Implicitly Animated Reorderable List

A Flutter ListView that implicitly calculates the changes between two lists using the MyersDiff algorithm and animates between them for you. The ImplicitlyAnimatedReorderableList adds reordering support to its items with fully custom animations.

Click here to view the full example.

Installing

Add it to your pubspec.yaml file:

dependencies:
  implicitly_animated_reorderable_list: ^0.1.1

Install packages from the command line

flutter packages get

Usage

The package contains two ListViews: ImplicitlyAnimatedList which is the base class and offers implicit animation for item insertions, removals as well as updates, and ImplicitlyAnimatedReorderableList which extends the ImplicitlyAnimatedList and adds reordering support to its items. See examples below on how to use them.

ImplicitlyAnimatedList

ImplicitlyAnimatedList is based on AnimatedList and uses the MyersDiff algorithm to calculate the difference between two lists and calls insertItem and removeItem on the AnimatedListState for you.

Example

// Specify the generic type of the data in the list.
ImplicitlyAnimatedList<MyGenericType>(
  // The current items in the list.
  items: items,
  // Called by the DiffUtil to decide whether two object represent the same item.
  // For example, if your items have unique ids, this method should check their id equality.
  areItemsTheSame: (a, b) => a.id == b.id,
  // Called, as needed, to build list item widgets.
  // List items are only built when they're scrolled into view.
  itemBuilder: (context, animation, item, index) {
    // Specifiy a transition to be used by the ImplicitlyAnimatedList.
    // In this case a custom transition.
    return SizeFadeTranstion(
      sizeFraction: 0.7,
      curve: Curves.easeInOut,
      animation: animation,
      child: Text(item.name),
    ); 
  },
  // An optional builder when an item was removed from the list.
  // If not specified, the List uses the itemBuilder with 
  // the animation reversed.
  removedItemBuilder: (context, animation, oldItem) {
    return FadeTransition(
      opacity: animation,
      child: Text(oldItem.name),
    );
  },
);

Note as AnimatedList doesn't support item moves, a move is handled by removing the item from the old index and inserting it at the new index.

ImplicitlyAnimatedReorderableList

ImplicitlyAnimatedReorderableList is based on ImplicitlyAnimatedList and adds reordering support to the list.

Example

ImplicitlyAnimatedReorderableList<MyGenericType>(
  items: items,
  areItemsTheSame: (oldItem, newItem) => oldItem.id == newItem.id,
  onReorderFinished: (item, from, to, newItems) {
    // Remember to update the underlying data when the list has been
    // reordered.
    setState(() {
      items
        ..clear()
        ..addAll(newItems);
    });
  },
  itemBuilder: (context, itemAnimation, item, index) {
    // Each item must be wrapped in a Reorderable widget.
    return Reorderable(
      // Each item must have an unique key.
      key: ValueKey(item),
      // The animation of the Reorderable builder can be used to
      // change to appearance of the item between dragged and normal
      // state. For example to add elevation when the item is being dragged.
      // This is not to be confused with the animation of the itemBuilder.
      // Implicit animations (like AnimatedContainer) are sadly not yet supported.
      builder: (context, dragAnimation, inDrag) {
        final t = dragAnimation.value;
        final elevation = lerpDouble(0, 8, t);
        final color = Color.lerp(Colors.white, Colors.white.withOpacity(0.8), t);

        return SizeFadeTranstion(
          sizeFraction: 0.7,
          curve: Curves.easeInOut,
          animation: itemAnimation,
          child: Material(
            color: color,
            elevation: elevation,
            type: MaterialType.transparency,
            child: ListTile(
              title: Text(item.name),
              // The child of a Handle can initialize a drag/reorder.
              // This could for example be an Icon or the whole item itself. You can
              // use the delay parameter to specify the duration for how long a pointer
              // must press the child, until it can be dragged.
              trailing: Handle(
                delay: const Duration(milliseconds: 100),
                child: Icon(
                  Icons.list,
                  color: Colors.grey,
                ),
              ),
            ),
          ),
        );
      },
    );
  },
);

For a more in depth example click here.

Caveats

Note that this package is still in its very early phase and not enough testing has been done to guarantee stability.
Also note that computing the diff between two very large lists my take significant amount of time (the computation is done on a background isolate though).

Acknowledgements

The diff algorithm that ImplicitlyAnimatedList uses was written by Dawid Bota at GitLab.