Windows Thumbnail Provider for Readest

This crate provides Windows Explorer thumbnail support for eBook files when Readest is set as the default application.

Features

• Automatic Cover Extraction: Extracts cover images from EPUB, MOBI, AZW, AZW3, FB2, CBZ, CBR files
• Readest Branding: Adds a small Readest icon overlay at the bottom-right corner
• Smart Caching: Caches generated thumbnails for faster subsequent loads
• File Association Aware: Only shows thumbnails when Readest is the default app for the file type
• COM Integration: Full Windows Shell extension implementation via IThumbnailProvider

Supported Formats

Building

Library Only

cargo build --release

COM DLL (for Windows Explorer integration)

cargo build --release --features com

CLI Tool

cargo build --release --features cli

Installation

The thumbnail provider DLL is automatically registered when Readest is installed via the NSIS installer.

Manual Registration (for development)

# Register the DLL
regsvr32 /s target\release\windows_thumbnail.dll

# Unregister the DLL
regsvr32 /s /u target\release\windows_thumbnail.dll

# Refresh Explorer to see changes
ie4uinit.exe -show

After registration, you may need to restart Windows Explorer or log out/in for changes to take effect.

Usage (Development / Manual testing)

For local development and testing, build the Windows DLL (or the library) from the Readest Tauri app folder and register it manually. The legacy CLI test harness used to live in the separate packages/tauri workspace, but the thumbnail handler implementation now lives inside Readest's Tauri app.

Build the DLL (for Windows explorer integration):

cd apps/readest-app/src-tauri
cargo build --release --manifest-path Cargo.toml --features com

The standalone CLI test harness is no longer distributed with the app. To test the thumbnail provider locally, build and register the DLL as shown above and use a small test harness that imports readestlib's thumbnail code or use Explorer after registering the handler.

Manual registration for development (register the generated DLL):

# Register the DLL
regsvr32 /s target\release\windows_thumbnail.dll

# Unregister the DLL
regsvr32 /s /u target\release\windows_thumbnail.dll

# Refresh Explorer to see changes
ie4uinit.exe -show

This generates a thumbnail with the Readest overlay at the specified size.

Architecture

┌─────────────────────────────────────────────────────────────────┐
│                    Windows Explorer                              │
├─────────────────────────────────────────────────────────────────┤
│                          │                                       │
│    IThumbnailProvider ───┼──► ThumbnailProvider                  │
│                          │         │                             │
│                          │         ▼                             │
│                          │    Check File Association             │
│                          │    (is Readest the default?)          │
│                          │         │                             │
│                          │         ▼ (if yes)                    │
│                          │    Extract Cover Image                │
│                          │         │                             │
│                          │         ▼                             │
│                          │    Add Readest Overlay                │
│                          │         │                             │
│                          │         ▼                             │
│                          │    Return HBITMAP                     │
│                          │                                       │
└─────────────────────────────────────────────────────────────────┘

COM Details

• CLSID: {A1B2C3D4-E5F6-7890-ABCD-EF1234567890}
• Shell Thumbnail Handler GUID: {e357fccd-a995-4576-b01f-234630154e96}
• Threading Model: Apartment

How It Works

1. When Windows Explorer needs a thumbnail, it queries the registered shell extension
2. The COM DLL implements IInitializeWithItem to receive the file path
3. It checks if Readest.exe is the default application for that file type using AssocQueryStringW
4. If Readest is the default, it extracts the cover and generates the thumbnail
5. If Readest is NOT the default, it returns S_FALSE to let Windows use other handlers

This ensures thumbnails only appear for files the user has associated with Readest.

License

MIT License - See LICENSE file for details.