Document GN rules and make targets that cannot be built on the platfo…

…rm be no-ops. (flutter#70)

impeller/compiler/BUILD.gn

@@ -30,7 +30,9 @@ impeller_component("compiler_lib") {
   ]
 }
 
-executable("impellerc") {
+impeller_component("impellerc") {
+  target_type = "executable"
+
   sources = [ "impellerc_main.cc" ]
 
   deps = [ ":compiler_lib" ]

impeller/display_list/display_list_dispatcher.cc

@@ -539,17 +539,7 @@ void DisplayListDispatcher::drawDisplayList(
 void DisplayListDispatcher::drawTextBlob(const sk_sp<SkTextBlob> blob,
                                          SkScalar x,
                                          SkScalar y) {
-  if (!blob) {
-    return;
-  }
-
-  auto bounds = blob->bounds();
-  bounds.fLeft += x;
-  bounds.fTop += y;
-
-  impeller::Paint paint;
-  paint.color = impeller::Color::Random().WithAlpha(0.2);
-  canvas_.DrawRect(ToRect(bounds), paint);
+  UNIMPLEMENTED;
 }
 
 // |flutter::Dispatcher|

impeller/renderer/backend/metal/render_pass_mtl.mm

@@ -397,7 +397,6 @@ static bool Bind(PassBindingsCache& pass,
   fml::closure pop_debug_marker = [encoder]() { [encoder popDebugGroup]; };
   for (const auto& command : commands_) {
     if (command.index_count == 0u) {
-      VALIDATION_LOG << "Zero index count in render pass command.";
       continue;
     }
 
@@ -505,6 +504,12 @@ static bool Bind(PassBindingsCache& pass,
     }
   }
 
+  if (command.index_count == 0u) {
+    // Essentially a no-op. Don't record the command but this is not necessary
+    // an error either.
+    return true;
+  }
+
   commands_.emplace_back(std::move(command));
   return true;
 }

impeller/renderer/renderer.h

@@ -4,8 +4,6 @@
 
 #pragma once
 
-#include <dispatch/dispatch.h>
-
 #include <functional>
 #include <memory>
 

impeller/tools/impeller.gni

@@ -8,38 +8,69 @@ import("//flutter/common/config.gni")
 declare_args() {
   # Whether playgrounds are enabled for unit tests.
   impeller_enable_playground = false
+
+  # Whether Impeller is supported on the platform.
+  impeller_supports_platform = is_mac || is_ios
 }
 
+# ------------------------------------------------------------------------------
+# @brief           Define an Impeller component. Components are different
+#                  Impeller subsystems part of the umbrella framework.
+#
+# @param[optional] target_type  The type of the component. This can be any of
+#                               the target types supported by GN. Defaults to
+#                               source_set. If Impeller is not supported on the
+#                               target platform, this target is a no-op.
+#
 template("impeller_component") {
-  source_set(target_name) {
-    forward_variables_from(invoker, "*")
-
-    if (!defined(invoker.public_configs)) {
-      public_configs = []
+  if (impeller_supports_platform) {
+    target_type = "source_set"
+    if (defined(invoker.target_type)) {
+      target_type = invoker.target_type
     }
+    target(target_type, target_name) {
+      forward_variables_from(invoker, "*")
 
-    if (!defined(invoker.cflags_objc)) {
-      cflags_objc = []
-    }
+      if (!defined(invoker.public_configs)) {
+        public_configs = []
+      }
 
-    if (!defined(invoker.cflags_objcc)) {
-      cflags_objcc = []
-    }
+      public_configs += [ "//flutter/impeller:impeller_public_config" ]
 
-    if (!defined(invoker.deps)) {
-      deps = []
-    }
+      if (!defined(invoker.cflags_objc)) {
+        cflags_objc = []
+      }
 
-    objc_warning_flags = [ "-Wno-unused-private-field" ]
+      if (is_ios || is_mac) {
+        cflags_objc += flutter_cflags_objc_arc
+      }
 
-    cflags_objc += flutter_cflags_objc_arc + objc_warning_flags
-    cflags_objcc += flutter_cflags_objcc_arc + objc_warning_flags
+      if (!defined(invoker.cflags_objcc)) {
+        cflags_objcc = []
+      }
 
-    public_configs += [ "//flutter/impeller:impeller_public_config" ]
+      if (is_ios || is_mac) {
+        cflags_objcc += flutter_cflags_objcc_arc
+      }
+    }
+  } else {
+    group(target_name) {
+      not_needed(invoker, "*")
+    }
   }
 }
 
+# ------------------------------------------------------------------------------
+# @brief           Build a Metal Library. The output is a single file. Use
+#                  get_target_outputs to get its location on build.
+#
+# @param[required] name     The name of the Metal library.
+#
+# @param[required] sources  The GLSL (4.60) sources to compiled into the Metal
+#                           library.
+#
 template("metal_library") {
+  assert(is_ios || is_mac)
   assert(defined(invoker.name), "Metal library name must be specified.")
   assert(defined(invoker.sources), "Metal source files must be specified.")
 
@@ -94,7 +125,17 @@ template("metal_library") {
   }
 }
 
-template("impeller_shaders") {
+# ------------------------------------------------------------------------------
+# @brief           Build and reflect shader information. Reflected shader
+#                  information will be added to a generated source set along
+#                  with the shader contents.
+#
+# @param[required] name     The name of the shader library.
+#
+# @param[required] sources  The GLSL (4.60) sources to compiled into the shader
+#                           library.
+#
+template("impeller_shaders_real") {
   assert(defined(invoker.shaders), "Impeller shaders must be specified.")
   assert(defined(invoker.name), "Name of the shader library must be specified.")
 
@@ -229,3 +270,27 @@ template("impeller_shaders") {
     ]
   }
 }
+
+# ------------------------------------------------------------------------------
+# @brief           Builds the shader library from shader sources, generates
+#                  reflected shader information as source set, and, generates a
+#                  translation unit added as a source set that allows embedding
+#                  shaders into the final binary. On platforms where Impeller is
+#                  not supported, this is a no-op.
+#
+# @note            For additional information about parameters, see
+#                  `impeller_shaders_real`
+#
+# @see             impeller_shaders_real
+#
+template("impeller_shaders") {
+  if (impeller_supports_platform) {
+    impeller_shaders_real(target_name) {
+      forward_variables_from(invoker, "*")
+    }
+  } else {
+    group(target_name) {
+      not_needed(invoker, "*")
+    }
+  }
+}