codegen: set up Gradle plugin for more maintable codegen build steps

Summary:
Instead of sourcing-in a .gradle file to setup codegen tasks in Gradle, let's define a proper `com.facebook.react.codegen` Gradle plugin, so that any Gradle project (lib/app) can include it via:

```
plugins {
    id 'com.facebook.react.codegen'
}
```

The idea (not yet implemented in this commit) is to then allow those projects to add this section in the projects:

```
codegen {
    enableCodegen = ...
    jsRootDir = ...
}
```

This is more scalable and less hacky.

Important notes:
* The Gradle plugin should be prepared during the build, we're not going to publish it to Maven or other repo at this point.
* This setup is inspired by composite build setup explained here: https://ncorti.com/blog/gradle-plugins-and-composite-builds
* All android specific setup is added under `packages/react-native-codegen/android/` dir, but long term, we may want to move it up to `packages/react-native-codegen/` along side setup for other platforms.
* As part of this setup, the plugin will have an option (to be validated) to produce Java specs using https://github.com/square/javapoet
  * This is the same library already used for React Native Android annotation processors
  * This generator will not deal with parsing Flow types into schema, it will just takes in the schema and produce Java code
  * We're evaluating whether JavaPoet is a better choice for Java code generation long term, vs building it in JS via string concatenation: https://github.com/facebook/react-native/blob/master/packages/react-native-codegen/src/generators/modules/GenerateModuleJavaSpec.js
  * This commit produces a sample Java code, not the actual codegen output

Changelog: [Internal]

To try this out, run this Gradle task:

```
USE_CODEGEN=1 ./gradlew :ReactAndroid:generateJava
```

Reviewed By: JoshuaGross, mdvacca

Differential Revision: D22917315

fbshipit-source-id: 0b79dba939b73ff1305b4b4fd86ab897c7a48d53

RNTester/android/app/build.gradle

@@ -7,6 +7,7 @@
 
 plugins {
     id("com.android.application")
+    id("com.facebook.react.codegen")
 }
 
 /**

ReactAndroid/build.gradle

@@ -9,6 +9,7 @@ plugins {
     id("com.android.library")
     id("maven")
     id("de.undercouch.download")
+    id("com.facebook.react.codegen")
 }
 
 import java.nio.file.Paths

packages/react-native-codegen/BUCK

@@ -97,7 +97,10 @@ fb_xplat_cxx_binary(
 
 rn_android_library(
     name = "rn_codegen_library_java",
-    srcs = glob(["**/*.java"]),
+    srcs = glob(
+        ["**/*.java"],
+        exclude = ["android/gradlePlugin-build/**/*"],
+    ),
     visibility = [
         "PUBLIC",
     ],

packages/react-native-codegen/android/.gitattributes

@@ -0,0 +1,6 @@
+#
+# https://help.github.com/articles/dealing-with-line-endings/
+#
+# These are explicitly windows files and should use crlf
+*.bat           text eol=crlf
+

packages/react-native-codegen/android/.gitignore

@@ -0,0 +1,5 @@
+# Ignore Gradle project-specific cache directory
+.gradle
+
+# Ignore Gradle build output directory
+build

packages/react-native-codegen/android/build.gradle

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+buildscript {
+    repositories {
+        mavenLocal()
+        google()
+        jcenter()
+    }
+    dependencies {
+        classpath("com.android.tools.build:gradle:3.5.3")
+    }
+}
+
+allprojects {
+    repositories {
+        mavenLocal()
+        google()
+        jcenter()
+    }
+}

packages/react-native-codegen/android/generator/build.gradle

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+plugins {
+    id 'java'
+    id 'application'
+}
+
+dependencies {
+    implementation 'com.squareup:javapoet:1.13.0'
+}
+
+application {
+    mainClassName = 'com.facebook.react.codegen.JavaGeneratorMain'
+}

packages/react-native-codegen/android/gradle/wrapper/gradle-wrapper.properties

@@ -0,0 +1,5 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.5-bin.zip
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists

packages/react-native-codegen/android/gradlePlugin-build/build.gradle

@@ -0,0 +1,14 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+allprojects {
+    repositories {
+        mavenLocal()
+        google()
+        jcenter()
+    }
+}

packages/react-native-codegen/android/gradlePlugin-build/gradlePlugin/build.gradle

