Wait for a function to end

Question

I have this code:

function getLocation() {
  //function executed if started by webview
  if (typeof Jinterface != 'undefined') {
    Jinterface.displayGPSRequest();
  }

  if (navigator.geolocation) {
    navigator.geolocation.getCurrentPosition(showPosition, showError, {
      enableHighAccuracy: true
    });
  } else {
    alert("Geolocation is not supported by this browser.");
  }
}

I need JavaScript to wait for the function Jinterface.displayGPSRequest() to end before continuing with the code. I tried async/await but being the function called from a Java file in Android Studio (where I’ve got a webview of my site) I can’t name it with async statement because Java doesn’t recognise it. Any help?

Java function:

 @JavascriptInterface
    public void displayGPSRequest() {
        Context context = getApplicationContext();
        GoogleApiClient googleApiClient = new GoogleApiClient.Builder(context)
                .addApi(LocationServices.API).build();
        googleApiClient.connect();
        final String TAG = "YOUR-TAG-NAME";
        final int REQUEST_CHECK_SETTINGS = 0x1;

        LocationRequest locationRequest = LocationRequest.create();
        locationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
        locationRequest.setInterval(10000);
        locationRequest.setFastestInterval(10000 / 2);

        LocationSettingsRequest.Builder builder = new LocationSettingsRequest.Builder().addLocationRequest(locationRequest);
        builder.setAlwaysShow(true);

        PendingResult<LocationSettingsResult> result = LocationServices.SettingsApi.checkLocationSettings(googleApiClient, builder.build());
        result.setResultCallback(new ResultCallback<LocationSettingsResult>() {
            @Override
            public void onResult(LocationSettingsResult result) {
                final Status status = result.getStatus();
                switch (status.getStatusCode()) {
                    case LocationSettingsStatusCodes.SUCCESS:
                        Log.i(TAG, "All location settings are satisfied.");
                        break;
                    case LocationSettingsStatusCodes.RESOLUTION_REQUIRED:
                        Log.i(TAG, "Location settings are not satisfied. Show the user a dialog to upgrade location settings ");

                        try {
                            // Show the dialog by calling startResolutionForResult(), and check the result
                            // in onActivityResult().
                            status.startResolutionForResult(MainActivity.this, REQUEST_CHECK_SETTINGS);
                        } catch (IntentSender.SendIntentException e) {
                            Log.i(TAG, "PendingIntent unable to execute request.");
                        }
                        break;
                    case LocationSettingsStatusCodes.SETTINGS_CHANGE_UNAVAILABLE:
                        Log.i(TAG, "Location settings are inadequate, and cannot be fixed here. Dialog not created.");
                        break;
                }
            }
        });
    }

Answer 1

displayGPSRequest will need to provide a means of knowing when it finishes. If it doesn't currently, you'll need to edit it to add one (or use polling, which isn't a good idea).

The usual ways are:

1. Via a promise.

2. Via a raw callback.

So if displayGPSRequest returns a promise (or you edit it to):

JInterface.displayGPSRequest()
    .then(function() {
        // The code that should run when it finishes
    })
    .catch(function() {
        // It failed
    });

If displayGPSRequest uses a raw callback (or you edit it to), then:

JInterface.displayGPSRequest(function() {
    // The code that should run when it finishes
    // It should also have some way of telling the callback it failed
});

If it doesn't provide a means of notifying you when it finishes and you can't add one, you'd have to use polling for some side-effect it produces, which is very much a last-resort situation:

var timer = setInterval(function() {
    if (/*...the side effect is present and so you know it's done..*/) {
        clearInterval(timer);
        timer = 0;
        // The code that should run when it finishes
    }
}, 100); // 100ms = ten times a second, adjust as appropriate
setTimeout(function() {
    if (timer) {
        // Give up
        clearInterval(timer);
        timer = 0;
    }
}, 5000); // 5000ms = five seconds