[Impeller] Document the EGL toolkit. (#52469)

No change in functionality. Just adds docs.

Author: chinmaygarde

impeller/toolkit/egl/config.h

@@ -52,10 +52,17 @@ struct ConfigDescriptor {
   SurfaceType surface_type = SurfaceType::kPBuffer;
 };
 
+class Display;
+
+//------------------------------------------------------------------------------
+/// @brief      An EGL config. These are returned by the display to indicate
+///             support for a specific config descriptor.
+///
+///             There is no ability to construct these manually except for
+///             testing.
+///
 class Config {
  public:
-  Config(ConfigDescriptor descriptor, EGLConfig config);
-
   ~Config();
 
   bool IsValid() const;
@@ -64,6 +71,9 @@ class Config {
 
   const EGLConfig& GetHandle() const;
 
+  // Do not use. Only for testing.
+  Config(ConfigDescriptor descriptor, EGLConfig config);
+
  private:
   const ConfigDescriptor desc_;
   EGLConfig config_ = nullptr;

impeller/toolkit/egl/context.h

@@ -16,38 +16,101 @@ namespace impeller {
 namespace egl {
 
 class Surface;
-
+class Display;
+
+//------------------------------------------------------------------------------
+/// @brief      An instance of an EGL context.
+///
+///             An EGL context can only be used on a single thread at a given
+///             time. A thread can only have a single context current at any
+///             given time.
+///
+///             Context cannot be created directly. Only a valid instance of an
+///             egl::Display can create a context.
+///
 class Context {
  public:
-  Context(EGLDisplay display, EGLContext context);
-
   ~Context();
 
+  //----------------------------------------------------------------------------
+  /// @brief      Determines if a valid context could be created. The context
+  ///             still needs to be made current on the thread for it to be
+  ///             useful.
+  ///
+  /// @return     True if valid, False otherwise.
+  ///
   bool IsValid() const;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Get the underlying handle to the EGL context.
+  ///
+  /// @return     The handle.
+  ///
   const EGLContext& GetHandle() const;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Make the context current on the calling thread. It is the
+  ///             caller responsibility to ensure that any context previously
+  ///             current on the thread must be cleared via `ClearCurrent`.
+  ///
+  /// @important  The config used to create the surface must match the config
+  ///             used to create this context instance.
+  ///
+  /// @param[in]  surface  The surface to use to make the context current.
+  ///
+  /// @return     If the context could be made current on the callers thread.
+  ///
   bool MakeCurrent(const Surface& surface) const;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Clear the thread association of this context.
+  ///
+  /// @return     If the thread association could be cleared.
+  ///
   bool ClearCurrent() const;
 
   enum class LifecycleEvent {
     kDidMakeCurrent,
     kWillClearCurrent,
   };
   using LifecycleListener = std::function<void(LifecycleEvent)>;
+  //----------------------------------------------------------------------------
+  /// @brief      Add a listener that gets invoked when the context is made and
+  ///             cleared current from the thread. Applications typically use
+  ///             this to manage workers that schedule OpenGL API calls that
+  ///             need to be careful about the context being current when
+  ///             called.
+  ///
+  /// @param[in]  listener  The listener
+  ///
+  /// @return     A unique ID for the listener that can used used in
+  ///             `RemoveLifecycleListener` to remove a previously added
+  ///             listener.
+  ///
   std::optional<UniqueID> AddLifecycleListener(
       const LifecycleListener& listener);
 
+  //----------------------------------------------------------------------------
+  /// @brief      Remove a previously added context listener.
+  ///
+  /// @param[in]  id    The identifier obtained via a previous call to
+  ///                   `AddLifecycleListener`.
+  ///
+  /// @return     True if the listener could be removed.
+  ///
   bool RemoveLifecycleListener(UniqueID id);
 
  private:
+  friend class Display;
+
   EGLDisplay display_ = EGL_NO_DISPLAY;
   EGLContext context_ = EGL_NO_CONTEXT;
   mutable RWMutex listeners_mutex_;
   std::map<UniqueID, LifecycleListener> listeners_ IPLR_GUARDED_BY(
       listeners_mutex_);
 
+  Context(EGLDisplay display, EGLContext context);
+
   void DispatchLifecyleEvent(LifecycleEvent event) const;
 
   Context(const Context&) = delete;

impeller/toolkit/egl/display.cc

@@ -66,7 +66,7 @@ std::unique_ptr<Context> Display::CreateContext(const Config& config,
     return nullptr;
   }
 
-  return std::make_unique<Context>(display_, context);
+  return std::unique_ptr<Context>(new Context(display_, context));
 }
 
 std::unique_ptr<Config> Display::ChooseConfig(ConfigDescriptor config) const {
@@ -182,7 +182,7 @@ std::unique_ptr<Surface> Display::CreateWindowSurface(
     IMPELLER_LOG_EGL_ERROR;
     return nullptr;
   }
-  return std::make_unique<Surface>(display_, surface);
+  return std::unique_ptr<Surface>(new Surface(display_, surface));
 }
 
 std::unique_ptr<Surface> Display::CreatePixelBufferSurface(const Config& config,
@@ -203,7 +203,7 @@ std::unique_ptr<Surface> Display::CreatePixelBufferSurface(const Config& config,
     IMPELLER_LOG_EGL_ERROR;
     return nullptr;
   }
-  return std::make_unique<Surface>(display_, surface);
+  return std::unique_ptr<Surface>(new Surface(display_, surface));
 }
 