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+plugins {
+    id 'java-gradle-plugin'
+}
+
+gradlePlugin {
+    plugins {
+        greeting {
+            id = 'com.facebook.react.codegen'
+            implementationClass = 'com.facebook.react.codegen.plugin.CodegenPlugin'
+        }
+    }
+}
+
+dependencies {
+    implementation 'com.squareup:javapoet:1.13.0'
+}

packages/react-native-codegen/android/gradlePlugin-build/gradlePlugin/src/main/java/com/facebook/react/codegen/generator/JavaGenerator.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.codegen.generator;
+
+import com.squareup.javapoet.JavaFile;
+import com.squareup.javapoet.MethodSpec;
+import com.squareup.javapoet.TypeSpec;
+import java.io.IOException;
+import java.nio.file.Paths;
+import javax.lang.model.element.Modifier;
+
+// TODO: Implement proper generator - this is a sample usage of JavaPoet
+public final class JavaGenerator {
+
+  private String mSchemaFilePath;
+  private String mOutputDir;
+
+  public JavaGenerator(String schemaFilePath, String outputDir) {
+    mSchemaFilePath = schemaFilePath;
+    mOutputDir = outputDir;
+  }
+
+  public void build() throws IOException {
+    MethodSpec main =
+        MethodSpec.methodBuilder("main")
+            .addModifiers(Modifier.PUBLIC, Modifier.STATIC)
+            .returns(void.class)
+            .addParameter(String[].class, "args")
+            .addStatement("$T.out.println($S)", System.class, "Hello, JavaPoet!")
+            .build();
+
+    TypeSpec helloWorld =
+        TypeSpec.classBuilder("ReactNativeCodegen")
+            .addModifiers(Modifier.PUBLIC, Modifier.FINAL)
+            .addMethod(main)
+            .build();
+
+    JavaFile javaFile = JavaFile.builder("com.facebook.react.codegen", helloWorld).build();
+
+    System.out.println(javaFile.toString());
+
+    if (!mOutputDir.isEmpty()) {
+      javaFile.writeToPath(Paths.get(mOutputDir));
+    }
+  }
+}

packages/react-native-codegen/android/gradlePlugin-build/gradlePlugin/src/main/java/com/facebook/react/codegen/plugin/CodegenPlugin.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.react.codegen.plugin;
+
+import com.facebook.react.codegen.generator.JavaGenerator;
+import java.io.IOException;
+import org.gradle.api.Plugin;
+import org.gradle.api.Project;
+
+/** A Gradle plugin to enable code generation from JavaScript in Gradle environment. */
+public class CodegenPlugin implements Plugin<Project> {
+  public void apply(final Project project) {
+    // Register a task
+    project
+        .getTasks()
+        .register(
+            "generateJava",
+            task -> {
+              task.doLast(
+                  s -> {
+                    if (System.getenv("USE_CODEGEN").isEmpty()) {
+                      return;
+                    }
+                    try {
+                      JavaGenerator generator = new JavaGenerator("", "");
+                      generator.build();
+                    } catch (IOException e) {
+                    }
+                  });
+            });
+  }
+}

packages/react-native-codegen/android/gradlePlugin-build/settings.gradle

@@ -0,0 +1,10 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+rootProject.name = 'react-native-codegen-gradlePlugin-build'
+
+include(":gradlePlugin")

