Signature pad on Android from Fragment

First thanks to the great job!!!

I tried to add a signature pad, on a android application using a custom view:

    public class Firma extends View{

// defines paint and canvas
private Paint drawPaint;
private Path path = new Path();


public Firma(Context context, AttributeSet attrs) {
    super(context);
    setFocusable(true);
    setFocusableInTouchMode(true);
    setupPaint();
}

private void setupPaint() {
    // Setup paint with color and stroke styles
    drawPaint = new Paint();
    int paintColor = Color.BLACK;
    drawPaint.setColor(paintColor);
    drawPaint.setAntiAlias(true);
    drawPaint.setStrokeWidth(5);
    drawPaint.setStyle(Paint.Style.STROKE);
    drawPaint.setStrokeJoin(Paint.Join.ROUND);
    drawPaint.setStrokeCap(Paint.Cap.ROUND);
    setDrawingCacheEnabled( true );
}

@Override
protected void onDraw(Canvas canvas) {
    canvas.drawPath(path, drawPaint);
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    float pointX = event.getX();
    float pointY = event.getY();
    // Checks for the event that occurs
    switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            path.moveTo(pointX, pointY);
            return true;
        case MotionEvent.ACTION_MOVE:
            path.lineTo(pointX, pointY);
            break;
        default:
            return false;
    }
    // Force a view to draw again
    postInvalidate();
    return true;
}

public void saveDrawing()
{
    Bitmap whatTheUserDrewBitmap = getDrawingCache();

    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    whatTheUserDrewBitmap.compress(Bitmap.CompressFormat.PNG, 0, baos);
// LATER I WILL STORE IN A SQLITE BLOB
}

public void borraDrawing() {
// THIS HOPEFULLY WILL ERASE THE SIGNATURE PAD 
    setDrawingCacheEnabled(false);
    invalidate();
    setDrawingCacheEnabled(true);
}

}

I use this view with this layout:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/disclaimersign"
    android:id="@+id/textView2"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:paddingEnd="10dp"
    android:paddingStart="10dp"
    android:layout_alignParentTop="true"
    android:layout_alignParentStart="true"
    android:layout_marginTop="48dp" />

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:text="@string/signature"
    android:id="@+id/textView3"
    android:paddingEnd="20dp"
    android:paddingStart="20dp"
    android:layout_marginTop="137dp"
    android:layout_below="@+id/textView2"
    android:layout_alignParentStart="true" />

<com.eternity.josep.eternityservice.views.Firma
    android:id="@+id/firma"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentEnd="true"
    android:layout_alignParentStart="true"
    android:layout_below="@+id/textView3"
    android:background="#e1fcf3"
    android:layout_above="@+id/email" />
<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:labelFor="@+id/email"
    android:text="@string/contactmail"
    android:id="@+id/l_email"
    android:layout_above="@+id/signReset"
    android:layout_alignStart="@+id/signReset" />
<EditText
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:inputType="textEmailAddress"
    android:ems="10"

    android:id="@+id/email"
    android:layout_above="@+id/signReset"
    android:layout_alignEnd="@+id/signSave"
    android:layout_toEndOf="@+id/signReset" />
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/SignReset"
    android:id="@+id/signReset"
    android:layout_alignParentBottom="true"
    android:layout_alignParentStart="true"
    android:layout_marginStart="38dp" />
<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/SaveSign"
    android:id="@+id/signSave"
    android:layout_alignParentBottom="true"
    android:layout_alignParentEnd="true"
    android:layout_marginEnd="40dp" />

I tried to manage from a fragment:

public class FragmentSign extends Fragment {

View view;
Firma firma;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    getActivity().setRequestedOrientation(
            ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

    view = inflater.inflate(R.layout.fragment_sign, container, false);
    super.onCreate(savedInstanceState);


    return view;
}

public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

}

@Override
public void onResume() {
    super.onResume();
    Button borra = (Button) view.findViewById(R.id.signReset);
    Button guarda = (Button) view.findViewById(R.id.signSave);
   firma=new Firma(getActivity(),null);

    borra.setOnClickListener(
            new View.OnClickListener() {
                public void onClick(View view) {
                    try {
                        firma.borraDrawing();
                    }catch(Exception e )
                    {
                        Log.d("LOGTAG",e.getMessage()  );
                    }
                }
            });

    guarda.setOnClickListener(
            new View.OnClickListener() {
                public void onClick(View view) {
                    try {
                        firma.saveDrawing();
                    }catch(Exception e )
                    {
                        Log.d("LOGTAG",e.getMessage()  );
                    }
                }
            });

    }

}

My problem is that when i debug here

public void saveDrawing()

{ Bitmap whatTheUserDrewBitmap = getDrawingCache();

getDrawingCache() is null, and not the same that when I debug here

        case MotionEvent.ACTION_DOWN:
            path.moveTo(pointX, pointY);
            return true;
        case MotionEvent.ACTION_MOVE:
            path.lineTo(pointX, pointY);
            break;

Could anybody gime an idea about the reason of this?

Thanks again.