Metal is a graphics API developed by Apple for the iOS and macOS platforms. It provides an object-oriented access to the GPU in an asynchronous way. Validation is performed ahead of execution, accumulating rendering commands in a buffer, which allows for concatenations, simplifications and reordering of commands.

Shaders are written in a subset of C++ with an additional library providing shader-specific functions. They are compiled at the same time as the CPU code. Documentation can be found on the Apple Developer site and in WWDC videos.

Visualizing the rendering passes and the data on the GPU with the Xcode graphics debugger.