Returns whether or not the field value is currently valid by {@link #getErrors validating} the field's current
value, and fires the {@link #validitychange} event if the field's validity has changed since the last validation.
**Note**: {@link #disabled} fields are always treated as valid.

Custom implementations of this method are allowed to have side-effects such as triggering error message display.
To validate without side-effects, use {@link #isValid}.

@return {Boolean} True if the value is valid, else false
function() {
        var me = this,
            isValid = me.isValid();
        if (isValid !== me.wasValid) {
            me.wasValid = isValid;
            me.fireEvent('validitychange', me, isValid);
        }
        return isValid;
    }