Button根据多个EditText输入条件改变背景颜色

et_usename = (EditText) findViewById(R.id.et_phone);
et_code = (EditText) findViewById(R.id.et_code);
bt_register = (Button) findViewById(R.id.bt_register);
bt_register.setOnClickListener(this);
WatchChange watch = new WatchChange();
et_usename.addTextChangedListener(watch);

et_code.addTextChangedListener(watch);

/**
 * 自定义监听EditText
 */
class WatchChange implements 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) {
        if(et_code.length()==4&&et_usename.length()==11){
            bt_register.setBackgroundResource(R.drawable.login_background_red);
            bt_register.setEnabled(true);
        }else {
            bt_register.setBackgroundResource(R.drawable.login_background);
            bt_register.setEnabled(false);
        }
    }
}