packages/react-native-codegen/android/gradlew

@@ -0,0 +1,185 @@
+#!/usr/bin/env sh
+
+#
+# Copyright 2015 the original author or authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+##############################################################################
+##
+##  Gradle start up script for UN*X
+##
+##############################################################################
+
+# Attempt to set APP_HOME
+# Resolve links: $0 may be a link
+PRG="$0"
+# Need this for relative symlinks.
+while [ -h "$PRG" ] ; do
+    ls=`ls -ld "$PRG"`
+    link=`expr "$ls" : '.*-> \(.*\)$'`
+    if expr "$link" : '/.*' > /dev/null; then
+        PRG="$link"
+    else
+        PRG=`dirname "$PRG"`"/$link"
+    fi
+done
+SAVED="`pwd`"
+cd "`dirname \"$PRG\"`/" >/dev/null
+APP_HOME="`pwd -P`"
+cd "$SAVED" >/dev/null
+
+APP_NAME="Gradle"
+APP_BASE_NAME=`basename "$0"`
+
+# Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+DEFAULT_JVM_OPTS='"-Xmx64m" "-Xms64m"'
+
+# Use the maximum available, or set MAX_FD != -1 to use that value.
+MAX_FD="maximum"
+
+warn () {
+    echo "$*"
+}
+
+die () {
+    echo
+    echo "$*"
+    echo
+    exit 1
+}
+
+# OS specific support (must be 'true' or 'false').
+cygwin=false
+msys=false
+darwin=false
+nonstop=false
+case "`uname`" in
+  CYGWIN* )
+    cygwin=true
+    ;;
+  Darwin* )
+    darwin=true
+    ;;
+  MINGW* )
+    msys=true
+    ;;
+  NONSTOP* )
+    nonstop=true
+    ;;
+esac
+
+CLASSPATH=$APP_HOME/gradle/wrapper/gradle-wrapper.jar
+
+
+# Determine the Java command to use to start the JVM.
+if [ -n "$JAVA_HOME" ] ; then
+    if [ -x "$JAVA_HOME/jre/sh/java" ] ; then
+        # IBM's JDK on AIX uses strange locations for the executables
+        JAVACMD="$JAVA_HOME/jre/sh/java"
+    else
+        JAVACMD="$JAVA_HOME/bin/java"
+    fi
+    if [ ! -x "$JAVACMD" ] ; then
+        die "ERROR: JAVA_HOME is set to an invalid directory: $JAVA_HOME
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+    fi
+else
+    JAVACMD="java"
+    which java >/dev/null 2>&1 || die "ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
+
+Please set the JAVA_HOME variable in your environment to match the
+location of your Java installation."
+fi
+
+# Increase the maximum file descriptors if we can.
+if [ "$cygwin" = "false" -a "$darwin" = "false" -a "$nonstop" = "false" ] ; then
+    MAX_FD_LIMIT=`ulimit -H -n`
+    if [ $? -eq 0 ] ; then
+        if [ "$MAX_FD" = "maximum" -o "$MAX_FD" = "max" ] ; then
+            MAX_FD="$MAX_FD_LIMIT"
+        fi
+        ulimit -n $MAX_FD
+        if [ $? -ne 0 ] ; then
+            warn "Could not set maximum file descriptor limit: $MAX_FD"
+        fi
+    else
+        warn "Could not query maximum file descriptor limit: $MAX_FD_LIMIT"
+    fi
+fi
+
+# For Darwin, add options to specify how the application appears in the dock
+if $darwin; then
+    GRADLE_OPTS="$GRADLE_OPTS \"-Xdock:name=$APP_NAME\" \"-Xdock:icon=$APP_HOME/media/gradle.icns\""
+fi
+
+# For Cygwin or MSYS, switch paths to Windows format before running java
+if [ "$cygwin" = "true" -o "$msys" = "true" ] ; then
+    APP_HOME=`cygpath --path --mixed "$APP_HOME"`
+    CLASSPATH=`cygpath --path --mixed "$CLASSPATH"`
+    
+    JAVACMD=`cygpath --unix "$JAVACMD"`
+
+    # We build the pattern for arguments to be converted via cygpath
+    ROOTDIRSRAW=`find -L / -maxdepth 1 -mindepth 1 -type d 2>/dev/null`
+    SEP=""
+    for dir in $ROOTDIRSRAW ; do
+        ROOTDIRS="$ROOTDIRS$SEP$dir"
+        SEP="|"
+    done
+    OURCYGPATTERN="(^($ROOTDIRS))"
+    # Add a user-defined pattern to the cygpath arguments
+    if [ "$GRADLE_CYGPATTERN" != "" ] ; then
+        OURCYGPATTERN="$OURCYGPATTERN|($GRADLE_CYGPATTERN)"
+    fi
+    # Now convert the arguments - kludge to limit ourselves to /bin/sh
+    i=0
+    for arg in "$@" ; do
+        CHECK=`echo "$arg"|egrep -c "$OURCYGPATTERN" -`
+        CHECK2=`echo "$arg"|egrep -c "^-"`                                 ### Determine if an option
+
+        if [ $CHECK -ne 0 ] && [ $CHECK2 -eq 0 ] ; then                    ### Added a condition
+            eval `echo args$i`=`cygpath --path --ignore --mixed "$arg"`
+        else
+            eval `echo args$i`="\"$arg\""
+        fi
+        i=`expr $i + 1`
+    done
+    case $i in
+        0) set -- ;;
+        1) set -- "$args0" ;;
+        2) set -- "$args0" "$args1" ;;
+        3) set -- "$args0" "$args1" "$args2" ;;
+        4) set -- "$args0" "$args1" "$args2" "$args3" ;;
+        5) set -- "$args0" "$args1" "$args2" "$args3" "$args4" ;;
+        6) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" ;;
+        7) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" ;;
+        8) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" "$args7" ;;
+        9) set -- "$args0" "$args1" "$args2" "$args3" "$args4" "$args5" "$args6" "$args7" "$args8" ;;
+    esac
+fi
+
+# Escape application args
+save () {
+    for i do printf %s\\n "$i" | sed "s/'/'\\\\''/g;1s/^/'/;\$s/\$/' \\\\/" ; done
+    echo " "
+}
+APP_ARGS=`save "$@"`
+
+# Collect all arguments for the java command, following the shell quoting and substitution rules
+eval set -- $DEFAULT_JVM_OPTS $JAVA_OPTS $GRADLE_OPTS "\"-Dorg.gradle.appname=$APP_BASE_NAME\"" -classpath "\"$CLASSPATH\"" org.gradle.wrapper.GradleWrapperMain "$APP_ARGS"
+
+exec "$JAVACMD" "$@"

