Reading qrcode with Ionic4: How to retrieve the value read from an asynchronous service provider

Ionic Framework ionic-v3
1

Hi,
I’d like to read the text of a QR code from a page, using a service provider that manages the scan features. Really I only have to get a number from this QR Code, but I have a function that does this job (getIdFromMessage) and it is working.

I don’t know how to retrieve the value read in the service provider because the method used to scan the QR code is asynchronous.
I thought to use a callback that sets in the page the text read from the service provider, so I’ have a page with this code:

// callback to set the deviceId property of the page
  public setDeviceId(deviceId) {
    alert('@@@ setDeviceId:' + deviceId);
    this.deviceId = deviceId;
  }

// the action that calls the services provider
  actionScanQrCode() {
    console.log('actionScanQrCode');
    // Passing a callback to the qrcode reader
    this.qrcodeManager.readScooterIdFromBarcode(this.setDeviceId);
  }

And a service provider with this code:

// the function that reads the QRCode
    readScooterIdFromBarcode(callback) {
      this.barcodeScanner
        .scan()
        .then(barcodeData => {
         let barcodeText: string;
          barcodeText = barcodeData.text;
          let deviceId: number;
          deviceId = Number(this.getIdFromMessage(barcodeText));:
          alert("@@@ deviceId: ", deviceId);
          callback(deviceId);
....

When I read the barcode, the function setDeviceId prints the right number, but after I have the error
“TypeError: Cannot set property ‘device’ of undefined”.

So the callback isn’t working.
Is it the right way to retrieve the read value from an asynchronous function?

Thank you very much

cld

2

I’ve solved using a Promise not a callback.

  actionScanQrCode() {
    this.qrcodeManager.readScooterIdFromBarcode()
      .then(result => {
        this.deviceId = result;
      });
  }

where:

  readScooterIdFromBarcode() {
      return this.barcodeScanner
          .scan()
          .then(barcodeData => {
            let barcodeText: string;
            barcodeText = barcodeData.text;
            return Number(this.getIdFromMessage(barcodeText)) || 0;
          })
          .catch(err => {
            console.log('Error', err);
            return 1;
          });
    }

barcodeScanner returns a Promise so I use the “then” branch to change the value of the property “deviceId” of the caller.

cld