SK58 Printer

Flutter library for SK58 thermal printer via Bluetooth.

Features

🔍 Bluetooth device scanning with permission handling
🔌 Connect/disconnect to SK58 thermal printer
📝 Print text with styles (bold, underline, sizes) and alignment
📊 Print barcodes (Code128, EAN-13, UPC-A, Code39)
📱 Print QR codes
🖼️ Print images with Floyd-Steinberg dithering
🏷️ Generic label templates (Sk58Label, Sk58TwoColumnLabel)
🔧 Fluent builder pattern for complex prints
✅ Support for Android and Linux

Screenshots

App Interface

Main Screen	Barcode Demo	Labels Demo

Print Examples

Barcode Print	QR Code	Label

Receipt Demo (Builder Pattern)

Receipt

Installation

Add to your pubspec.yaml:

dependencies:
  sk58_printer: ^0.2.0

Then run:

flutter pub get

From Git (development version)

If you want to use the latest development version from GitHub:

dependencies:
  sk58_printer:
    git:
      url: https://github.com/mosulache/sk58_printer.git
      ref: main

Quick Start

import 'package:sk58_printer/sk58_printer.dart';

// Create scanner and scan for devices
final scanner = Sk58Scanner();
await scanner.startScan();

// Listen for discovered devices
scanner.deviceStream.listen((device) {
  print('Found: ${device.name}');
});

// Connect to a device
final printer = await Sk58Printer.connect(device);

// Print text
await printer.printText('Hello World!');
await printer.printText('Centered', align: Sk58Align.center);

// Print QR code
await printer.printQrCode('https://example.com');

// Feed paper and disconnect
await printer.feedLines(3);
await printer.disconnect();

Advanced Usage

Print Barcodes

// Code 128 (alphanumeric)
await printer.printBarcode('ABC-12345', type: BarcodeType.code128);

// EAN-13 (13 digits)
await printer.printBarcode('5901234123457', type: BarcodeType.ean13);

// UPC-A (12 digits)
await printer.printBarcode('012345678905', type: BarcodeType.upcA);

// Code 39
await printer.printBarcode('CODE39', type: BarcodeType.code39);

Print Images

final imageBytes = await File('logo.png').readAsBytes();

// Print with dithering (better for photos/grayscale)
await printer.printImage(imageBytes, dithering: true);

// Print with threshold (better for logos/line art)
await printer.printImage(imageBytes, dithering: false, threshold: 128);

// Resize to specific width
await printer.printImage(imageBytes, maxWidth: 200);

// For mobile printers or small labels, use bandMode for reliable printing
await printer.printImage(imageBytes, maxWidth: 280, bandMode: true);

Note: Use bandMode: true for mobile thermal printers or when printing on small labels (e.g., 40x15mm). This sends the image in smaller chunks using ESC * 33 (24-dot double density) which is more reliable for printers with limited buffers.

Use Templates

// Simple label
await printer.printTemplate(
  Sk58Label(
    title: 'TORX 4x50',
    subtitle: 'Cap T20 - Inox A2',
    qrData: 'SKU-12345',
  ),
);

// Two-column label
await printer.printTemplate(
  Sk58TwoColumnLabel(
    title: 'Product Info',
    rows: [
      ('Type', 'TORX'),
      ('Size', '4x50mm'),
      ('Head', 'T20'),
    ],
  ),
);

Builder Pattern (for complex prints)

await printer.build()
  .header('DEMO STORE')
  .text('123 Main Street', align: Sk58Align.center)
  .doubleLine()
  .text('RECEIPT', style: Sk58TextStyle.boldStyle, align: Sk58Align.center)
  .line()
  .row('Coffee', '\$3.50')
  .row('Sandwich', '\$8.00')
  .line()
  .row('TOTAL', '\$11.50')
  .feed(1)
  .qrCode('https://receipt.example.com/12345')
  .feed(3)
  .execute();

Text Styles

// Bold text
await printer.printText('Bold', style: Sk58TextStyle.boldStyle);

// Large text
await printer.printText('Large', style: const Sk58TextStyle(size: Sk58FontSize.large));

// Combined styles
await printer.printText(
  'Bold + Underline',
  style: const Sk58TextStyle(bold: true, underline: true),
);

// Available sizes: normal, wide, tall, large

Printer Specifications (SK58)

Print Area

Specification	Value
Paper width	58mm
Effective print width	48mm (384 dots)
Resolution	203 DPI (8 dots/mm)
Max characters per line	32 (normal font)

Label Printing - Dead Zone

⚠️ Important: When printing on label paper, the printer has a non-printable "dead zone" at the top of each label where the print head cannot reach.

Label Type	Dead Zone	Notes
Gap labels	~4mm (32 dots)	Empirically measured
Black mark labels	~3-5mm	Varies by printer

Example for 40x15mm label:

Total label height: 15mm (120 dots)
Dead zone: ~4mm (32 dots)
Actual printable height: ~11mm (88 dots)

Image sizing recommendation:

For 40x15mm label → Image should be max 320 x 88 pixels
For 50x25mm label → Image should be max 384 x 168 pixels
For 50x30mm label → Image should be max 384 x 208 pixels

The example app includes a realistic label preview that shows:

The dead zone (grey area marked "NO PRINT")
The actual printable area (white)
How your image fits on the label

Label Commands

// Print image on label (no extra feed after image)
await printer.printImage(imageBytes, bandMode: true, feedAfter: false);

// Eject to next label (GS FF command)
await printer.printAndPeel();

// Or use form feed
await printer.formFeed();

Platform Setup

Android

Add to android/app/src/main/AndroidManifest.xml:

<uses-permission android:name="android.permission.BLUETOOTH_SCAN" />
<uses-permission android:name="android.permission.BLUETOOTH_CONNECT" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

Linux

Ensure BlueZ is installed and your user has bluetooth permissions:

sudo apt install bluez
sudo usermod -a -G bluetooth $USER

!! linux support requires more testing / documentation

Example App

The example/ folder contains a complete demo app with:

Text printing with styles
Barcode printing (all types)
Image printing with dithering options
Simple Label templates
Builder pattern demos

Run it:

cd example
flutter run

API Reference

Sk58Printer

Method	Description
`connect(device)`	Connect to a BLE device
`disconnect()`	Disconnect from printer
`printText(text, {style, align})`	Print text
`printQrCode(data, {size})`	Print QR code
`printBarcode(data, {type, height, width})`	Print barcode
`printImage(bytes, {maxWidth, dithering, bandMode})`	Print image
`printTemplate(template)`	Print a template
`build()`	Get a print builder
`feedLines(n)`	Feed n lines
`printLine({char})`	Print horizontal line

Sk58Scanner

Method	Description
`startScan()`	Start scanning for devices
`stopScan()`	Stop scanning
`deviceStream`	Stream of discovered devices

Third-Party Licenses

This project uses the following open-source libraries:

universal_ble - BSD-3-Clause License (license)
permission_handler - MIT License (license)
image - MIT License (license)
flutter_lints - BSD-3-Clause License (license)

License

MIT License