getContentView() {
    let overlay: React.Element<void, void> = null;

    if (this.isOpen) {
      overlay = (
        <TouchableWithoutFeedback onPress={() => this.openMenu(false)}>
          <View style={styles.overlay} />
        </TouchableWithoutFeedback>
      );
    }

    const { width, height } = this.state;
    const ref = sideMenu => (this.sideMenu = sideMenu);
    const style = [
      styles.frontView,
      { width, height },
      this.props.animationStyle(this.state.left),
    ];

    return (
      <Animated.View style={style} ref={ref} {...this.responder.panHandlers}>
        {this.props.children}
        {overlay}
      </Animated.View>
    );
  }
Get content view. This view will be rendered over menu
@return {React.Component}