How to let the user pick a location on the map

Purpose. Open a map screen (Google) with a draggable pin and GPS tracking, so the user can confirm a position. The result is a LatLon delivered via callback.

APIs used

• LocationPicker(RunnableWith<LatLon> onCapture, double minPdop, boolean canDragPin, boolean highPrecision, boolean streetview) — full constructor (it’s the actual source signature).
  • onCapture — callback called with the confirmed LatLon (may be null).
  • minPdop — minimum acceptable PDOP; the confirm button stays hidden until the fix reaches that precision (on desktop it’s always enabled).
  • canDragPin — true to allow dragging the pin to a custom position.
  • highPrecision — true for high-precision GPS.
  • streetview — true to enable street view on the map.
• LatLon getAnswer() — confirmed position, or null if cancelled.
• Configurable statics: LocationPicker.defaultZoom, Instrucoes, GpsLocation, CustomLocation, Precision, Meters.

Divergence vs. the request description. LocationPicker does not use show(onAnswer) for the result: the coordinate arrives through the RunnableWith<LatLon> onCapture passed in the constructor (and it’s also available in getAnswer()). onCapture is only called when the user confirms with a non-null position. Internally the picker runs its own GPSThread (inner class) and already dispatches the updates via Component.onUI(...), so your callback runs on the paint thread.

Complete example (copy, paste, and run)

package com.exemplo;

import dev.crossfly.ui.*;                   // Window, Button, Label
import dev.crossfly.ui.layout.*;            // VerticalLayout
import dev.crossfly.ui.picker.*;            // LocationPicker
import dev.crossfly.map.*;                  // LatLon
import dev.crossfly.util.*;                 // RunnableWith

/** Runnable main screen. Point the launcher (classname) and run: gradlew :lwjgl3:run */
public class MainWindow extends Window
{
   @Override
   public void onShow() throws Exception
   {
      super.onShow();
      setLayout(new VerticalLayout());

      Label resultado = new Label("");
      add(LAY_AUTO, resultado);

      Button acao = new Button("Escolher no mapa");
      acao.onAction(c ->
      {
         RunnableWith<LatLon> aoConfirmar = coord ->
         {
            // Already arrives on the paint thread (the picker uses Component.onUI internally)
            resultado.setText("Lat " + coord.lat + " / Lon " + coord.lon);
         };

         LocationPicker picker = new LocationPicker(
            aoConfirmar,   // callback with the confirmed LatLon
            30.0,          // minPdop: enables confirm when accuracy <= 30m
            true,          // canDragPin: user can drag the pin
            true,          // highPrecision
            false);        // streetview
         picker.show();
      });
      add(LAY_AUTO, acao);
   }
}

The snippet below shows only the core. To run it, place it in the MainWindow skeleton above.

Minimal snippet

// verified at source: LocationPicker.java (constructor lines 129-141, onCapture/closeWith, getAnswer)
import dev.crossfly.ui.picker.*;           // LocationPicker
import dev.crossfly.map.*;                 // LatLon
import dev.crossfly.util.*;                // RunnableWith

RunnableWith<LatLon> aoConfirmar = coord -> {
   // Already arrives on the paint thread (the picker uses Component.onUI internally)
   double lat = coord.lat, lon = coord.lon;
   salvarLocal(lat, lon);
};

LocationPicker picker = new LocationPicker(
   aoConfirmar,   // callback with the confirmed LatLon
   30.0,          // minPdop: enables confirm when accuracy <= 30m
   true,          // canDragPin: user can drag the pin
   true,          // highPrecision
   false);        // streetview
picker.show();

// Alternative: read the result later via getAnswer() (null if cancelled)

Go deeper

• Location guide: Location & GPS · Widget: LocationPicker
• Sibling recipe: escolher-local-no-mapa.md