src/docs/android_accessing_cpp_enums_in_java.md

Accessing C++ Enums In Java

[TOC]

Introduction

Accessing C++ enums in Java is implemented via a Python script which analyzes the C++ enum and spits out the corresponding Java class. The enum needs to be annotated in a particular way. By default, the generated class name will be the same as the name of the enum. If all the names of the enum values are prefixed with the MACRO_CASED_ name of the enum those prefixes will be stripped from the Java version.

Features

• Customize the package name of the generated class using the GENERATED_JAVA_ENUM_PACKAGE directive (required)
• Customize the class name using the GENERATED_JAVA_CLASS_NAME_OVERRIDE directive (optional)
• Strip enum entry prefixes to make the generated classes less verbose using the GENERATED_JAVA_PREFIX_TO_STRIP directive (optional)
• Supports @IntDef
• Copies comments that directly precede enum entries into the generated Java class

Usage

1. Add directives to your C++ enum

   // GENERATED_JAVA_ENUM_PACKAGE: org.chromium.chrome
   // GENERATED_JAVA_CLASS_NAME_OVERRIDE: FooBar
   // GENERATED_JAVA_PREFIX_TO_STRIP: BAR_
   enum SomeEnum {
     BAR_A,
     BAR_B,
     BAR_C = BAR_B,
   };
   

2. Add a new build target

   import("//build/config/android/rules.gni")
   
   java_cpp_enum("foo_generated_enum") {
     sources = [
       "base/android/native_foo_header.h",
     ]
   }
   

3. Add the new target to the desired android_library targets srcjar_deps:

   android_library("base_java") {
     srcjar_deps = [
       ":foo_generated_enum",
     ]
   }
   

4. The generated file org/chromium/chrome/FooBar.java would contain:

   package org.chromium.chrome;
   
   import android.support.annotation.IntDef;
   
   import java.lang.annotation.Retention;
   import java.lang.annotation.RetentionPolicy;
   
   public class FooBar {
     @IntDef({
         A, B, C
     })
     @Retention(RetentionPolicy.SOURCE)
     public @interface FooBarEnum {}
     public static final int A = 0;
     public static final int B = 1;
     public static final int C = 1;
   }
   

Formatting Notes

• Handling long package names:

  // GENERATED_JAVA_ENUM_PACKAGE: (
  //   org.chromium.chrome.this.package.is.too.long.to.fit.on.a.single.line)
  

• Enum entries

  • Single line enums should look like this:

      // GENERATED_JAVA_ENUM_PACKAGE: org.foo
      enum NotificationActionType { BUTTON, TEXT };
    

  • Multi-line enums should have one enum entry per line, like this:

      // GENERATED_JAVA_ENUM_PACKAGE: org.foo
      enum NotificationActionType {
        BUTTON,
        TEXT
      };
    

  • Multi-line enum entries are allowed but should be formatted like this:

      // GENERATED_JAVA_ENUM_PACKAGE: org.foo
      enum NotificationActionType {
        LongKeyNumberOne,
        LongKeyNumberTwo,
        ...
        LongKeyNumberThree =
            LongKeyNumberOne | LongKeyNumberTwo | ...
      };
    

• Preserving comments

  // GENERATED_JAVA_ENUM_PACKAGE: org.chromium
  enum CommentEnum {
    // This comment will be preserved.
    ONE,
    TWO, // This comment will NOT be preserved.
    THREE
  }
  

  ...
  public class CommentEnum {
    ...
    /**
     * This comment will be preserved.
     */
    public static final int ONE = 0;
    public static final int TWO = 1;
    public static final int THREE = 2;
  }
  

Code

• Generator code and Tests
• GN template