安卓 GPS : how to check if a certain point has been reached?

Question

我正在开发一个 Android 应用程序，我需要确保设备已达到用纬度和经度描述的某个点。我唯一能想到的就是拥有这样的东西:

    Location initLoc = theManager.getLastKnownLocation(LocationManager.GPS_PROVIDER);
    double lat = initLoc.getLatitude();
    double lon = initLoc.getLongitude();
    MyPoint firstPoint = getPoints().get(0);

    double dist = CalcHelper.getDistance1(lat, lat, firstPoint.getLat(), firstPoint.getLon());

    while(dist > 30){

        initLoc = theManager.getLastKnownLocation(LocationManager.GPS_PROVIDER);
        lat = initLoc.getLatitude();
        lon = initLoc.getLongitude();
        dist = CalcHelper.getDistance1(lat, lon, firstPoint.getLat(), firstPoint.getLon());

    }

但这会导致程序崩溃。如果您能引导我走向正确的方向，我将非常感激。

让我借此机会再问一个问题。正如我所说，我是 Android 和 GPS 的新手，并且考虑到几乎没有关于如何正确开发与 GPS 配合使用的应用程序的文档和信息，我基本上是在盲目工作。所以我的问题是:

这是 onLocationChanged 方法的样子:

            public void onLocationChanged(Location location) {

            double lat = location.getLatitude();
            double lon = location.getLongitude();
            MyPoint firstPoint = MainScreen.dataset.getPoints().get(0);

            double dist = CalcHelper.getDistance1(lat, firstPoint.getLat(),lon, firstPoint.getLon());

            if(dist < 10){

                Context context = getApplicationContext();
                CharSequence text = "Start reached. Starting moving on track";
                int duration = 6000;
                Toast toast = Toast.makeText(context, text, duration);
                toast.show();   

            }else{

                Context context = getApplicationContext();
                CharSequence text = "Distance until start: "+dist;
                int duration = 6000;
                Toast toast = Toast.makeText(context, text, duration);
                toast.show();

            }

    }

我想做的是让程序确定我何时到达我作为一组点提供的轨道的起点。因此，当我启动该程序时，我会收到对距离的合理估计。问题是，当我开始移动时，距离似乎没有更新，它会更新，但移动 50 米后，它说我只移动了 5 米。另一方面，当我启动应用程序并且距离起点不到 10m 时，它会正确检测到它。所以基本上，当我带着设备移动时， onLocationChanged 方法似乎没有给我现在所在的正确位置。你能告诉我我可能做错了什么吗？

Answer 1

我认为你想要做的是让你的 Activity 实现 LocationListener，即订阅 GPS 修复。例如。

public class MyActivity extends Activity implements LocationListener {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // request current location updates
        LocationManager locMan = (LocationManager) getSystemService(LOCATION_SERVICE);
        // set here the criteria for location provider accuracy
        Criteria locationProviderCriteria = new Criteria();
        locationProviderCriteria.setAccuracy(Criteria.ACCURACY_FINE);
        String locationProvider = locMan.getBestProvider(locationProviderCriteria, true);
        locMan.requestLocationUpdates(locationProvider, MIN_TIME_BETWEEN_UPDATES_IN_MILLIS, MIN_DISTANCE_BETWEEN_UPDATES_IN_METERS, this);
    }

    /*
     * This method will be called on each location update
     */
    @Override
    public void onLocationChanged(Location loc) {
        //put here your logic to see whether the user reached the destination
    }
}