javascript - Wait until a condition is true?

Question

I'm using navigator.geolocation.watchPosition in JavaScript, and I want a way to deal with the possibility that the user might submit a form relying on location before watchPosition has found its location.

Ideally the user would see a 'Waiting for location' message periodically until the location was obtained, then the form would submit.

However, I'm not sure how to implement this in JavaScript given its lack of a wait function.

Current code:

var current_latlng = null;
function gpsSuccess(pos){
    //console.log('gpsSuccess');  
    if (pos.coords) { 
        lat = pos.coords.latitude;
        lng = pos.coords.longitude;
    }
    else {
        lat = pos.latitude;
        lng = pos.longitude;
    }
    current_latlng = new google.maps.LatLng(lat, lng);
}
watchId = navigator.geolocation.watchPosition(gpsSuccess,
                  gpsFail, {timeout:5000, maximumAge: 300000});
$('#route-form').submit(function(event) {
    // User submits form, we need their location...
    while(current_location==null) {
        toastMessage('Waiting for your location...');
        wait(500); // What should I use instead?
    }
    // Continue with location found...
});

Answer 1

Personally, I use a waitfor() function which encapsulates a setTimeout():

//**********************************************************************
// function waitfor - Wait until a condition is met
//        
// Needed parameters:
//    test: function that returns a value
//    expectedValue: the value of the test function we are waiting for
//    msec: delay between the calls to test
//    callback: function to execute when the condition is met
// Parameters for debugging:
//    count: used to count the loops
//    source: a string to specify an ID, a message, etc
//**********************************************************************
function waitfor(test, expectedValue, msec, count, source, callback) {
    // Check if condition met. If not, re-check later (msec).
    while (test() !== expectedValue) {
        count++;
        setTimeout(function() {
            waitfor(test, expectedValue, msec, count, source, callback);
        }, msec);
        return;
    }
    // Condition finally met. callback() can be executed.
    console.log(source + ': ' + test() + ', expected: ' + expectedValue + ', ' + count + ' loops.');
    callback();
}

I use my waitfor() function in the following way:

var _TIMEOUT = 50; // waitfor test rate [msec]
var bBusy = true;  // Busy flag (will be changed somewhere else in the code)
...
// Test a flag
function _isBusy() {
    return bBusy;
}
...

// Wait until idle (busy must be false)
waitfor(_isBusy, false, _TIMEOUT, 0, 'play->busy false', function() {
    alert('The show can resume !');
});