EditText with masked input for russian (+7) phone numbers

PhoneEditText.java

import android.content.Context;
import android.text.Editable;
import android.text.TextWatcher;
import android.util.AttributeSet;
import android.view.View;
import android.widget.EditText;

public class PhoneEditText extends EditText implements TextWatcher, View.OnFocusChangeListener {

    private static final String PATTERN_NOT_NUMBERS = "[^0-9]*";
    private static final String PATTERN_NUMBERS = "[0-9]*";
    private static final String EMPTY = "";

    private static final int COUNTRY_CODE = 7;
    private static final String PLUS = "+";
    private static final String MINUS = "-";
    private static final String OPEN_BRACKET = "(";
    private static final String CLOSE_BRACKET = ")";
    private static final String SPACE = " ";

    private static final int MAX_LENGTH = 11;

    private static final int PART1_START = 1;
    private static final int PART2_START = 4;
    private static final int PART3_START = 7;
    private static final int PART4_START = 9;

    private boolean isUpdating;
    private int currentPos;

    public PhoneEditText(Context context, AttributeSet attrs) {
        super(context, attrs);
        addTextChangedListener(this);
        setOnFocusChangeListener(this);
    }

    @Override
    public void afterTextChanged(Editable s) {
        if (s.length() == 0 || isUpdating) {
            isUpdating = false;
            return;
        }

        // extract number from text
        String number = s.toString().replaceAll(PATTERN_NOT_NUMBERS, EMPTY);
        if (number.length() > MAX_LENGTH)
            number = number.substring(0, MAX_LENGTH);

        int length = number.length();

        StringBuilder result = new StringBuilder();

        if (length > 0) {
            result.append(PLUS + COUNTRY_CODE);
            currentPos++;
        }

        if (length > PART1_START) {
            result.append(SPACE + OPEN_BRACKET);
            if (length >= PART2_START)
                result.append(number.substring(PART1_START, PART2_START));
            else
                result.append(number.substring(PART1_START));

            currentPos += 2;
        }

        if (length > PART2_START) {
            result.append(CLOSE_BRACKET + SPACE);
            if (length >= PART3_START)
                result.append(number.substring(PART2_START, PART3_START));
            else
                result.append(number.substring(PART2_START));

            currentPos += 2;
        }

        if (length > PART3_START) {
            result.append(MINUS);
            if (length >= PART4_START)
                result.append(number.substring(PART3_START, PART4_START));
            else
                result.append(number.substring(PART3_START));

            currentPos++;
        }

        if (length > PART4_START) {
            result.append(MINUS);
            if (length >= MAX_LENGTH)
                result.append(number.substring(PART4_START, MAX_LENGTH));
            else
                result.append(number.substring(PART4_START));

            currentPos++;
        }

        isUpdating = true;
        setText(result);
    }

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {
    }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {
        if (isUpdating) {
            setSelection(currentPos);
        } else {
            final String number = s.toString().replaceAll(PATTERN_NUMBERS, EMPTY);
            currentPos = start + count - number.length();
            currentPos = (currentPos > MAX_LENGTH ? MAX_LENGTH : currentPos);
        }
    }

    @Override
    public void onFocusChange(View v, boolean hasFocus) {
        if (isEnabled()) {
            String text = getText().toString();
            if (text.length() == (PART1_START + 1)) {
                setText(EMPTY);
            }
        }
    }
}