Auto scrolling TextView in constraint layout

MainActivity.java

// in your onCreate of MainActivity.java
        final TextView txtMsgView = findViewById(R.id.textView);
        txtMsgView.setMovementMethod(new ScrollingMovementMethod()); // Let users use the mouse or touchpad to move the scroll
        txtMsgView.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence charSequence, int i, int i1, int i2) {
            }
            @Override
            public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {
            }
            @Override
            public void afterTextChanged(Editable editable) {
                txtMsgView.post(new Runnable() {
                    @Override
                    public void run() {
                        if (txtMsgView.getLineHeight() * txtMsgView.getLineCount() >= txtMsgView.getHeight()) {
                            txtMsgView.setGravity(Gravity.BOTTOM | Gravity.LEFT);
                        } else {
                            txtMsgView.setGravity(Gravity.TOP | Gravity.LEFT);
                        }
                    }
                });
            }
        });
        // a buttton for testing auto scrolling
        Button btn = findViewById(R.id.button);
        btn.setOnClickListener(new View.OnClickListener(){
            int _counter = 0;
            @Override
            public void onClick(View view) {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        txtMsgView.append("this is test " + _counter + "\n");
                        Log.d("TEST","text" + _counter);
                        _counter++;
                    }
                });
            }
        });

/*
in your activity_main.xml
<TextView
        android:id="@+id/textView"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintBottom_toTopOf="@id/nav_view"
        android:background="@color/colorAccent"
        android:focusable="false"
        android:scrollbars="vertical"
        android:gravity="top|left"
        android:text="msg\n" />
    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintRight_toRightOf="parent" />
*/