packages/react-native-codegen/android/gradlew.bat

@@ -0,0 +1,104 @@
+@rem
+@rem Copyright 2015 the original author or authors.
+@rem
+@rem Licensed under the Apache License, Version 2.0 (the "License");
+@rem you may not use this file except in compliance with the License.
+@rem You may obtain a copy of the License at
+@rem
+@rem      https://www.apache.org/licenses/LICENSE-2.0
+@rem
+@rem Unless required by applicable law or agreed to in writing, software
+@rem distributed under the License is distributed on an "AS IS" BASIS,
+@rem WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+@rem See the License for the specific language governing permissions and
+@rem limitations under the License.
+@rem
+
+@if "%DEBUG%" == "" @echo off
+@rem ##########################################################################
+@rem
+@rem  Gradle startup script for Windows
+@rem
+@rem ##########################################################################
+
+@rem Set local scope for the variables with windows NT shell
+if "%OS%"=="Windows_NT" setlocal
+
+set DIRNAME=%~dp0
+if "%DIRNAME%" == "" set DIRNAME=.
+set APP_BASE_NAME=%~n0
+set APP_HOME=%DIRNAME%
+
+@rem Resolve any "." and ".." in APP_HOME to make it shorter.
+for %%i in ("%APP_HOME%") do set APP_HOME=%%~fi
+
+@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
+set DEFAULT_JVM_OPTS="-Xmx64m" "-Xms64m"
+
+@rem Find java.exe
+if defined JAVA_HOME goto findJavaFromJavaHome
+
+set JAVA_EXE=java.exe
+%JAVA_EXE% -version >NUL 2>&1
+if "%ERRORLEVEL%" == "0" goto init
+
+echo.
+echo ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
+echo.
+echo Please set the JAVA_HOME variable in your environment to match the
+echo location of your Java installation.
+
+goto fail
+
+:findJavaFromJavaHome
+set JAVA_HOME=%JAVA_HOME:"=%
+set JAVA_EXE=%JAVA_HOME%/bin/java.exe
+
+if exist "%JAVA_EXE%" goto init
+
+echo.
+echo ERROR: JAVA_HOME is set to an invalid directory: %JAVA_HOME%
+echo.
+echo Please set the JAVA_HOME variable in your environment to match the
+echo location of your Java installation.
+
+goto fail
+
+:init
+@rem Get command-line arguments, handling Windows variants
+
+if not "%OS%" == "Windows_NT" goto win9xME_args
+
+:win9xME_args
+@rem Slurp the command line arguments.
+set CMD_LINE_ARGS=
+set _SKIP=2
+
+:win9xME_args_slurp
+if "x%~1" == "x" goto execute
+
+set CMD_LINE_ARGS=%*
+
+:execute
+@rem Setup the command line
+
+set CLASSPATH=%APP_HOME%\gradle\wrapper\gradle-wrapper.jar
+
+
+@rem Execute Gradle
+"%JAVA_EXE%" %DEFAULT_JVM_OPTS% %JAVA_OPTS% %GRADLE_OPTS% "-Dorg.gradle.appname=%APP_BASE_NAME%" -classpath "%CLASSPATH%" org.gradle.wrapper.GradleWrapperMain %CMD_LINE_ARGS%
+
+:end
+@rem End local scope for the variables with windows NT shell
+if "%ERRORLEVEL%"=="0" goto mainEnd
+
+:fail
+rem Set variable GRADLE_EXIT_CONSOLE if you need the _script_ return code instead of
+rem the _cmd.exe /c_ return code!
+if  not "" == "%GRADLE_EXIT_CONSOLE%" exit 1
+exit /b 1
+
+:mainEnd
+if "%OS%"=="Windows_NT" endlocal
+
+:omega

packages/react-native-codegen/android/settings.gradle

@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+rootProject.name = 'react-native-codegen'
+
+include(":generator")
+
+includeBuild("gradlePlugin-build")

settings.gradle.kts

@@ -9,3 +9,6 @@ include(
     ":ReactAndroid",
     ":RNTester:android:app"
 )
+
+// Include this to enable codegen Gradle plugin.
+includeBuild("packages/react-native-codegen/android")