PhoneGap条形码扫描仪结果未传递给变量

Question

I have a JavaScript function in a PhoneGap app that makes a successful call to the barcode scanner (using the Cordova plugin). I then form a JSON string and make a 'return' call in an attempt to pass the string back to the function call assignment. I successfully alert the JSON string in the scanning function but then get an undefined value for the variable that's been assigned the function result. I'm thinking that this might have to do with scope but declaring the variable outside of the function didn't make any difference.

var myscan = null;
var myclueJSON = null;

var myscan = getScan(); //call scanning function and assign result JSON to myscan variable
alert(myscan); //returns undefined

//call PhoneGap barcode scanner function
//and form JSON to return
function getScan()
{
    var scanner = cordova.require("cordova/plugin/BarcodeScanner");
    scanner.scan( function (result)
    { 
        var myresult = result.text;
        var obj= JSON.parse(myresult);
        //fetch event id from barcode
        var myeventid = obj.eventid;
        //fetch clue sequence from barcode
        var mycluesequence = obj.cluesequence;
        //form JSON string
        var myscanJSON = '{"eventid":"' + myeventid + '","cluesequence":"' + mycluesequence + '"}';
        //return JSON string
        return myscanJSON;
    }, function (error)
    { 
        console.log("Scanning failed: ", error); 
    });

Answer 1

It may be due to the fact you're trying to return myscanJSON within the callback function. You could try declaring an empty string outside the callback then append to it like so:

var myscan = null;
var myclueJSON = null;

var myscan = getScan(); //call scanning function and assign result JSON to myscan variable
alert(myscan); //returns undefined

//call PhoneGap barcode scanner function
//and form JSON to return
function getScan()
{
    var scanner = cordova.require("cordova/plugin/BarcodeScanner"),
    myscanJSON = '';
    scanner.scan( function (result)
    { 
        var myresult = result.text;
        var obj= JSON.parse(myresult);
        //fetch event id from barcode
        var myeventid = obj.eventid;
        //fetch clue sequence from barcode
        var mycluesequence = obj.cluesequence;
        //form JSON string
        myscanJSON += '{"eventid":"' + myeventid + '","cluesequence":"' + mycluesequence + '"}';

    }, function (error)
    { 
        console.log("Scanning failed: ", error); 
    });

    //return JSON string
    return myscanJSON;
}

Or you could refactor your callback function:

var myscan = null;
var myclueJSON = null;

var myscan = scanner.scan(getScan(result), handleError(error)); //call scanning function and assign result JSON to myscan variable
alert(myscan); //returns undefined

//call PhoneGap barcode scanner function
//and form JSON to return
function getScan(result)
{

  var myresult = result.text;
  var obj= JSON.parse(myresult);
  //fetch event id from barcode
  var myeventid = obj.eventid;
  //fetch clue sequence from barcode
  var mycluesequence = obj.cluesequence;
  //form JSON string
  var myscanJSON = '{"eventid":"' + myeventid + '","cluesequence":"' + mycluesequence + '"}';
  //return JSON string
  return myscanJSON;

}

function handleError(error){
  return error; 
}

Answer 2

I refactored the code. It did have a bit of a challenge that I didn't mention. The barcode scanner call needed to be initiated after a successful geolocation call. Since these want to complete asynchronously, I had to nest the getScan function inside the geolocation success function, passing in a simple lat/lng JSON object to the scan function which eventually forms a longer JSON string and makes a jQuery AJAX call to write to a database.

//get current position function
function fetchGeolocation()
{
    navigator.geolocation.getCurrentPosition(fetchCoords, handle_error);
}

//extract current coords
function fetchCoords(p)
{
    mylocjson = '{"lat":"' + p.coords.latitude + '","lng":"' + p.coords.longitude + '"}';

//fire up scanner, passing in lat/lng JSON string
    getScan(mylocjson);
}

//fire up PG scanner and form JSON result object
function getScan(incominglocjson)
{
    //parse lat/lng
    var clueobj = JSON.parse(incominglocjson);

    var scanner = cordova.require("cordova/plugin/BarcodeScanner");
    scanner.scan(function (result)
    { 
        var myresult = result.text;
        var obj= JSON.parse(myresult);
        //fetch event id from barcode
        var myclueJSON = '{"eventid":"' + obj.eventid + '","cluesequence":"' + obj.cluesequence + '","lat":"' + clueobj.lat + '","lng":"' + clueobj.lng + '"}';
    }//end scanner.scan()

    //make AJAX call to save data
    $.ajax(
    {
        url: "http://myurlhere/myfilename.php",
        type: 'post',
        data:{data:myclueJSON},
        success: function(returndata)
        {
             //process success returndata
        },
        fail: function(returndata){
             //process fail returndata
        }
    });//end AJAX call to save data
}//end getScan()

//handle current position fetch error
function handle_error(error)
{
    switch(error.code)
    {
        case error.PERMISSION_DENIED:
            alert("User denied the request for geolocation.  Please allow access to your GPS system when prompted.");
            break;
        case error.POSITION_UNAVAILABLE:
            alert("Location information is unavailable.  Check to see if geolocation is enabled on your device.");
            break;
        case error.TIMEOUT:
            alert("The request to get user location timed out.  Try moving to a slightly different location to better access the satellite network.");
            break;
        case error.UNKNOWN_ERROR:
            alert("An unknown error occurred.  Call tech support at (999) 999-9999.");
        break;
    }//end error code switch
}//end handle_error()