jnigen should offer efficient methods to create and extract data from ByteBuffers

[brianquinlan]

For example:

// Get the data out of a byte buffer without having to index into it's array 1,000,000 times.
final b = java.ByteBuffer.allocateDirect(1000000);
UInt8List d = b.toUInt8List();

// Populate a ByteBuffer without having to index into an array 1,000,000 times
final d = UInt8List(1000000);
java.ByteBuffer b = d.toJByteBuffer();

The JNI functions that can help with this are:

• NewDirectByteBuffer
• GetDirectBufferAddress
• GetDirectBufferCapacity

[brianquinlan]

I'm creating bindings for Cronet and have two use cases:

I need to convert a complete java.nio.ByteBuffer into a List<int>. The code currently looks like:

      onReadCompleted: (urlRequest, responseInfo, byteBuffer) {
        byteBuffer.flip();

        final remaining = byteBuffer.remaining();
        final data = Uint8List(remaining);
        // TODO: Use a more efficient approach when
        // https://github.com/dart-lang/native/issues/563 is fixed.
        for (var i = 0; i < remaining; ++i) {
          data[i] = byteBuffer.get1(i);
        }
        responseStream!.add(data);
        byteBuffer.clear();
        cronetRequest.read(byteBuffer);
      },

I have a List<int> and need to convert it to a java.nio.ByteBuffer (or byte[]). The code currently looks like:

    if (body.isNotEmpty) {
      // TODO: Use a more efficient approach when
      // https://github.com/dart-lang/native/issues/563 is fixed.
      final bodyBytes = JArray(jbyte.type, body.length);
      for (var i = 0; i < body.length; ++i) {
        bodyBytes[i] = body[i];
      }
      builder.setUploadDataProvider(
          // This also accepts a `java.nio.ByteBuffer`
          jb.UploadDataProviders.create4(bodyBytes), _executor);

Eventually I'll want to implement by own UploadDataProvider, which will mean copying the data from a List<int> to an offset in a java.nio.ByteBuffer

[HosseinYousefi]

Fixed by dart-lang/jnigen#390.