Why have multiple constructors when they all call the same internal constructor with the same arguments

[moda20]

I found out that you are creating constructors for no reason when you don't do anything special for each constructor : here check these constructors they have the same implementation but with different constructor arguments :

  /// Constructor for creating [FadingEdgeScrollView] with [PageView] as child
  /// child must have [PageView.controller] set
  factory FadingEdgeScrollView.fromPageView({
    Key key,
    @required PageView child,
    double gradientFractionOnStart = 0.1,
    double gradientFractionOnEnd = 0.1,
    bool shouldDisposeScrollController = false,
  }) {
    assert(child.controller != null, "Child must have controller set");

    return FadingEdgeScrollView._internal(
      key: key,
      child: child,
      scrollController: child.controller,
      scrollDirection: child.scrollDirection,
      reverse: child.reverse,
      gradientFractionOnStart: gradientFractionOnStart,
      gradientFractionOnEnd: gradientFractionOnEnd,
      shouldDisposeScrollController: shouldDisposeScrollController,
    );
  }

  /// Constructor for creating [FadingEdgeScrollView] with [AnimatedList] as child
  /// child must have [AnimatedList.controller] set
  factory FadingEdgeScrollView.fromAnimatedList({
    Key key,
    @required AnimatedList child,
    double gradientFractionOnStart = 0.1,
    double gradientFractionOnEnd = 0.1,
    bool shouldDisposeScrollController = false,
  }) {
    assert(child.controller != null, "Child must have controller set");

    return FadingEdgeScrollView._internal(
      key: key,
      child: child,
      scrollController: child.controller,
      scrollDirection: child.scrollDirection,
      reverse: child.reverse,
      gradientFractionOnStart: gradientFractionOnStart,
      gradientFractionOnEnd: gradientFractionOnEnd,
      shouldDisposeScrollController: shouldDisposeScrollController,
    );
  }

why is that ?

[mponkin]

All constructors are extracting required parameters (scrollController, scrollDirection, reverse) from child widgets. Unfortunately scrollable widgets don't share same interface for this purpose even if their methods are looking similar.

[timukasr]

You could allow any Widget and extract based on type:

  factory FadingEdgeScrollView({
    Key? key,
    required Widget child,
    double gradientFractionOnStart = 0.1,
    double gradientFractionOnEnd = 0.1,
  }) {
    final (controller, scrollDirection, reverse) = _extractFromChild(child);

    if (controller == null) {
      throw Exception("Child must have controller set");
    }

    return FadingEdgeScrollView._internal(
      key: key,
      scrollController: controller,
      scrollDirection: scrollDirection,
      reverse: reverse,
      gradientFractionOnStart: gradientFractionOnStart,
      gradientFractionOnEnd: gradientFractionOnEnd,
      child: child,
    );
  }

  static (ScrollController? controller, Axis scrollDirection, bool reverse) _extractFromChild(Widget widget) {
    switch (widget) {
      case (ScrollView widget):
        return (widget.controller, widget.scrollDirection, widget.reverse);
      case (ReorderableListView widget):
        return (widget.scrollController, widget.scrollDirection, widget.reverse);
      case (SingleChildScrollView widget):
        return (widget.controller, widget.scrollDirection, widget.reverse);
      case (PageView widget):
        return (widget.controller, widget.scrollDirection, widget.reverse);
      case (AnimatedList widget):
        return (widget.controller, widget.scrollDirection, widget.reverse);
      case (ListWheelScrollView widget):
        return (widget.controller, Axis.vertical, false);
      default:
        throw Exception(
          "Child must be ScrollView, ReorderableListView, SingleChildScrollView, PageView, AnimatedList or ListWheelScrollView",
        );
    }
  }