タッチに追従してViewを動かす

今までどうやったらいいかわからなかったのですが、意外に簡単でした。

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/HelloWorld"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

 MainActivity.java

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        TextView helloWorld = findViewById(R.id.HelloWorld);
        helloWorld.setOnTouchListener(new moveOnTouchListener());

    }

    public static class moveOnTouchListener implements View.OnTouchListener {

        private static float dX;
        private static float dY;

        @Override
        public boolean onTouch(View view, MotionEvent motionEvent) {


            switch (motionEvent.getAction()) {
                case MotionEvent.ACTION_DOWN :
                    dX = view.getX() - motionEvent.getRawX();
                    dY = view.getY() - motionEvent.getRawY();
                    break;
                case MotionEvent.ACTION_MOVE :
                    view.setY(motionEvent.getRawY() + dY);
                    view.setX(motionEvent.getRawX() + dX);
                    break;
                case MotionEvent.ACTION_UP :
                    break;
            }

            return true;
        }

    }
}