[linux] Improve example Makefile (#244)

- Clearly separates out and documents the variables that are most likely
  to need to change if copying the example app.
- Better differentiate between paths that are relative to the example
  and paths that are relative to the FDE repository, since apps using
  FDE would not be expected to be in the same directory the way the
  example is.
- Add a minimal "release" mode that enables that flag to disable Dart
  asserts, as on macOS and Windows.
- Remove the output directory override that puts the example app in the
  same output directory as the GN build (when building with GN).
  Combining them makes it less clear what's specific to the example,
  and what's part of building the library itself.

Author: stuartmorgan

example/README.md

@@ -36,7 +36,14 @@ Run `make -C example/linux_fde/`. The example binary and its resources will be
 in `example/build/linux_fde`, and can be run from there:
 
 ```
-$ ./example/build/linux_fde/flutter_embedder_example
+$ ./example/build/linux_fde/debug/flutter_embedder_example
+```
+
+To build a version with Dart asserts disabled (and thus no DEBUG banner),
+run `make BUILD=release` instead, then launch it with:
+
+```
+$ ./example/build/linux_fde/release/flutter_embedder_example
 ```
 
 ### macOS

example/linux_fde/Makefile

@@ -12,13 +12,30 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# Example-specific variables.
+# To modify this Makefile for a different application, these are the values
+# that are mostly likely to need to be changed.
+
+# The location of the flutter-desktop-embedding repository.
+FDE_ROOT=$(CURDIR)/../..
+# The C++ code for the embedder application.
+SOURCES=flutter_embedder_example.cc
+# Plugins to include (from the flutter-desktop-embedding plugins/ directory).
+PLUGIN_NAMES=color_panel file_chooser menubar
+
+
+# Default build type. For a release build, set BUILD=release.
+# Currently this only sets NDEBUG, which is used to control the flags passed
+# to the Flutter engine in the example shell, and not the complation settings
+# (e.g., optimization level) of the C++ code.
+BUILD:=debug
+
 # Dependency locations
-PROJECT_ROOT=$(CURDIR)/../..
-FLUTTER_EMBEDDER_LIB_DIR=$(PROJECT_ROOT)/library/linux
 FLUTTER_APP_DIR=$(CURDIR)/..
 FLUTTER_APP_BUILD_DIR=$(FLUTTER_APP_DIR)/build
-PLUGINS_DIR=$(PROJECT_ROOT)/plugins
-TOOLS_DIR=$(PROJECT_ROOT)/tools
+FLUTTER_EMBEDDER_LIB_DIR=$(FDE_ROOT)/library/linux
+PLUGINS_DIR=$(FDE_ROOT)/plugins
+TOOLS_DIR=$(FDE_ROOT)/tools
 FLUTTER_DIR=$(shell $(TOOLS_DIR)/flutter_location)
 
 # Libraries
@@ -28,7 +45,6 @@ FLUTTER_EMBEDDER_LIB=$(FLUTTER_EMBEDDER_LIB_DIR)/lib$(FLUTTER_EMBEDDER_LIB_NAME)
 FLUTTER_ENGINE_LIB_NAME=flutter_engine
 FLUTTER_ENGINE_LIB=$(FLUTTER_EMBEDDER_LIB_DIR)/lib$(FLUTTER_ENGINE_LIB_NAME).so
 
-PLUGIN_NAMES=color_panel file_chooser menubar
 PLUGIN_LIB_NAME_PREFIX=flutter_embedder_
 PLUGIN_LIBS=$(foreach plugin,$(PLUGIN_NAMES)\
 	,$(PLUGINS_DIR)/$(plugin)/linux/lib$(PLUGIN_LIB_NAME_PREFIX)$(plugin).so)
@@ -38,7 +54,7 @@ ALL_LIBS=$(FLUTTER_EMBEDDER_LIB) $(FLUTTER_ENGINE_LIB) $(PLUGIN_LIBS)
 # Headers
 PLUGIN_DIRS=$(patsubst %,$(PLUGINS_DIR)/%/linux,$(PLUGIN_NAMES))
 LIBRARY_DIRS=$(FLUTTER_EMBEDDER_LIB_DIR) $(PLUGIN_DIRS)
-INCLUDE_DIRS=$(patsubst %,%/include,$(LIBRARY_DIRS)) $(PROJECT_ROOT)/library/include
+INCLUDE_DIRS=$(patsubst %,%/include,$(LIBRARY_DIRS)) $(FDE_ROOT)/library/include
 
 # Tools
 BUILD_ASSETS_BIN=$(TOOLS_DIR)/build_flutter_assets
@@ -51,7 +67,7 @@ FLUTTER_ASSETS_NAME=flutter_assets
 FLUTTER_ASSETS_SOURCE=$(FLUTTER_APP_BUILD_DIR)/$(FLUTTER_ASSETS_NAME)
 
 # Output bundle structure and targets
-OUT_DIR=$(FLUTTER_APP_BUILD_DIR)/linux_fde
+OUT_DIR=$(FLUTTER_APP_BUILD_DIR)/linux_fde/$(BUILD)
 OUT_DATA_DIR=$(OUT_DIR)/data
 OUT_LIB_DIR=$(OUT_DIR)/lib
 
@@ -61,7 +77,7 @@ ALL_LIBS_OUT=$(foreach lib,$(ALL_LIBS),$(OUT_LIB_DIR)/$(notdir $(lib)))
 
 # Overrides for the optional GN build.
 ifdef USE_GN
-GN_OUT_DIR=$(PROJECT_ROOT)/out
+GN_OUT_DIR=$(FDE_ROOT)/out
 
 # The GN build places all libraries at the top level of the output directory.
 FLUTTER_EMBEDDER_LIB=$(GN_OUT_DIR)/lib$(FLUTTER_EMBEDDER_LIB_NAME).so
@@ -82,7 +98,9 @@ endif
 
 # Build settings
 CXX=g++ -std=c++14
-CXXFLAGS=-Wall -Werror $(shell pkg-config --cflags jsoncpp glfw3)
+CXXFLAGS.release=-DNDEBUG
+CXXFLAGS=-Wall -Werror $(shell pkg-config --cflags jsoncpp glfw3) \
+				 $(CXXFLAGS.$(BUILD))
 CPPFLAGS=$(patsubst %,-I%,$(INCLUDE_DIRS))
 ifdef USE_GN
 CPPFLAGS+=-DUSE_FLATTENED_INCLUDES
@@ -94,8 +112,6 @@ LDFLAGS=-L$(OUT_LIB_DIR) \
 	$(patsubst %,-l$(PLUGIN_LIB_NAME_PREFIX)%,$(PLUGIN_NAMES)) \
 	-Wl,-rpath=\$$ORIGIN/lib
 
-SOURCES=flutter_embedder_example.cc
-
 # Targets
 
 .PHONY: all