Answers

• Pedro_Moniz Member ✭✭

  April 2018 Accepted Answer

  I was able to work around this problem but the solution is probably on the side of the creator of the lib.

  Instead of having the variable public, he should have a getter for it.

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• JamesHarris.7547 GBMember ✭

  June 2018

  I think we might be implementing the same library! I've hit exactly the same issue - could you let me know how you worked around it?

  (...and I agree a getter would be much better!)

  0
• tailsu Member ✭

  April 2019 Accepted Answer

  byte[] Java fields are wrapped as IList<byte> on the C# side. The conversion with ToArray()/CopyTo() is really, really slow, because the copying process is [insanely inefficient] github.com/xamarin/xamarin-android/blob/master/src/Mono.Android/Android.Runtime/JNIEnv.cs#L1291 . Basically, what the JNIEnv.CopyArray method does is create a byte[1] array for every element in the source array, copy each element into its respective byte[1] array using _GetByteArrayRegion, then copy the single element of each byte[1] array at the corresponding index in the target array. I kid you not.

  The efficient solution is to use _GetByteArrayRegion to copy the entire java array into the managed byte[] array once. We can't use the _GetByteArrayRegion method directly, because it's internal, but we can copy its code:

          public static unsafe byte[] ToByteArray(IList<byte> javaArray)
          {
              IList<byte> array = javaArray;
              var arrayHandle = ((IJavaObject)array).Handle;
              byte[] copy = new byte[array.Count];
              fixed (byte* p = copy)
                  JniEnvironment.Arrays.GetByteArrayRegion(new JniObjectReference(arrayHandle), 0, array.Count, (sbyte*)p);
              return copy;
          }
  

  This method is basically instantaneous. Note that the above method is necessarily marked as unsafe, so you'll need to switch the "Allow unsafe code" option in your project's properties. This version requires the Java.Interop library. Alternatively, you can copy the [other implementation] github.com/xamarin/xamarin-android/blob/master/src/Mono.Android/Android.Runtime/JNIEnv.cs#L1147 that doesn't depend on Java.Interop.

  Welcome to Xamarin World, where common sense makes no sense.

  5
• tailsu Member ✭

  April 2019

  I posted a detailed workaround with an explanation, but the entire post got mysteriously deleted.

  Here's just the working function:

          public static unsafe byte[] ToByteArray(IList<byte> javaArray)
          {
              IList<byte> array = javaArray;
              var arrayHandle = ((IJavaObject)array).Handle;
              byte[] copy = new byte[array.Count];
              fixed (byte* p = copy)
                  JniEnvironment.Arrays.GetByteArrayRegion(new JniObjectReference(arrayHandle), 0, array.Count, (sbyte*)p);
              return copy;
          }
  

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• Pedro_Moniz Member ✭✭

  June 2019

  I still see the detailed explanation and marked it.

  Thank you for your time and the detailed explanation.

  0