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Paulo Pinheiro de9aa0cdee Add basic Kotlin support (#5409 )

* [Kotlin] Add kotlin generate code for tests and add
kotlin test to TestAll.sh

* [Kotlin] Add Kotlin generator

This change adds support for generating Kotlin classes.

The approach of this generator is to keep it as close
as possible to the java generator for now, in order
to keep the change simple.

It uses the already implemented java runtime,
so we don't support cross-platform nor js Kotlin yet.

Kotlin tests are just a copy of the java tests.

* Add optional ident support for CodeWriter

Identation is important for some languages and
different projects have different ways of ident
code, e.g. tabs vs spaces, so we are adding optional
support on CodeWriter for identation.

* [Kotlin] Add Documentation for Kotlin

* [Kotlin] Modify generated code to use experimental Unsigned types.

2019-07-22 16:05:15 -07:00

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Use in Kotlin

Before you get started

Before diving into the FlatBuffers usage in Kotlin, it should be noted that the [Tutorial](@ref flatbuffers_guide_tutorial) page has a complete guide to general FlatBuffers usage in all of the supported languages (including K).

This page is designed to cover the nuances of FlatBuffers usage, specific to Kotlin.

You should also have read the [Building](@ref flatbuffers_guide_building) documentation to build flatc and should be familiar with [Using the schema compiler](@ref flatbuffers_guide_using_schema_compiler) and [Writing a schema](@ref flatbuffers_guide_writing_schema).

Kotlin and FlatBuffers Java code location

Code generated for Kotlin currently uses the flatbuffers java runtime library. That means that Kotlin generated code can only have Java virtual machine as target architecture (which includes Android). Kotlin Native and Kotlin.js are currently not supported.

The code for the FlatBuffers Java library can be found at flatbuffers/java/com/google/flatbuffers. You can browse the library on the [FlatBuffers GitHub page](https://github.com/google/flatbuffers/tree/master/ java/com/google/flatbuffers).

Testing FlatBuffers Kotlin

The test code for Java is located in [KotlinTest.java](https://github.com/google /flatbuffers/blob/master/tests/KotlinTest.kt).

To run the tests, use [KotlinTest.sh](https://github.com/google/ flatbuffers/blob/master/tests/KotlinTest.sh) shell script.

Note: These scripts require that Kotlin is installed.

Using the FlatBuffers Kotlin library

Note: See [Tutorial](@ref flatbuffers_guide_tutorial) for a more in-depth example of how to use FlatBuffers in Kotlin.

FlatBuffers supports reading and writing binary FlatBuffers in Kotlin.

To use FlatBuffers in your own code, first generate Java classes from your schema with the --kotlin option to flatc. Then you can include both FlatBuffers and the generated code to read or write a FlatBuffer.

For example, here is how you would read a FlatBuffer binary file in Kotlin: First, import the library and generated code. Then, you read a FlatBuffer binary file into a ByteArray. You then turn the ByteArray into a ByteBuffer, which you pass to the getRootAsMyRootType function:

    import MyGame.Example.*
    import com.google.flatbuffers.FlatBufferBuilder

    // This snippet ignores exceptions for brevity.
    val data = RandomAccessFile(File("monsterdata_test.mon"), "r").use {
        val temp = ByteArray(it.length().toInt())
        it.readFully(temp)
        temp
    }

    val bb = ByteBuffer.wrap(data)
    val monster = Monster.getRootAsMonster(bb)

Now you can access the data from the Monster monster:

    val hp = monster.hp
    val pos = monster.pos!!;

Differences between Kotlin and Java code

Kotlin generated code was designed to be as close as possible to the java counterpart, as for now, we only support kotlin on java virtual machine. So the differences in implementation and usage are basically the ones introduced by the Kotlin language itself. You can find more in-depth information here.

The most obvious ones are:

Fields as accessed as Kotlin properties
Static methods are accessed in companion object

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