 }  // namespace egl

impeller/toolkit/egl/display.h

@@ -18,23 +18,83 @@ namespace egl {
 class Context;
 class Surface;
 
+//------------------------------------------------------------------------------
+/// @brief      A connection to an EGL display. Only one connection per
+///             application instance is sufficient.
+///
+///             The display connection is used to first choose a config from
+///             among the available, create a context from that config, and then
+///             use that context with a surface on one (and only one) thread at
+///             a time.
+///
 class Display {
  public:
   Display();
 
   virtual ~Display();
 
+  //----------------------------------------------------------------------------
+  /// @return     True if the display connection is valid.
+  ///
   virtual bool IsValid() const;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Choose a config that most closely matches a given descriptor.
+  ///             If there are no matches, this method returns `nullptr`.
+  ///
+  /// @param[in]  config  The configuration
+  ///
+  /// @return     A config that matches a descriptor if one is available.
+  ///             `nullptr` otherwise.
+  ///
   virtual std::unique_ptr<Config> ChooseConfig(ConfigDescriptor config) const;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Create a context with a supported config. The supported config
+  ///             can be obtained via a successful call to `ChooseConfig`.
+  ///
+  /// @param[in]  config         The configuration.
+  /// @param[in]  share_context  The share context. Context within the same
+  ///                            share-group use the same handle table. The
+  ///                            contexts should still only be used exclusively
+  ///                            on each thread however.
+  ///
+  /// @return     A context if one can be created. `nullptr` otherwise.
+  ///
   virtual std::unique_ptr<Context> CreateContext(const Config& config,
                                                  const Context* share_context);
 
+  //----------------------------------------------------------------------------
+  /// @brief      Create a window surface. The window is an opaque pointer whose
+  ///             value value is platform specific. For instance, ANativeWindow
+  ///             on Android.
+  ///
+  /// @param[in]  config  A valid configuration. One can be obtained via
+  ///                     `ChooseConfig`.
+  /// @param[in]  window  An opaque pointer to a platform specific window
+  ///                     handle.
+  ///
+  /// @return     A valid window surface if one can be created. `nullptr`
+  ///             otherwise.
+  ///
   virtual std::unique_ptr<Surface> CreateWindowSurface(
       const Config& config,
       EGLNativeWindowType window);
 
+  //----------------------------------------------------------------------------
+  /// @brief      Create an offscreen pixelbuffer surface. These are of limited
+  ///             use except in the context where applications need to render to
+  ///             a texture in an offscreen context. In such cases, a 1x1 pixel
+  ///             buffer surface is created to obtain a surface that can be used
+  ///             to make the context current on the background thread.
+  ///
+  /// @param[in]  config  The configuration
+  /// @param[in]  width   The width
+  /// @param[in]  height  The height
+  ///
+  /// @return     A valid pixel buffer surface if one can be created. `nullptr`
+  ///             otherwise.
+  ///
   virtual std::unique_ptr<Surface>
   CreatePixelBufferSurface(const Config& config, size_t width, size_t height);
 

impeller/toolkit/egl/egl.h

@@ -14,6 +14,12 @@
 namespace impeller {
 namespace egl {
 
+//------------------------------------------------------------------------------
+/// @brief      Creates a proc address resolver that resolves function pointers
+///             to EGL and OpenGL (ES) procs.
+///
+/// @return     The resolver if one can be created.
+///
 std::function<void*(const char*)> CreateProcAddressResolver();
 
 #define IMPELLER_LOG_EGL_ERROR LogEGLError(__FILE__, __LINE__);

impeller/toolkit/egl/surface.h

@@ -11,22 +11,41 @@
 namespace impeller {
 namespace egl {
 
+//------------------------------------------------------------------------------
+/// @brief      An instance of an EGL surface. There is no ability to create
+///             surfaces directly. Instead, one must be created using a Display
+///             connection.
+///
 class Surface {
  public:
-  Surface(EGLDisplay display, EGLSurface surface);
-
   ~Surface();
 
+  //----------------------------------------------------------------------------
+  /// @return     True if this is a valid surface.
+  ///
   bool IsValid() const;
 
+  //----------------------------------------------------------------------------
+  /// @return     Get the handle to the underlying surface.
+  ///
   const EGLSurface& GetHandle() const;
 
+  //----------------------------------------------------------------------------
+  /// @brief      Present the surface. For an offscreen pixel buffer surface,
+  ///             this is a no-op.
+  ///
+  /// @return     True if the surface could be presented.
+  ///
   bool Present() const;
 
  private:
+  friend class Display;
+
   EGLDisplay display_ = EGL_NO_DISPLAY;
   EGLSurface surface_ = EGL_NO_SURFACE;
 
+  Surface(EGLDisplay display, EGLSurface surface);
+
   Surface(const Surface&) = delete;
 
   Surface& operator=(const Surface&) = delete;