[Impeller] Create a libImpeller dylib and expose symbols. (flutter/engine#55526)

engine/src/flutter/BUILD.gn

@@ -210,6 +210,7 @@ group("unittests") {
         "//flutter/impeller:impeller_dart_unittests",
         "//flutter/impeller:impeller_unittests",
         "//flutter/impeller/toolkit/interop:example",
+        "//flutter/impeller/toolkit/interop:library",
       ]
     }
 

engine/src/flutter/impeller/toolkit/interop/BUILD.gn

@@ -4,12 +4,6 @@
 
 import("//flutter/impeller/tools/impeller.gni")
 
-impeller_component("interop_public") {
-  public = [ "impeller.h" ]
-
-  sources = [ "impeller_c.c" ]
-}
-
 impeller_component("interop") {
   sources = [
     "color_filter.cc",
@@ -27,6 +21,8 @@ impeller_component("interop") {
     "image_filter.cc",
     "image_filter.h",
     "impeller.cc",
+    "impeller.h",
+    "impeller_c.c",
     "mask_filter.cc",
     "mask_filter.h",
     "object.cc",
@@ -43,8 +39,6 @@ impeller_component("interop") {
     "texture.h",
   ]
 
-  public_deps = [ ":interop_public" ]
-
   deps = [
     "../../base",
     "../../display_list",
@@ -55,6 +49,14 @@ impeller_component("interop") {
   ]
 }
 
+impeller_component("library") {
+  target_type = "shared_library"
+
+  output_name = "impeller"
+
+  deps = [ ":interop" ]
+}
+
 impeller_component("example") {
   target_type = "executable"
 

engine/src/flutter/impeller/toolkit/interop/impeller.h

@@ -19,9 +19,17 @@
 #define IMPELLER_EXTERN_C_END
 #endif  // defined(__cplusplus)
 
-#ifndef IMPELLER_EXPORT
+#ifdef _WIN32
+#define IMPELLER_EXPORT_DECORATION __declspec(dllexport)
+#else
+#define IMPELLER_EXPORT_DECORATION __attribute__((visibility("default")))
+#endif
+
+#ifndef IMPELLER_NO_EXPORT
+#define IMPELLER_EXPORT IMPELLER_EXPORT_DECORATION
+#else  // IMPELLER_NO_EXPORT
 #define IMPELLER_EXPORT
-#endif  // IMPELLER_EXPORT
+#endif  // IMPELLER_NO_EXPORT
 
 #ifdef __clang__
 #define IMPELLER_NULLABLE _Nullable