained. In these cases, you will be responsible for making the appropriate `release()` call on the object after you no longer need it.
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You can find a more-detailed introduction to the memory management rules here: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmRules.html.
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For more details about the application's RunLoop, please find its documentation here: https://developer.apple.com/documentation/foundation/nsrunloop
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### Use and debug AutoreleasePools
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When you create an autoreleased object and there is no enclosing `AutoreleasePool`, the object is leaked.
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To prevent this, you normally create an `AutoreleasePool` in your program's `main` function, and in the entry function for every thread you create. You may also create additional `AutoreleasePool`s to avoid growing your program's high memory watermark when you create several autoreleased objects, such as when rendering.
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Use the Environment Variable `OBJC_DEBUG_MISSING_POOLS=YES` to print a runtime warning when an autoreleased object is leaked because no enclosing `AutoreleasePool` is available for its thread.
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You can also run `leaks --autoreleasePools` on a memgraph file or a process ID (macOS only) to view a listing of your program's `AutoreleasePool`s and all objects they contain.
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### nullptr
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Similar to Objective-C, it is legal to call any method, including `retain()` and `release()`, on `nullptr` "objects". While calling methods on `nullptr` still does incur in function call overhead, the effective result is equivalent of a NOP.
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Conversely, do not assume that because calling a method on a pointer did not result in a crash, that the pointed-to object is valid.
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## Adding metal-cpp to a Project
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Simply include `Metal/Metal.hpp`. To ensure that the selector and class symbols are linked, add to one of your cpp files:
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```cpp
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#define NS_PRIVATE_IMPLEMENTATION
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#define MTL_PRIVATE_IMPLEMENTATION
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#include "Metal/Metal.hpp"
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```
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If you want to use the QuartzCore wrapper, add:
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```cpp
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#define CA_PRIVATE_IMPLEMENTATION
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#include "QuartzCore/QuartzCore.hpp"
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```
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## Generating a Single Header File
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Purely optional: You can generate a single header file that contains all **metal-cpp** headers via:
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```shell
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./SingleHeader/MakeSingleHeader.py Foundation/Foundation.hpp QuartzCore/QuartzCore.hpp Metal/Metal.hpp
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```
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By default the generated header file will be written to `./SingleHeader/Metal.hpp`
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## Examples
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#### Creating the device
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###### Objective-C (with automatic reference counting)
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```objc
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id< MTLDevice > device = MTLCreateSystemDefaultDevice();
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// ...
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```
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###### Objective-C
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```objc
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id< MTLDevice > device = MTLCreateSystemDefaultDevice();
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// ...
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[device release];
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```
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###### C++
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```cpp
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MTL::Device* pDevice = MTL::CreateSystemDefaultDevice();
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// ...
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pDevice->release();
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```
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#### Metal function calls map directly to C++
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###### Objective-C (with automatic reference counting)
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```objc
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MTLSamplerDescriptor* samplerDescriptor = [[MTLSamplerDescriptor alloc] init];
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[samplerDescriptor setSAddressMode: MTLSamplerAddressModeRepeat];
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[samplerDescriptor setTAddressMode: MTLSamplerAddressModeRepeat];
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[samplerDescriptor setRAddressMode: MTLSamplerAddressModeRepeat];
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[samplerDescriptor setMagFilter: MTLSamplerMinMagFilterLinear];
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[samplerDescriptor setMinFilter: MTLSamplerMinMagFilterLinear];
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[samplerDescriptor setMipFilter: MTLSamplerMipFilterLinear];
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[samplerDescriptor setSupportArgumentBuffers: YES];
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id< MTLSamplerState > samplerState = [device newSamplerStateWithDescriptor:samplerDescriptor];
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```
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###### Objective-C
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```objc
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MTLSamplerDescriptor* samplerDescriptor = [[MTLSamplerDescriptor alloc] init];
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[samplerDescriptor setSAddressMode: MTLSamplerAddressModeRepeat];
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[samplerDescriptor setTAddressMode: MTLSamplerAddressModeRepeat];
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[samplerDescriptor setRAddressMode: MTLSamplerAddressModeRepeat];
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[samplerDescriptor setMagFilter: MTLSamplerMinMagFilterLinear];
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[samplerDescriptor setMinFilter: MTLSamplerMinMagFilterLinear];
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[samplerDescriptor setMipFilter: MTLSamplerMipFilterLinear];
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[samplerDescriptor setSupportArgumentBuffers: YES];
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id< MTLSamplerState > samplerState = [device newSamplerStateWithDescriptor:samplerDescriptor];
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[samplerDescriptor release];
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// ...
