Transit / Djinni

Djinni is a project originally created by Dropbox that generates bridging code between C++ and other programming languages.

After Dropbox dropped the support of Djinni, Snapchat took over the maintenance.

Since we needed some changes we created a fork based on our needs. This fork is not inteded to be maintained for all languages (we only need Djinni for our view models in C++, Kotlin and Objc).

You should probably use the official Djinni tool from Snap.

Original snapchat readme for the full Snapchat Djinni documentation.

Original dropbox readme for the full Djinni documentation.

Djinni

Errors

If you ever get this error

clang: error: unknown argument: '-fno-canonical-system-headers'

Please run bazel clean --expunge at the root where the WORKSPACE file is located

Modifications

• Added Kotlin support
• Added inheritance support (only in C++, Objc, Kotlin, Java)
• Cpp default values

Using new features

Kotlin support

In your script replace --java-out by --kotlin-out:

Inheritance

You can add inheritence to your records. Here is an example:

Vehicle = record {
   id : string;
} 

Bus = record extends Vehicle {
  headsign : string;
}

Inheritance limitations with collections

When using a vector in C++, a parent class should be a shared_ptr in order to be able to do a dynamic_cast.

The JNI and Objc generated code should then try to cast items to every possible children types and call the proper fromCpp methods.

This is not done yet.

If you want to use inherited records in lists here is what you can do:

Instead of declaring items : list<Vehicle> create a special type like this items : VehicleListItems

Then, in a YAML file, define something like this:

---
name: VehicleListItems
typedef: 'record deriving(eq)'
params: []
prefix: ''
cpp:
    typename: 'std::vector<std::shared_ptr<Vehicle>>'
    header: '"Vehicle.h"'
    byValue: false
objc:
    typename: 'NSArray<NSVehicle *>'
    header: '"NSVehicle.h"'
    boxed: 'NSArray<NSVehicle *>'
    pointer: true
    hash: '%s.hash'
objcpp:
    translator: 'MyDjinniTranslator:: VehicleListItems'
    header: '"MyDjinniTranslator.h"'
java:
    typename: 'ArrayList<Vehicle>'
    hash: '%s.hashCode()'
    boxed: 'ArrayList<Vehicle>'
    reference: false
    generic: true
jni:
    typename: jobject
    typeSignature: 'Ljava/util/ArrayList;'
    translator: 'djinniTranslator::VehicleListItems'
    header: '"DjinniTranslator.h"'
---

Then in you translators, implements the methods where you will iterate threw your vector of Vehicles and do dynamic cast to properly call the right djinni methods.

example for JNI:

    ::djinni::LocalRef<jobject> VehicleListItems::fromCpp(JNIEnv* jniEnv, const VehicleListItems::CppType& items) {
     		//create a jobject of ArrayList
     		//then iterate
       for (auto const& item : items) {
			            if (auto bus = dynamic_pointer_cast<Bus>(item)) {
			            jobject obj = djinni_generated::NativeBus::fromCpp(jniEnv, *bus).release();
			            // add obj to your array list
			}
		}
		
		  auto ref = ::djinni::LocalRef<jobject>{myJniArrayList};
        ::djinni::jniExceptionCheck(jniEnv);
        return ref;
    }

Properties default values

You can add a default value to properties in C++

Bike = record {
    text: string = "default string";
    count: i32 = 0;
}

will generate

struct Bike final {
    std::string text = "default string";
    int32_t count = 0;
};