Avalonia/src/Web/Avalonia.Web.Blazor/Interop/Storage/WriteableStream.cs

using System.Buffers;
using System.Text.Json.Serialization;

using Microsoft.JSInterop;

namespace Avalonia.Web.Blazor.Interop.Storage
{
    // Loose wrapper implementaion of a stream on top of FileAPI FileSystemWritableFileStream
    internal sealed class JSWriteableStream : Stream
    {
        private IJSInProcessObjectReference? _jSReference;

        // Unfortunatelly we can't read current length/position, so we need to keep it C#-side only.
        private long _length, _position;

        internal JSWriteableStream(IJSInProcessObjectReference jSReference, long initialLength)
        {
            _jSReference = jSReference;
            _length = initialLength;
        }

        private IJSInProcessObjectReference JSReference => _jSReference ?? throw new ObjectDisposedException(nameof(JSWriteableStream));

        public override bool CanRead => false;

        public override bool CanSeek => true;

        public override bool CanWrite => true;

        public override long Length => _length;

        public override long Position
        {
            get => _position;
            set => Seek(_position, SeekOrigin.Begin);
        }

        public override void Flush()
        {
            // no-op
        }

        public override int Read(byte[] buffer, int offset, int count)
        {
            throw new NotSupportedException();
        }

        public override long Seek(long offset, SeekOrigin origin)
        {
            var position = origin switch
            {
                SeekOrigin.Current => _position + offset,
                SeekOrigin.End => _length + offset,
                _ => offset
            };
            JSReference.InvokeVoid("seek", position);
            return position;
        }

        public override void SetLength(long value)
        {
            _length = value;

            // See https://docs.w3cub.com/dom/filesystemwritablefilestream/truncate
            // If the offset is smaller than the size, it remains unchanged. If the offset is larger than size, the offset is set to that size
            if (_position > _length)
            {
                _position = _length;
            }

            JSReference.InvokeVoid("truncate", value);
        }

        public override void Write(byte[] buffer, int offset, int count)
        {
            throw new NotSupportedException("Synchronous writes are not supported.");
        }

        public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
        {
            if (offset != 0 || count != buffer.Length)
            {
                // TODO, we need to pass prepared buffer to the JS
                // Can't use ArrayPool as it can return bigger array than requested
                // Can't use Span/Memory, as it's not supported by JS interop yet.
                // Alternatively we can pass original buffer and offset+count, so it can be trimmed on the JS side (but is it more efficient tho?)
                buffer = buffer.AsMemory(offset, count).ToArray();
            }
            return WriteAsyncInternal(buffer, cancellationToken).AsTask();
        }

        public override ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default)
        {
            return WriteAsyncInternal(buffer.ToArray(), cancellationToken);
        }

        private ValueTask WriteAsyncInternal(byte[] buffer, CancellationToken _)
        {
            _position += buffer.Length;

            return JSReference.InvokeVoidAsync("write", buffer);
        }

        protected override void Dispose(bool disposing)
        {
            if (_jSReference is { } jsReference)
            {
                _jSReference = null;
                jsReference.InvokeVoid("close");
                jsReference.Dispose();
            }
        }

        public override async ValueTask DisposeAsync()
        {
            if (_jSReference is { } jsReference)
            {
                _jSReference = null;
                await jsReference.InvokeVoidAsync("close");
                await jsReference.DisposeAsync();
            }
        }
    }
}