'NFC' external object was designed to help users create applications that can interact with NFC tags. 

This feature requires a device with compatible hardware.

Compatibility: Android

Structure

This external object includes the following functions:

StartListening

Starts listening for NFC tag events in the background. Users can continue executing any logic, but if the application goes to the background or a new Panel is invoked, NFC events will not be received.

After the API starts listening, if an NFC Tag is put near the device, the event 'OnTagFound' will be fired.

NFC.StartListening()

StopListening

If the API was listening in the background, this method will stop listening for NFC events. This means that if an NFC Tag is put near the device, the API will not react.

Should be called when the screen no longer needs to read tags (e.g., when leaving the screen or switching to write mode).

NFC.StopListening()

WriteTag

Starts listening for NFC tag, but on 'write' mode. This means that if an NFC Tag is put near the device, the API will try to write down the provided information in the Tag.

This will fire the 'OnWriteTagEnded' event with the result of the 'write' action, either it is successful or not.

NFC.WriteTag(&textToWrite)

The app can only be in read mode or write mode at a time, not both simultaneously.

IsAvailable  → Boolean

Checks whether the device has NFC hardware and whether it is currently enabled.

&nfcAvailable = NFC.IsAvailable()
If &nfcAvailable = False
    Msg("This device does not have NFC or it is disabled")
EndIf

Always call this before starting any NFC operation.

Available Events

OnTagFound

Fires when an NFC tag is successfully read. The TagInformation parameter contains the text stored on the tag.

Event NFC.OnTagFound(&tagInformation)
    &lastTagRead = &tagInformation
    Refresh
EndEvent

Behavior:

• If the tag has NDEF text content, that text is returned.
• If the tag is empty or has no text records, TagInformation arrives as an empty string "".

OnWriteTagEnded

Fires when the write operation has completed, whether successfully or with an error.

Event NFC.OnWriteTagEnded(&result)
    If &result = ""
        Msg("Tag written successfully")
    Else
        Msg("Write error: " + &result)
    EndIf
EndEvent

You can also use &result.IsEmpty()

Result parameter values:

Content Format for Writing

The API always writes in NDEF plain text format (RTD_TEXT, UTF-8). No special formatting is needed from GeneXus — pass the text string directly.

// Correct — simple text
NFC.WriteTag("ITEM-001")

// Correct — with separators if the receiver expects them
NFC.WriteTag("ID:001|LOC:A3")

// Correct — numbers only
NFC.WriteTag("12345678")

// Correct — variable
 NFC.WriteTag(&Content)

Native URL writing (RTD_URI) is not supported. If a URL is passed as text it is stored and read back as plain text, not as a link automatically opened by the system.

Character Limits and Special Characters

Each character does not necessarily occupy 1 byte on the tag:

In addition, the library adds a fixed 10-byte NDEF overhead per write (protocol headers, language code).

Available space by tag type
 

Practical recommendation: Using standard ASCII characters (no accents or special letters) guarantees compatibility with all supported tags, including MIFARE Ultralight.

Tags That Require Prior NDEF Formatting

Some tags come from the factory without NDEF format (called "blank" or NdefFormatable tags). Typical examples: freshly purchased MIFARE Classic, some industrial tags.

The API handles this automatically. If the detected tag is NdefFormatable, the library formats it and writes the content in a single operation, with no intervention required from the user or the GeneXus developer. The result is delivered through OnWriteTagEnded exactly the same way.

There is no extra step for the user. Simply bringing the tag close to the device while the app is in write mode is enough.

Recommended Usage Flow

Read only

// When the screen loads
Event Start
    If NFC.IsAvailable()
        NFC.StartListening()
    EndIf
EndEvent

// When a tag is detected
Event NFC.OnTagFound(&tagInformation)
    &data = &tagInformation
    Refresh
EndEvent

// When leaving the screen
Event Back
    NFC.StopListening()
EndEvent

Write only

// User taps "Write Tag"
Event 'WriteTag'
    If NFC.IsAvailable()
        NFC.WriteTag(&textToWrite)
        Msg("Hold an NFC tag close to the device...")
    EndIf
EndEvent

// Write result
Event NFC.OnWriteTagEnded(&result)
    If &result = ""
        Msg("Written successfully")
    Else
        Msg("Error: " + &result)
    EndIf
    // Return to read mode if needed
    NFC.StartListening()
EndEvent

Supported Android versions

NFC must be enabled on the device under Settings → Connections → NFC.

Tags supported by NFC API

Templates

The following templates are distributed:

NFC Tag scanner

Shows an example of how to use the API to scan and show a message with the TAG information

NFC Tag writer

Shows an example of how to write content in the TAG