How to convert JArray<jbyte> to Uint8List efficiently?

[yanshouwang]

When convert Uint8List to JArray, we can use the JArray<jbyte>.setRange extension method.
But I don't know how to do the opposite way.
Currently I can do this by use a for-loop, but I think this will spend a lot when convert a large byte array.

[dcharkes]

Hi @yanshouwang!

If the Struct containing the Array is created from a Pointer, you can use asTypedList on that pointer if you know the offset of the array in the struct.

In general we should support something like what you're asking for. I've filed dart-lang/sdk#55170 to track that feature.

Edit: wait, this is asking for JArray not Array!

@HosseinYousefi over to you!

[mkustermann]

Right now we have JArray:

class JArray<E> extends JObject { ... }
extension ByteArray on JArray<jbyte> {
  int operator [](int index) {
    return elementAt(index, JniCallType.byteType).byte;
  }

  void operator []=(int index, int value) {
    RangeError.checkValidIndex(index, this);
    _allocate<JByteMarker>(sizeOf<JByteMarker>(), (ptr) {
      ptr.value = value;
      Jni.env.SetByteArrayRegion(reference.pointer, index, 1, ptr);
    });
  }

  void setRange(int start, int end, Iterable<int> iterable,
      [int skipCount = 0]) {
      ...
      Jni.env.SetByteArrayRegion(reference.pointer, start, size, ptr);
  }
}

Maybe this just needs a

void getRange(int start, int end, Uint8List output) { ... }
// or 
Uint8List getRange(int start, int end) { ... }

that is then implemented via Jni.env.GetByteArrayRegion to get C pointers to memory and then copy it to a dart typed list.

[HosseinYousefi]

What @mkustermann said is possible. We can support the getRange operation as well.

I first wrote something wrong about using JByteBuffers, that's not currently possible, because we can only convert direct JByteBuffers into Uint8Lists.

---

If you don't have to use an array, you can use JByteBuffer directly, maybe that simplifies your code.

final buffer = JByteBuffer.fromList(list);
final list = buffer.asUint8List();

[yanshouwang]

What @mkustermann said is possible. We can support the getRange operation as well.

I first wrote something wrong about using JByteBuffers, that's not currently possible, because we can only convert direct JByteBuffers into Uint8Lists.

If you don't have to use an array, you can use JByteBuffer directly, maybe that simplifies your code.

final buffer = JByteBuffer.fromList(list);
final list = buffer.asUint8List();

The JByteArray is returned by a Kotlin method, so I can't use JByteBuffer instead, I think provide a getRange extension will be nice.