[Impeller] Document impeller::Context. (flutter#43389)

Author: chinmaygarde

impeller/renderer/context.h

@@ -19,31 +19,122 @@ class CommandBuffer;
 class PipelineLibrary;
 class Allocator;
 
+//------------------------------------------------------------------------------
+/// @brief      To do anything rendering related with Impeller, you need a
+///             context.
+///
+///             Contexts are expensive to construct and typically you only need
+///             one in the process. The context represents a connection to a
+///             graphics or compute accelerator on the device.
+///
+///             If there are multiple context in a process, it would typically
+///             be for separation of concerns (say, use with multiple engines in
+///             Flutter), talking to multiple accelerators, or talking to the
+///             same accelerator using different client APIs (Metal, Vulkan,
+///             OpenGL ES, etc..).
+///
+///             Contexts are thread-safe. They may be created, used, and
+///             collected (though not from a thread used by an internal pool) on
+///             any thread. They may also be accessed simultaneously from
+///             multiple threads.
+///
+///             Contexts are abstract and a concrete instance must be created
+///             using one of the subclasses of `Context` in
+///             `//impeller/renderer/backend`.
 class Context {
  public:
+  //----------------------------------------------------------------------------
+  /// @brief      Destroys an Impeller context.
+  ///
   virtual ~Context();
 
+  // TODO(129920): Refactor and move to capabilities.
   virtual std::string DescribeGpuModel() const = 0;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Determines if a context is valid. If the caller ever receives
+  ///             an invalid context, they must discard it and construct a new
+  ///             context. There is no recovery mechanism to repair a bad
+  ///             context.
+  ///
+  ///             It is convention in Impeller to never return an invalid
+  ///             context from a call that returns an pointer to a context. The
+  ///             call implementation performs validity checks itself and return
+  ///             a null context instead of a pointer to an invalid context.
+  ///
+  ///             How a context goes invalid is backend specific. It could
+  ///             happen due to device loss, or any other unrecoverable error.
+  ///
+  /// @return     If the context is valid.
+  ///
   virtual bool IsValid() const = 0;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Get the capabilities of Impeller context. All optionally
+  ///             supported feature of the platform, client-rendering API, and
+  ///             device can be queried using the `Capabilities`.
+  ///
+  /// @return     The capabilities. Can never be `nullptr` for a valid context.
+  ///
   virtual const std::shared_ptr<const Capabilities>& GetCapabilities()
       const = 0;
 
+  // TODO(129920): Refactor and move to capabilities.
   virtual bool UpdateOffscreenLayerPixelFormat(PixelFormat format);
 
+  //----------------------------------------------------------------------------
+  /// @brief      Returns the allocator used to create textures and buffers on
+  ///             the device.
+  ///
+  /// @return     The resource allocator. Can never be `nullptr` for a valid
+  ///             context.
+  ///
   virtual std::shared_ptr<Allocator> GetResourceAllocator() const = 0;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Returns the library of shaders used to specify the
+  ///             programmable stages of a pipeline.
+  ///
+  /// @return     The shader library. Can never be `nullptr` for a valid
+  ///             context.
+  ///
   virtual std::shared_ptr<ShaderLibrary> GetShaderLibrary() const = 0;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Returns the library of combined image samplers used in
+  ///             shaders.
+  ///
+  /// @return     The sampler library. Can never be `nullptr` for a valid
+  ///             context.
+  ///
   virtual std::shared_ptr<SamplerLibrary> GetSamplerLibrary() const = 0;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Returns the library of pipelines used by render or compute
+  ///             commands.
+  ///
+  /// @return     The pipeline library. Can never be `nullptr` for a valid
+  ///             context.
+  ///
   virtual std::shared_ptr<PipelineLibrary> GetPipelineLibrary() const = 0;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Create a new command buffer. Command buffers can be used to
+  ///             encode graphics, blit, or compute commands to be submitted to
+  ///             the device.
+  ///
+  ///             A command buffer can only be used on a single thread.
+  ///             Multi-threaded render, blit, or compute passes must create a
+  ///             new command buffer on each thread.
+  ///
+  /// @return     A new command buffer.
+  ///
   virtual std::shared_ptr<CommandBuffer> CreateCommandBuffer() const = 0;
 
-  /// @brief Force all pending async work to finish by deleting any owned
-  /// concurrent message loops.
+  //----------------------------------------------------------------------------
+  /// @brief      Force all pending asynchronous work to finish. This is
+  ///             achieved by deleting all owned concurrent message loops.
+  ///
   virtual void Shutdown() = 0;
 
  protected: