Allow building the embedder API for the target toolchains. (#6069)

BUILD.gn

@@ -10,6 +10,7 @@ group("flutter") {
   public_deps = [
     "$flutter_root/lib/snapshot:generate_snapshot_bin",
     "$flutter_root/lib/snapshot:kernel_platform_files",
+    "$flutter_root/shell/platform/embedder:flutter_engine",
     "$flutter_root/sky",
   ]
 
@@ -45,7 +46,6 @@ group("flutter") {
       "$flutter_root/runtime:runtime_unittests",
       "$flutter_root/shell/common:shell_unittests",
       "$flutter_root/shell/platform/embedder:embedder_unittests",
-      "$flutter_root/shell/platform/embedder:flutter_engine",
       "$flutter_root/synchronization:synchronization_unittests",
       "$flutter_root/third_party/txt:txt_unittests",
     ]

shell/platform/embedder/BUILD.gn

@@ -4,6 +4,7 @@
 
 import("$flutter_root/testing/testing.gni")
 import("$flutter_root/shell/gpu/gpu.gni")
+import("$flutter_root/shell/platform/embedder/embedder.gni")
 
 shell_gpu_configuration("embedder_gpu_configuration") {
   enable_software = false
@@ -205,14 +206,21 @@ if (is_mac) {
 }
 
 group("flutter_engine") {
-  # All platforms require the embedder dylib and headers.
-  deps = [
-    ":copy_headers",
-    ":flutter_engine_library",
-  ]
+  deps = []
+
+  build_embedder_api =
+      current_toolchain == host_toolchain || embedder_for_target
+
+  if (build_embedder_api) {
+    # All platforms require the embedder dylib and headers.
+    deps += [
+      ":copy_headers",
+      ":flutter_engine_library",
+    ]
 
-  # For the Mac, the dylib is packaged in a framework with the appropriate headers.
-  if (is_mac) {
-    deps += [ ":flutter_embedder_framework" ]
+    # For the Mac, the dylib is packaged in a framework with the appropriate headers.
+    if (is_mac) {
+      deps += [ ":flutter_embedder_framework" ]
+    }
   }
 }

shell/platform/embedder/embedder.gni

+# Copyright 2018 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+declare_args() {
+  # By default, the dynamic library target exposing the embedder API is only
+  # built for the host. The reasoning is that platforms that have target
+  # definitions would not need an embedder API because an embedder
+  # implementation is already provided for said target. This flag allows tbe
+  # builder to obtain a shared library exposing the embedder API for alternative
+  # embedder implementations.
+  embedder_for_target = false
+}

tools/gn

@@ -89,6 +89,8 @@ def to_gn_args(args):
     gn_args['is_debug'] = args.unoptimized
     gn_args['android_full_debug'] = args.target_os == 'android' and args.unoptimized
     gn_args['is_clang'] = not sys.platform.startswith(('cygwin', 'win'))
+    
+    gn_args['embedder_for_target'] = args.embedder_for_target
 
     enable_lto = args.lto
     if args.unoptimized:
@@ -234,9 +236,10 @@ def parse_args(args):
   parser.add_argument('--target-sysroot', type=str)
   parser.add_argument('--toolchain-prefix', type=str)
 
-  parser.add_argument('--use-glfw', action='store_true', default=False)
   parser.add_argument('--enable-vulkan', action='store_true', default=False)
 
+  parser.add_argument('--embedder-for-target', dest='embedder_for_target', action='store_true', default=False)
+
   return parser.parse_args(args)
 
 def main(argv):