Dynamic Sliver FloatingActionButton

In this post, I will walk through the process of creating FloatingActionButton that is pinned to the edge of FlexibleSpaceBar (shown below). Let’s get to it!

TLDR: I exported pub package, so you can achieve similar FAB behavior. Feel free to use and expand it. 🙂

The problem

Main problem with implementing pinned FloatingActionButton like this, is that there is no place inside the CustomScrollView or SliverList to put a Widget that would be on top of SliverAppBar and Slivers below it. In order to do it, we have to put our FloatingActionButton outside CustomScrollView and place it inside a Stack. Here is our starting point:

class MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new Stack(
        children: <Widget>[
          new CustomScrollView(
            slivers: [
              new SliverAppBar(
                expandedHeight: 256.0,
                pinned: true,
                flexibleSpace: new FlexibleSpaceBar(
                  title: new Text("SliverFab Example"),
                  background: new Image.asset(
                    "img.jpg",
                    fit: BoxFit.cover,
                  ),
                ),
              ),
              new SliverList(
                delegate: new SliverChildListDelegate(
                  new List.generate(
                    20,
                    (int index) => new ListTile(title: new Text("Item $index")),
                  ),
                ),
              ),
            ],
          ),
          new Positioned(
            top: 256.0,
            right: 16.0,
            child: new FloatingActionButton(
              onPressed: () {},
              child: new Icon(Icons.add),
            ),
          ),
        ],
      ),
    );
  }
}

It looks like this:

Pinning FloatingActionButton

As you can see, we have a beautiful FloatingActionButton pinned to the edge of the AppBarbut only if the user will not scroll the content. Not the best use case for us. In order to pin FloatingActionButton to edge of AppBar, we need to use ScrollController to keep track of scrolling offset and change position of fab. First, let’s add ScrollController that will update our Widget’s state:

class MyHomePageState extends State<MyHomePage> {
  ScrollController _scrollController;

  @override
  void initState() {
    super.initState();
    _scrollController = new ScrollController();
    _scrollController.addListener(() => setState(() {}));
  }

  @override
  void dispose() {
    _scrollController.dispose();
    super.dispose();
  }
  ...
}

Then let’s add it to the CustomScrollView:

new CustomScrollView(
  controller: _scrollController,
  slivers: [
    ...
  ]
)

And then let’s change FloatingActionButton position depending on scrolling offset. To do that I extracted creating FloatingActionButton to a new method. A thing that is worth mentioning is that on the first build _scrollController is not connected to scrollViewyet, so we can’t directly ask it for offset.

Widget _buildFab() {
  double top = 256.0 - 4.0; //default top margin, -4 for exact alignment
  if (_scrollController.hasClients) {
    top -= _scrollController.offset;
  }
  return new Positioned(
    top: top,
    right: 16.0,
    child: new FloatingActionButton(
      onPressed: () => {},
      child: new Icon(Icons.add),
    ),
  );
}

Current result should look like this:

Hiding FloatingActionButton

As you can see, our FloatingActionButton is scrolling way out of the screen. Let’s hide it when it comes close to the top. To do that we could use Opacity widget, however visual aspect of that solution is not great since Fab would be just vanishing. In my opinion better solution would be to scale FloatingActionButton down, so let’s try to do that:

Widget _buildFab() {
  //starting fab position
  final double defaultTopMargin = 256.0 - 4.0;
  //pixels from top where scaling should start
  final double scaleStart = 96.0;
  //pixels from top where scaling should end
  final double scaleEnd = scaleStart / 2;

  double top = defaultTopMargin;
  double scale = 1.0;
  if (_scrollController.hasClients) {
    double offset = _scrollController.offset;
    top -= offset;
    if (offset < defaultTopMargin - scaleStart) {
      //offset small => don't scale down
      scale = 1.0;
    } else if (offset < defaultTopMargin - scaleEnd) {
      //offset between scaleStart and scaleEnd => scale down
      scale = (defaultTopMargin - scaleEnd - offset) / scaleEnd;
    } else {
      //offset passed scaleEnd => hide fab
      scale = 0.0;
    }
  }

  return new Positioned(
    top: top,
    right: 16.0,
    child: new Transform(
      transform: new Matrix4.identity()..scale(scale),
      alignment: Alignment.center,
      child: new FloatingActionButton(
        onPressed: () => {},
        child: new Icon(Icons.add),
      ),
    ),
  );
}

ScaleStart and ScaleEnd values can be changed depending on what behavior you expect. I find these values to satisfy my aesthetic needs 😀

And that’s it! Now we have nice FloatingActionButton placed on the edge of AppBar which looks like this:

Since this kind of view might be needed by other people, I exported it as a pub package. I designed it only for my use case so if you think, there are any bugs or missing features I would highly appreciate any pull requests 🙂

You can find source code created in this post here and current repo with the code here.

Cheers 🙂