Ndtyjky

I'm trying to call Android's class GeomagneticField method getDeclination() from my Delphi 10.2.3 app.

First I imported the class GeomagneticField using Java2OP:

unit Androidapi.JNI.Interfaces.JGeomagneticField;

interface

uses

  Androidapi.JNIBridge,

  Androidapi.JNI.JavaTypes;

type

// ===== Forward declarations =====

  JGeomagneticField = interface;//android.hardware.GeomagneticField

// ===== Interface declarations =====

  JGeomagneticFieldClass = interface(JObjectClass)

    ['{77F2155B-1F9A-40E0-89FA-FE3422336577}']

    {class} function init(gdLatitudeDeg: Single; gdLongitudeDeg: Single; altitudeMeters: Single; timeMillis: Int64): JGeomagneticField; cdecl;

    {class} function getHorizontalStrength: Single; cdecl;//Deprecated

    {class} function getInclination: Single; cdecl;//Deprecated

    {class} function getX: Single; cdecl;//Deprecated

  end;



  [JavaSignature('android/hardware/GeomagneticField')]

  JGeomagneticField = interface(JObject)

    ['{47CF41EC-AAAB-4EE2-867A-884A3EF00407}']

    function getDeclination: Single; cdecl;//Deprecated

    function getFieldStrength: Single; cdecl;//Deprecated

    function getY: Single; cdecl;

    function getZ: Single; cdecl;

  end;

  TJGeomagneticField = class(TJavaGenericImport<JGeomagneticFieldClass, JGeomagneticField>) end;



implementation



initialization

end.

Then I called the method this way:

procedure TForm.Button1Click(Sender: TObject);

var GeoField: JGeomagneticField; tm:int64; t0,t:TDatetime; 

begin

  t0 := EncodeDatetime(1970,1,1,0,0,0,0);

  t  := now;

  tm := Trunc( (t-t0)*24*3600*1000);   // <-- ms since 1/1/1970



  GeoField := TJGeomagneticField.JavaClass.init(-23,-46,750,tm );

  Label1.Text := FloatToStr( GeoField.getDeclination );

end;

When I run this and click, Label1 shows -174.3635711, which is wrong

On Android Studio I built a similar app in Java:

@Override

protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);



    Button btn = (Button) findViewById(R.id.btnShowDecl);

    btn.setOnClickListener(new View.OnClickListener() {

        @Override

        public void onClick(View view) {

            GeomagneticField geoField = new GeomagneticField(

                    (float) -23,

                    (float) -46,

                    (float) 750,

                    System.currentTimeMillis()

            );

            float f = geoField.getDeclination();

            String s = Float.toString(f);



            Toast.makeText(getApplicationContext(),s,Toast.LENGTH_LONG)

                    .show();

        }

    });

}

When I run this and click the button, I got -21.839144, which is correct.

Why is my Delphi app getting this wrong result?

I narrowed the problem to the "timeMillis" parameter.
It is a Java long, which corresponds to Delphi's int64.
I suspected of some kind of endian problem, so I
tried to switch the long parameter high and lo dwords.

And it worked !

function switchDWords(n:int64):int64;    // switch hi <--> lo dwords of an int64

var i: Integer;

    nn :int64; nnA:array[0..7]  of byte absolute nn;

    nn1:int64; nn1A:array[0..7] of byte absolute nn1;

begin

  nn1 := n;           // copy n

  for i := 0 to 3 do  // switch bytes  hidword <--> lodword

    begin

      nnA[i]   := nn1A[i+4];

      nnA[i+4] := nn1A[i];

    end;

  Result := nn;

end;



procedure TForm.Button1Click(Sender: TObject);

var GeoField: JGeomagneticField; tm:int64;  aD:Single; 

begin

  tm := System.DateUtils.DateTimeToUnix( Now, {InputAsUTC:} false )*1000;  



  tm := switchDWords(tm);    // <--  hack tm



  GeoField := TJGeomagneticField.JavaClass.init({Lat:}-23, {Lon:}-46, {Alt:}750, tm );

  aD := GeoField.getDeclination();   // <-- this shows -21.8416 which is correct !

  Label1.Text := FloatToStr( aD );

end;

Java2OP translation seems to be ok, so I wonder where the problem is.
Anyway, passing this hacked int64 parameter fixed it.