Android Map OnClick Tracker

Question

好的，所以我有一张简单的地图，在你移动时跟随你的位置。

我试图让它只有在点击一个切换按钮时才会这样做。

从长远来看，我希望制作一款能够跟踪旅程并将其存储在数据库中的应用程序。因此，当我按下开始时，我将把当前位置绘制到我当前的新位置，然后当我按下停止时，它将保存行程。

然而，当我添加一个onClick监听器时，我的应用程序只是在启动时崩溃。有什么想法吗？

private void setUpMapIfNeeded() {

    if (mMap == null) {
        mMap = ((SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map)).getMap();

    }

    if(mMap != null) {
        //Enable GPS
        mMap.setMyLocationEnabled(true);

        Location previousLocation = mMap.getMyLocation();
        final LatLng previousPos = new LatLng(previousLocation.getLatitude(), previousLocation.getLongitude());

        final ToggleButton button = (ToggleButton) findViewById(R.id.startStop);
        button.setOnClickListener(new View.OnClickListener() 
        {

            public void onClick(View v) {

                if(button.isChecked()) {

                    //Set the map to current location
                    mMap.setOnMyLocationChangeListener(new OnMyLocationChangeListener() {

                        public void onMyLocationChange(Location location) {
                            LatLng position = new LatLng(location.getLatitude(), location.getLongitude());


                            //Zoom parameter is set to 17
                            CameraUpdate update = CameraUpdateFactory.newLatLngZoom(position, 17);


                            //Use map.animateCamera(update) if you want moving effect
                            mMap.moveCamera(update);

                        }

                    });

                }
                else {

                }

            }
        });



    }

}
 }

这是logcat错误

  11-09 19:06:38.143: E/AndroidRuntime(12275): FATAL EXCEPTION: main
  11-09 19:06:38.143: E/AndroidRuntime(12275): java.lang.RuntimeException: 
  Unable to start activity                                                                                         ComponentInfo{com.example.hellomap/com.example.hellomap.MainActivity}: java.lang.NullPointerException
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2110)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2135)
   11-09 19:06:38.143: E/AndroidRuntime(12275):     at android.app.ActivityThread.access$700(ActivityThread.java:140)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1237) 
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at android.os.Handler.dispatchMessage(Handler.java:99)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at android.os.Looper.loop(Looper.java:137)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at android.app.ActivityThread.main(ActivityThread.java:4921)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at java.lang.reflect.Method.invokeNative(Native Method)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at java.lang.reflect.Method.invoke(Method.java:511)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1027)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at dalvik.system.NativeStart.main(Native Method)
  11-09 19:06:38.143: E/AndroidRuntime(12275): Caused by: java.lang.NullPointerException
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at com.example.hellomap.MainActivity.setUpMapIfNeeded(MainActivity.java:50)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at com.example.hellomap.MainActivity.onCreate(MainActivity.java:29)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at android.app.Activity.performCreate(Activity.java:5206)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1094)
  11-09 19:06:38.143: E/AndroidRuntime(12275):  at          android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2074)

Answer 1

取出两行代码49和50.它现在有效！ ：）

  //    Location previousLocation = mMap.getMyLocation();
        //final LatLng previousPos = new LatLng(previousLocation.getLatitude(), previousLocation.getLongitude());