LehrerApp/LehrerApp.Desktop/Services/ExportService.cs

using Avalonia.Platform.Storage;
using System.Text;

namespace LehrerApp.Desktop.Services;

public sealed record ExportFormat(string Label, string Extension, string MimeType)
{
    public static ExportFormat Csv { get; } = new("CSV-Dateien", ".csv", "text/csv");
    public static ExportFormat Json { get; } = new("JSON-Dateien", ".json", "application/json");
    public static ExportFormat Pdf { get; } = new("PDF-Dateien", ".pdf", "application/pdf");
}

public sealed record ExportFile(
    string DialogTitle,
    string SuggestedFileName,
    ExportFormat Format,
    ReadOnlyMemory<byte> Content)
{
    public static ExportFile Csv(string dialogTitle, string suggestedFileName, string content) =>
        FromText(dialogTitle, suggestedFileName, ExportFormat.Csv, content, includeUtf8Bom: true);

    public static ExportFile Json(string dialogTitle, string suggestedFileName, string content) =>
        FromText(dialogTitle, suggestedFileName, ExportFormat.Json, content);

    public static ExportFile Pdf(string dialogTitle, string suggestedFileName, byte[] content) =>
        new(dialogTitle, EnsureExtension(SanitizeFileName(suggestedFileName), ExportFormat.Pdf.Extension),
            ExportFormat.Pdf, content);

    private static ExportFile FromText(string dialogTitle, string suggestedFileName,
        ExportFormat format, string content, bool includeUtf8Bom = false)
    {
        var encoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: includeUtf8Bom);
        var preamble = encoding.GetPreamble();
        var text = encoding.GetBytes(content);
        var bytes = new byte[preamble.Length + text.Length];
        preamble.CopyTo(bytes, 0);
        text.CopyTo(bytes, preamble.Length);
        return new ExportFile(dialogTitle,
            EnsureExtension(SanitizeFileName(suggestedFileName), format.Extension), format, bytes);
    }

    private static string EnsureExtension(string fileName, string extension) =>
        fileName.EndsWith(extension, StringComparison.OrdinalIgnoreCase) ? fileName : fileName + extension;

    private static string SanitizeFileName(string fileName)
    {
        var invalidCharacters = Path.GetInvalidFileNameChars();
        return string.Concat(fileName.Select(character =>
            invalidCharacters.Contains(character) ? '_' : character));
    }
}

/// <summary>Zentrale Dateiauswahl und Stream-Ausgabe für alle Exportformate.</summary>
public sealed class ExportService
{
    public async Task<bool> SaveAsync(IStorageProvider storageProvider, ExportFile export)
    {
        var file = await storageProvider.SaveFilePickerAsync(new FilePickerSaveOptions
        {
            Title = export.DialogTitle,
            SuggestedFileName = export.SuggestedFileName,
            DefaultExtension = export.Format.Extension.TrimStart('.'),
            FileTypeChoices =
            [
                new FilePickerFileType(export.Format.Label)
                {
                    Patterns = [$"*{export.Format.Extension}"],
                    MimeTypes = [export.Format.MimeType],
                },
            ],
        });

        if (file is null) return false;

        await using var stream = await file.OpenWriteAsync();
        if (stream.CanSeek) stream.SetLength(0);
        await stream.WriteAsync(export.Content);
        return true;
    }
}