_onFocus() {
        if (this.settings.isCancellable) {
            // Save the current unformatted value for later use by the 'cancellable' feature
            this._saveCancellableValue();
        }
    }
Handler for the 'focus' event.
We update the info of the focused state in the `this.isFocused` variable when the element gets focused.
@private