Backends

Backends provide transport I/O for printer classes.

Backend Contract

Printer code relies on:

• write(Uint8Array | ArrayLike<number>): Promise<void>
• read(count: number): Promise<Uint8Array | null>

Optional helpers:

• open()
• close()
• getStatus() (custom shortcut; if present, printer uses it)

WebUSBBackend

WebUSBBackend.requestDevice({ filters }):

• Prompts user to select a USB device.
• Opens device and claims interface.
• Defaults to USB printer class filter (classCode: 7).

Options:

• outEndpoint default 0x02
• inEndpoint default 0x81
• interfaceNumber default 0

WebBluetoothBackend

WebBluetoothBackend.requestDevice(...) requires:

• serviceUuid
• writeCharacteristicUuid

Optional:

• notifyCharacteristicUuid
• readCharacteristicUuid
• filters

Behavior:

• If notify characteristic is used, incoming bytes are buffered from notifications.
• read(count) resolves from that buffer (or direct read characteristic if configured).

Environment Notes

• WebUSB and WebBluetooth require browser support and secure context.
• Device selection must run from a user gesture (click/tap).