Avalonia/src/Android/Avalonia.Android/Platform/Input/TextEditBuffer.cs

using System;
using Android.Text;
using Android.Views;
using Android.Views.InputMethods;
using Avalonia.Android.Platform.SkiaPlatform;
using Avalonia.Input.TextInput;

namespace Avalonia.Android.Platform.Input
{
    internal class TextEditBuffer
    {
        private readonly IAndroidInputMethod _textInputMethod;
        private readonly TopLevelImpl _topLevel;
        private TextSelection? _composition;

        public TextEditBuffer(IAndroidInputMethod textInputMethod, TopLevelImpl topLevel)
        {
            _textInputMethod = textInputMethod;
            _topLevel = topLevel;
        }

        public bool HasComposition => Composition is { } composition && composition.Start != composition.End;

        public TextSelection Selection
        {
            get
            {
                var selection = _textInputMethod.Client?.Selection ?? default;
                return new TextSelection(Math.Min(selection.Start, selection.End), Math.Max(selection.Start, selection.End));
            }

            set
            {
                if (_textInputMethod.Client is { } client)
                    client.Selection = value;
            }
        }

        public TextSelection? Composition
        {
            get => _composition; set
            {
                if (value is { } v)
                {
                    var text = Text;
                    var start = Math.Clamp(v.Start, 0, text.Length);
                    var end = Math.Clamp(v.End, 0, text.Length);
                    _composition = new TextSelection(Math.Min(start, end), Math.Max(start, end));
                }
                else
                    _composition = null;
            }
        }

        public string? SelectedText
        {
            get
            {
                if(_textInputMethod.Client is not { } client || Selection.Start < 0 || Selection.End >= client.SurroundingText.Length)
                {
                    return "";
                }

                var selection = Selection;

                return client.SurroundingText.Substring(selection.Start, selection.End - selection.Start);
            }
        }

        public string? ComposingText
        {
            get => !HasComposition ? null : Text?.Substring(Composition!.Value.Start, Composition!.Value.End - Composition!.Value.Start); set
            {
                if (HasComposition)
                {
                    var start = Composition!.Value.Start;
                    Replace(Composition!.Value.Start, Composition!.Value.End, value ?? "");
                    Composition = new TextSelection(start, start  + (value?.Length ?? 0));
                }
                else
                {
                    var selection = Selection;
                    Replace(selection.Start, selection.End, value ?? "");
                    Composition = new TextSelection(selection.Start, selection.Start + (value?.Length ?? 0));
                }
            }
        }

        public string Text => _textInputMethod.Client?.SurroundingText ?? "";

        public ExtractedText? ExtractedText => new ExtractedText
        {
            Flags = Text.Contains('\n') ? 0 : ExtractedTextFlags.SingleLine,
            PartialStartOffset = -1,
            PartialEndOffset = Text.Length,
            SelectionStart = Selection.Start,
            SelectionEnd = Selection.End,
            StartOffset = 0,
            Text = new Java.Lang.String(Text)
        };

        internal void Remove(int index, int length)
        {
            if (_textInputMethod.Client is { } client)
            {
                client.Selection = new TextSelection(index, index + length);
                if (length > 0)
                    _textInputMethod?.View.DispatchKeyEvent(new KeyEvent(KeyEventActions.Down, Keycode.ForwardDel));
            }
        }

        internal void Replace(int start, int end, string text)
        {
            if (_textInputMethod.Client is { } client)
            {
                var realStart = Math.Min(start, end);
                var realEnd = Math.Max(start, end);
                if (realEnd > realStart)
                {
                    client.Selection = new TextSelection(realStart, realEnd);
                    _textInputMethod?.View.DispatchKeyEvent(new KeyEvent(KeyEventActions.Down, Keycode.ForwardDel));
                }
                _topLevel.TextInput(text);
                var index = realStart + text.Length;
                client.Selection = new TextSelection(index, index);
                Composition = null;
            }
        }
    }
}