Decentralize schema lib cmake build file (#151)

Summary:
Pull Request resolved: #151

Split out schema header generation logic into `schema/CMakeLists.txt`.

Reviewed By: huydhn

Differential Revision: D48712734

fbshipit-source-id: d2456bd756de8f2257b32ff5d2e0e68bf64f917b

Author: larryliu0820

CMakeLists.txt

@@ -127,38 +127,7 @@ add_subdirectory(third-party/gflags)
 #
 # program_schema: Generated .h files from schema/*.fbs inputs
 #
-
-# The include directory that will contain the generated schema headers.
-set(_program_schema__include_dir "${CMAKE_CURRENT_BINARY_DIR}/schema/include")
-
-# Paths to headers generated from the .fbs files.
-set(_program_schema__outputs)
-foreach(fbs_file ${_program_schema__srcs})
-  string(REGEX REPLACE "[.]fbs$" "_generated.h" generated "${fbs_file}")
-  list(APPEND _program_schema__outputs
-       "${_program_schema__include_dir}/executorch/${generated}")
-endforeach()
-
-# Generate the headers from the .fbs files.
-add_custom_command(
-  OUTPUT ${_program_schema__outputs}
-  COMMAND
-    flatc --cpp --cpp-std c++11 --gen-mutable --scoped-enums
-    # Add a subdirectory to the include dir so the files can be included as
-    # <executorch/schema/x_generated.h>
-    -o "${_program_schema__include_dir}/executorch/schema"
-    ${_program_schema__srcs}
-  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
-  DEPENDS flatc ${_program_schema__srcs}
-  COMMENT "Generating program_schema headers"
-  VERBATIM)
-
-add_library(program_schema INTERFACE ${_program_schema__outputs})
-set_target_properties(program_schema PROPERTIES LINKER_LANGUAGE CXX)
-target_include_directories(
-  program_schema
-  INTERFACE ${_program_schema__include_dir}
-            ${CMAKE_CURRENT_SOURCE_DIR}/third-party/flatbuffers/include)
+add_subdirectory(schema)
 
 #
 # executorch: Core runtime library

schema/CMakeLists.txt

@@ -0,0 +1,48 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+# Flatbuffer schema header lib. Please this file formatted by running:
+# ~~~
+# cmake-format --first-comment-is-literal=True CMakeLists.txt
+# ~~~
+
+# The include directory that will contain the generated schema headers.
+set(_program_schema__include_dir "${CMAKE_CURRENT_BINARY_DIR}/schema/include")
+
+# Source root directory for executorch.
+if(NOT EXECUTORCH_ROOT)
+  set(EXECUTORCH_ROOT ${CMAKE_CURRENT_SOURCE_DIR}/..)
+endif()
+
+# Paths to headers generated from the .fbs files.
+set(_program_schema__srcs program.fbs scalar_type.fbs)
+
+set(_program_schema__outputs)
+foreach(fbs_file ${_program_schema__srcs})
+  string(REGEX REPLACE "[.]fbs$" "_generated.h" generated "${fbs_file}")
+  list(APPEND _program_schema__outputs
+       "${_program_schema__include_dir}/executorch/${generated}")
+endforeach()
+
+# Generate the headers from the .fbs files.
+add_custom_command(
+  OUTPUT ${_program_schema__outputs}
+  COMMAND
+    flatc --cpp --cpp-std c++11 --gen-mutable --scoped-enums
+    # Add a subdirectory to the include dir so the files can be included as
+    # <executorch/schema/x_generated.h>
+    -o "${_program_schema__include_dir}/executorch/schema"
+    ${_program_schema__srcs}
+  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+  DEPENDS flatc ${_program_schema__srcs}
+  COMMENT "Generating program_schema headers"
+  VERBATIM)
+
+add_library(program_schema INTERFACE ${_program_schema__outputs})
+set_target_properties(program_schema PROPERTIES LINKER_LANGUAGE CXX)
+target_include_directories(
+  program_schema INTERFACE ${_program_schema__include_dir}
+                           ${EXECUTORCH_ROOT}/third-party/flatbuffers/include)