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[samplerState release];
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```
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###### C++
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```cpp
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MTL::SamplerDescriptor* pSamplerDescriptor = MTL::SamplerDescriptor::alloc()->init();
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pSamplerDescriptor->setSAddressMode( MTL::SamplerAddressModeRepeat );
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pSamplerDescriptor->setTAddressMode( MTL::SamplerAddressModeRepeat );
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pSamplerDescriptor->setRAddressMode( MTL::SamplerAddressModeRepeat );
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pSamplerDescriptor->setMagFilter( MTL::SamplerMinMagFilterLinear );
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pSamplerDescriptor->setMinFilter( MTL::SamplerMinMagFilterLinear );
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pSamplerDescriptor->setMipFilter( MTL::SamplerMipFilterLinear );
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pSamplerDescriptor->setSupportArgumentBuffers( true );
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MTL::SamplerState* pSamplerState = pDevice->newSamplerState( pSamplerDescriptor );
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pSamplerDescriptor->release();
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// ...
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pSamplerState->release();
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```
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#### A subset of bindings for Foundation classes is provided for seamless integration
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###### Objective-C (with automatic reference counting)
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```objc
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NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
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NSString* string = [NSString stringWithCString: "Hello World" encoding: NSASCIIStringEncoding];
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printf( "string = \"%s\"\n", [string cStringUsingEncoding: NSASCIIStringEncoding] );
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```
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###### Objective-C
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```objc
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NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];
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NSString* string = [NSString stringWithCString: "Hello World" encoding: NSASCIIStringEncoding];
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printf( "string = \"%s\"\n", [string cStringUsingEncoding: NSASCIIStringEncoding] );
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[pool release];
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```
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###### C++
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```cpp
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NS::AutoreleasePool* pPool = NS::AutoreleasePool::alloc()->init();
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NS::String* pString = NS::String::string( "Hello World", NS::ASCIIStringEncoding );
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printf( "pString = \"%s\"\n", pString->cString( NS::ASCIIStringEncoding ) );
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pPool->release();
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```
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#### Interoperability with CoreFoundation
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```cpp
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MTL::AccelerationStructureTriangleGeometryDescriptor* pGeoDescriptor = MTL::AccelerationStructureTriangleGeometryDescriptor::alloc()->init();
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CFTypeRef descriptors[] = { ( CFTypeRef )( pGeoDescriptor ) };
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NS::Array* pGeoDescriptors = ( NS::Array* )( CFArrayCreate( kCFAllocatorDefault, descriptors, SIZEOF_ARRAY( descriptors), &kCFTypeArrayCallBacks ) );
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// ...
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pGeoDescriptors->release();
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```
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#### Accessing a CAMetalDrawable
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```objc
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#import <QuartzCore/CAMetalLayer.h>
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#import <QuartzCore/QuartzCore.hpp>
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// ...
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CAMetalLayer* metalLayer = /* get your layer from your view */;
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id< CAMetalDrawable > metalDrawable = [metalLayer nextDrawable];
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CA::MetalDrawable* pMetalCppDrawable = ( __bridge CA::MetalDrawable* ) metalDrawable;
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// ...
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```
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