等待方法完成后再返回值

Question

I've found quite a few answers to similar questions, but have yet to find one that works for me. I have a method in a helper class that returns a String value of a users present latitude. here is the code.

public String getLatitude(){
    createLocationListener();
    defineCriteria();
    createLocationManager();
    getLocation();
    return String.valueOf(d_latitude);
}

My issue is that class variable d_latitude is updating correctly in the getLocation() or more specifically the onLocationChanged() method, but getLatitude() is returning a null d_latitude before getLocation() is complete. Like I said, the solutions I have found didn't work for me for whatever reason or they seemed totally too complex for my situation (200 line special classes and such).

private void getLocation(){
    //get single shot location update, null looper sets single shot
    locationManager.requestSingleUpdate(criteria, locationListener, null);
}


private void createLocationListener(){
    locationListener = new LocationListener() {
        @Override
        public void onLocationChanged(android.location.Location location){
            d_latitude = location.getLatitude();
            Toast.makeText(Location.this, "olc lat= " +d_latitude, Toast.LENGTH_SHORT).show();
            stopLocationListener();
        }

        @Override
        public void onStatusChanged(String provider, int status, Bundle extras){
        }
        @Override
        public void onProviderEnabled(String provider){
        }
        @Override
        public void onProviderDisabled(String provider){
        }
    };
}

Answer 1

You need to understand how async functions and listeners work together.

For example, if you changed your method

private void getLocation(LocationListener l) {
    locationManager.requestSingleUpdate(criteria, l, null);
}

Then you could pass in the listener, which calls back to the method, and "waits" for a response on a different thread.

You could define your own callback interface to get the latitude externally.

An example would be

public interface MyLocatonListener {
    void onLatChanged(Latitude lat);
    void onLongChanged(Longitude lng);
} 

And you pass an instance of that around, but, continuing on without that, since you already have a builtin listener...

Since you're not guaranteed an immediate response, you cannot return from this method, so make it void

public void getLatitude(){
    defineCriteria();
    createLocationManager();
    getLocation(new LocationListener() {
        @Override
        public void onLocationChanged(android.location.Location location){
            // latitude is "returned" here 
            d_latitude = location.getLatitude();
            Toast.makeText(Location.this, "olc lat= " +d_latitude, Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onStatusChanged(String provider, int status, Bundle extras){
        }
        @Override
        public void onProviderEnabled(String provider){
        } 
        @Override
        public void onProviderDisabled(String provider){
        }
    });
    // d_latitude == null here, most likely 
}

For another example, you can extract the LocationListener parameter up to the getLatitude method, then pass it down into getLocation

public void getLatitude(LocationListener l){
    defineCriteria();
    createLocationManager();
    getLocation(l);
}

By the way, I think you can define criteria and location manager before this method